Geochemistry, mineralogy, petrology and volcanology

Crustal sagging during major volcanism

May 2010

Ice sheets during the last glaciation reached more than 2 km in thickness over vast high-latitude areas of the Northern Hemisphere. Even though ice has less than half the density of continental crust, their sheer mass forced the lithosphere down into the asthenosphere by up to several hundred metres. The displaced asthenosphere resulted in a corresponding bulge around the glacial fringe. Continental flood basalts are about three times as dense as ice and reach thicknesses up to 2-3 km, so they would have produced even more subsidence, although set against that is the uplifting effect of reduced density of the crust as a result of magmatic heating. The loading effects of individual volcanoes are well known. Yet surprisingly, there have been few accounts of subsidence caused by CFB loading, and the prevailing view is that plume-related large igneous provinces are preceded by doming and even erosion. Geophysicists at the University of Colorado modelled the effects of plumes and CFB eruption and reverse the general view decisively (Leng, W. & Zhong, S 2010. Surface subsidence caused by mantle plumes and volcanic loading in large igneous provinces. Earth and Planetary Science Letters, v. 291, p. 207-214). They found that phase changes in the rising mantle plume at the 660 km deep discontinuity cause subsidence themselves, so that even before volcanism begins the surface subsides. This is borne out by preservation of basinal sediments beneath some CFB provinces, such as the Siberian and Deccan Traps. Effectively, flood basalts may fill shallow basins that they recreate and maintain due to their loading effect on the crust during successive eruptions. The high elevations of many ancient CFB provinces are a product of later tectonic processes rather than being 'built' by volcanism.

'Microdating' sedimentary sequences

May 2010

There are few minerals amenable to radiometric dating that are found in all sedimentary rock types. To give ages that are stratigraphically useful they would have had to form authigenically while the sediment itself was accumulating - glauconite in 'greensands' is an example. Calibrated stratigraphy largely depends on dateable igneous minerals found in volcanic rocks interlayered with sediments, the most common being zircon that can be dated precisely using U-Pb methods. The vast bulk of high quality ages of this kind depend on being able to collect sufficient volcanic ash or lava to yield zircon grains. So only volcanic layers thicker than a few centimetres have been used, and they are haphazard in their occurrence in sedimentary sequences. Much thinner ash layers do occur more commonly and uniformly in sequences from arc-related sedimentary basins, and being able to date those would permit much better control over rates of sedimentation and correlation between different sequences. The key is being able to date zircons in thin section (Rasmussen, B. & Fletcher, I.R. 2010. Dating sedimentary rocks using in situ U-Pb geochronology of syneruptive zircon in ash-fall tiffs <1 mm thick. Geology, v. 38, p. 299-302). Rasmussen and Fletcher (Curtin University, Western Australia) applied ion-microprobe methods to polished this sections of diamond drill core through Archaean sediments of the Pilbara craton in Western Australia, specifically to date a thin sediment layer that contains spherules formed by a major asteroid impact. They were able to narrow its age down to that of a thin ash only 15 mm above the spherules, about 2632±7 Ma. Though with a specialised objective, they demonstrate that semi-continuous stable isotope data in sediments can be calibrated sufficiently precisely to allow global correlations

Geochemical clue to environmental effects of large igneous provinces

January 2010

Several flood volcanism events seem to link to mass extinctions, and they have been seen as the culprits for global environmental change. Since flood volcanism is outside human experience, geologists have little conception of what they do other than amass up to millions of cubic kilometres of lavas both mafic and silicic. They all probably emitted CO₂ and contributed to global warming, but whether they are able to deliver sulfate and particulate aerosols to the stratosphere to trigger cooling is hard to judge. But it seems there is a proxy for their global influence (Peate, D. 2009. Global dispersal of Pb by large-volume silicic eruptions in the Paraná-Etendeka large igneous province. Geology, v. 37, p. 1071-1074). Lead is potentially a volatile element that would accompany large volcanic gas and dust emissions, and it also bears unique isotopic signatures. Lead isotope proportions in sediments in contemporaneous marine sediments could be matched with those of large igneous provinces (LIPs). Should their signature occur globally, then it would be a fair bet that the products of volcanism did reach cloud-free stratospheric altitudes, there to be mixed globally and to remain aloft for many years. Below the tropopause gas and dust would soon be rained out, so that signatures would remain local.

Dave Peate of the University of Iowa found that the ²⁰⁸Pb/²⁰⁴Pb and ²⁰⁶Pb/²⁰⁶Pb ratios of 132 Ma sediments from an Ocean Drilling Program core in the mid-Pacific fall in the same field as those of the Paraná-Etendeka large igneous province. The sediments occur just below and within a prominent d¹³C anomaly that geochemists believe to signify a major change in the biosphere, and the site is almost at the antipode of the Paraná-Etendeka large igneous province. Sediments from below the shift in carbon isotopes show lead-isotope ratios that can be explained by derivation from the oceanic crust underlying them, whereas those that witness a profound change in the biosphere overlap with the field of the P-E LIP. Specifically, they match the lead ‘signature’ of silicic volcanics rather than basalts, and in particular those with low titanium contents. So it seems that in this case basalt floods may not have been implicated in global environmental change, but the much less voluminous but probably far more violent ignimbrite do seem likely culprits. There were more than 20 such events within an interval of less than 2 Ma that emitted >100 km³ of silicic magma, most exceeding 1000 km³.

Did mantle chemistry change after the late heavy bombardment?

September 2009

During the Hadean the Inner Solar System was subject to a high flux of asteroidal debris, culminating in a dramatic increase in the rate of cratering on planetary surfaces between 4.0 and 3.8 Ga known as the late heavy bombardment. It left a subtle mark in tungsten isotopes of the Earth’s continental crust that formed during and shortly after the cataclysm (see Tungsten and Archaean heavy bombardment, August 2002 EPN). It has also been suggested that it enriched the mantle in elements, such as those of the platinum group, that have an affinity for metallic iron, a major constituent of many meteorites. The most likely rocks of the Archaean crust to show hints of such enrichment are ultramafic lavas known as komatiites, though to have formed by high degrees of partial melting of plumes rising from deep in the Archaean mantle. Komatiites from their type locality in South Africa and from the Pilbara area of Western Australia do indeed suggest that there was significant effects (Maier, W. D. et al. 2009. Progressive mixing of meteoritic veneer into the early Earth’s deep mantle. Nature, v. 460, p. 620-623). The Finnish-Australian-Canadian team found that the older komatiites (3.2-3.4 Ga) contain less platinum-group elements (PGE) than do those from the later Archaean and early Proterozoic (2.0-2.9 Ga). This they ascribe to a surface layer of meteoritic debris gradually being mixed into the mantle by convection. In their discussion they suggest that once the Earth’s core formed (almost certainly very soon after the Moon-forming event at 4.45 Ga) it effectively leached all PGE from the lower mantle, and could only have achieved higher concentrations by mixing of later meteoritic debris. Their results suggest that this went on through the Hadean, but reached its acme and then stabilised in the late Archaean once the earlier Archaean alien debris had been churned in.

The swaddled mantle

May 2009

A great deal of both theoretical petrology and tectonics hinges on how temperature changes with depth within the Earth. The geotherm, as this variation is termed, depends on how heat is conducted – by conduction, convection or radiation – and where it is produced – either as a relic of original heat of Earth’s accretion or through decay of radioactive isotopes. There are plenty of imponderables, and it would be safe to say that, below the depths at which we can measure temperature (a few km), geotherms are guesswork. Metamorphism, partial melting in crust and mantle, and the rigidity of rock depend on temperature and pressure. Rocks that are too cool to act in a plastic manner tend only to conduct heat, and they are poor conductors. This applies to most of the crust, especially the lower continental crust, which is also low in heat producing radioactive K, U and Th isotopes and rigid. The upshot of this is that the crust acts to insulate the mantle, and that implies build-up of heat and temperature just below the crust. A new means of measuring a rock’s thermal conductivity has revealed that thermal conductivity actually decreases as temperature rises (Whittington, A.G et al. 2009. Temperature dependent thermal diffusivity of the Earth’s crust and implications for magmatism. Nature, v. 458, p. 319-321). The range of crustal temperatures in both continental and oceanic crust roughly halves conduction in the lower crust from previously measured values. This further increases insulation of the mantle, boosting the chances of partial melting. This tallies with a coincidentally published account of how seismic shear waves change speed with depth beneath the oceanic crust (Kawakatsu, H. et al. 2009. Seismic evidence for sharp lithosphere-asthenosphere boundaries of oceanic plates. Science, v. 324, p. 499-502). As well as sharply showing up the lithosphere-asthenosphere boundary, thought to be a transition from brittle to ductile behaviour, it detects thin layers of partially melted peridotite, which facilitates plate tectonics. A further coincidence is publication of an analysis of 15 years of global earthquake records that focuses on the base of the lithosphere (Rychert, C.A. & Shearer, P.M 2009. A global view of the lithosphere-asthenosphere boundary. Science, v. 324, p. 495-498). As well as its thickness this effectively maps the top of the asthenosphere and therefore the thickness of tectonic plates across the planet, albeit crudely (previously both had been estimated from surface heat flow and theoretical models). Beneath cratons that have remained sluggish for more than a billion years, the asthenosphere is deep (~95 km) and thin, shallowing and thickening appreciably beneath more recently active continental belts. Despite being the uppermost Earth and the stuff of plates and the medium upon which they move, respectively, the lithosphere and asthenosphere are less-well known than the mantle and even the core in terms of the mechanical properties. That may sound odd, but there is a good reason why it is so: more deeply travelled seismic waves are a great deal easier to record by the global network of seismic stations than are shallow regions.

At last, 4.0 Ga barrier broken

November 2008

Since the 1960s when Stephen Moorbath of the University of Oxford determined a date of 3.8 Ga for metamorphic rocks in West Greenland discovered by Vic McGregor of the Geological Survey of Greenland, pushing the age of tangible rocks towards that of the Earth itself has been slow. Indeed, geologists found only one geological terrain that pushed the ‘vestige of a beginning’ significantly back in time beyond the famous Isua rocks: the Acasta Gneiss east of Great Slave Lake in northern Canada, dated at 30 Ma more than 4 Ga. In fact in the 30 years between Moorbath’s Greenland date and that for the Acasta Gneiss, stratigraphers seem to have become resigned to a maximum 3.8 Ga age for rocks, and the start of the Archaean was set at that age. All earlier time, some 750 Ma of it, became known as the Hadean – a hellish time from which nothing had survived. Some geochemists perked up with the discovery, sifted from a much younger sandstone in the late 1980s, by Australians Bill Compston and Bob Pidgeon of 17 zircon grains that formed up to 4.4 Ga ago; but they tell us very little about the early world. What had become the lost cause of seeking pre-4 Ga rocks, has suddenly become revitalised with the discovery of a voluminous suite of rocks that are 200 million years closer to Earth’s origin in the eastern part of Arctic Canada (O’Neil, J. et al. 2008. Neodymium-142 evidence for Hadean mafic crust. Science, v. 321, p. 1828-1831).

The rocks are part of a recently mapped greenstone belt on the east shore of Hudson Bay, which contains a variety of mafic igneous rocks along with metasedimentary banded iron formations and cherts. The most dominant of the mafic rocks has yielded a ¹⁴⁶Sm-^142Nd isochron age of almost 4.3 Ga, and they are intruded by mafic and ultramafic sills dated at around 4.0 Ga. The older meta-igneous rock’s geochemistry suggests that it formed by partial melting of undepleted mantle rocks to produce magmas similar to those forming at modern convergent plate margins. Its major element variability, reflected in very diverse metamorphic mineral assemblages, suggests it to have originally formed as a mafic pyroclastic rock. It would be hard to prove that the BIFs and cherts are the same age in such a structurally complex belt, but that they are as old as the dated material is a distinct possibility. In that case they push back tangible evidence for surface water a great deal more convincingly than the arcane isotopic evidence derived from the oldest known zircons (see Zircon and the quest for life’s origin in the May 2005 issue of EPN). That such a substantial piece of very old crust has turned up a record age owes a great deal to advances in the Sm-Nd dating technique; the use of ¹⁴⁶Sm decay to ^142Nd (1/2 life of ~10⁸ years), rather than the more readily addressed ¹⁴⁷Sm to ^143Nd decay (1/2 life of ~10¹¹ years). This proof of concept may unleash a reappraisal of rocks that seem to be the oldest relative to others in Precambrian shields on every continent. It may eventually become possible to show that, apart from its cataclysmic experience that formed the Moon and probably a global magma ocean shortly after accretion, the Earth was by no means a totally hellish period during the ‘Hadean’.

Banding in BIFs

November 2008

Banded iron formations, or BIFs, from the late Archaean and early Proterozoic are made of interlayered accumulations of iron oxides (and occasionally sulfides) and chert, and are the world’s most important iron ores. The BIFs of the Hammersley Range in Western Australia produce 26 % of the western world’s iron ore, and are hundreds of metres thick. The banding extends down to the scale of a few micrometres, and in some cases seems to record cyclic events. It has been claimed that, sun-spot, tidal, Milankovich and other nature cycles can be discerned. Few dispute that the iron oxides formed by oxidation of dissolved iron(II) ions through the influence of micro-organisms in shallow seawater. A popular candidate is photosynthetic blue-green bacteria, which produce oxygen; abundant reduced iron dissolved in Archaean seawater would have consumed the oxygen to become insoluble iron (III) oxides, delaying the development of an oxygen-bearing environment util about 2.2 Ga. There are other possibilities, such as anoxygenic photosynthesising bacteria, or photoferrotrophs, that could have achieved the Fe(II) to Fe(III) oxidation directly, without the need for free oxygen.. The puzzle is the on-off mechanism needed to produce the banding itself. That may have been resolved by experimental work under simulated Archaean conditions (Posth, N.R. et al. 2008. Alternating Si and Fe deposition caused by temperature fluctuations in Precambrian oceans. Nature Geoscience, v. 1, p. 703-708). The authors based their experiments on primitive, but living photoferrotrophs in conditions that chemically mimic likely Archaean seawater. They discovered that the critical factor in this form of biogenic precipitation of iron is sea-surface temperature: the microbes reproduce fastest to maximise iron-oxide formation at 20-25ºC. Temperatures above or below this range shut down productivity. However, temperatures above 25ºC favour silica remaining in solution, so the alternation of Fe- and Si-rich bands favours cooler sea temperatures for the latter. As well as providing a means of producing the enigmatic BIF banding, the experiments help resolve the controversy over prevailing sea-surface temperatures in the Archaean, which have been suggested by some to be as high as 85ºC. At least for the late Archaean, ocean temperatures seem to have been much the same as at present.

Great surprise: Deccan flood volcanism emitted gases

May 2008

The only documented volcanic eruption resembling those thought to characterise effusion of flood basalts was of the Icelandic Laki fissure in 1783. At 14 km3 its lava volume was minuscule compared with those of ancient flood-basalt flows, but it did have a remarkable effect on the atmosphere and climate of the Northern Hemisphere. A bluish, ground-hugging dry fog spread over much of Europe and North America. The fog caused severe chest ailments and was probably full of sulfuric acid aerosols. Such droplets also serve to increase the reflectivity of the atmosphere, thereby reducing solar heating. In fact, witnesses remarked on how dim the summer sun appeared that year, although it seems not to be particularly chilly. The climatic effects emerged the following winter with the average temperature in Paris falling by almost 5°C from the long-term average. On Iceland itself, crops failed during the eruption, but worse was to come. Both livestock and humans developed the awful bone lesions associated with fluorosis, for the basalt magma emitted hydrogen fluoride as well as SO2. Human and animal skeletons from the time show gross bone deformities, often like fibrous needles that would have grown through living flesh. Gas emissions from modern basalt flows chemically similar to those of Laki and far larger flood basalts are well documented, and the potential climate effects of continental flood basalt magmatism have been modelled repeatedly using those data.

Measuring actual gas contents of the magmas that fed ancient lava flows is difficult, simply because most magma degasses before it finally crystallises. Even vesicles are devoid of pristine gas that formed them, due to later percolation of fluids. In a few extremely fresh flows some of the original magma may have been preserved as glassy blobs trapped within phenocrysts such as olivine or Ca-plagioclase that formed in magma chambers before eruption. A group from the Open University, UK has analysed sulfur and chlorine content in four such minute samples by electron probe and XRF, finding levels up to 1400 and 900 ppm respectively (Self, S. et al. 2008. Sulfur and chlorine in late Cretaceous Deccan Magmas and eruptive gas release. Science, v. 319, p. 1654-1657).  The sulfur values are not unusual compared with modern basaltic glasses that have not lost their magmatic gases, though chlorine concentrations are somewhat high in the known range.

The climatic and environmental implications of both gases are noteworthy, mainly because each basalt flood would have emitted hundreds to thousands of teragrams of each annually – vastly more than modern emissions by both humanity and active volcanoes. In the lower atmosphere effects would have been like those of Laki – locally choking fogs acid rain, and cooling. Had chlorine reached the stratosphere it would have destroyed ozone to increase exposure of terrestrial life to UV radiation. So quite a few large-scale kill mechanisms may be ascribed to continental flood basalts such as the Deccan province.

This may well be the first direct evidence for actual gas-emission potential of ancient basalt magma samples. Sadly, however, the specimens containing glass were erupted some time before the K-T extinction event – the on-line data supplement reports ages of 66-68 Ma for the lower Deccan flows in which glass inclusions occur, between 0.5 to 2.5 Ma earlier than the end of the Cretaceous. That undermines, to some extent, the need to have analysed the glasses in the first place, when modern data serve well for modelling the effects of CFBs.  Still, even at the low end of S and Cl contents of modern undegassed basalt magmas, the stupendous volume of any flood basalt province – up to millions of km3 – would have repeatedly placed great stresses on the biosphere. The wonder is that not all CFBs are associated with mass extinctions, so maybe the environmentally less-destructive CFB provinces since 250 Ma ago (8 out of 11) involved magmas with extremely low S and Cl contents…

What becomes of all the sediments?

March 2008

It used to be widely thought that sediment of the ocean floor and that at active continental margins or ahead of volcanic arcs were scraped off subducting lithosphere and simply added to continental growth. If that didn’t happen, then perhaps continents could be recycled by a combination of erosion and tectonics? Geochemists know better now, for a variety of compositional anomalies in volcanic rocks do suggest a measure of recycling of subducted lithosphere, and it is becoming clear that part of the oddity has a sedimentary source. “Which one?” is the question.

Hafnium and neodymium isotopes have become choice tracers of whether basaltic magmas formed from pristine mantle, that depleted by previously sourcing magma or some kind of mixture with recycled materials. . Catherine Chauvel and colleagues from the University of Grenoble have pondered on the sizeable amount of Hf and Nd isotopic data that has emerged from a couple of decades of fancy mass spectrometry of ocean-island and mid-ocean-ridge basalts, and a variety of sediments (Chauvel, C. et al. 2008. Role of recycled oceanic basalt and sediment in generating the Hf-Nd mantle array. Nature Geoscience, v. 1, p. 64-67). By modelling how various reasonable mixtures of isotopes of the two elements might fit the simple Hf-Nd relationship for the source mantle of all oceanic basalts they discovered that it couldn’t be derived from just the crystalline oceanic lithosphere, but must involve a substantial contribution from subducted sediments. Moreover, they seem to have demonstrated that much of the mantle involved in producing ocean-island, hot-spot basalts is a product of this recycling – both oceanic crust and its sedimentary cover get down to the levels where the mantle involved in hot-spot melting originates. Although there is a good probability of separation of sediment and crystalline components of subducted slabs according to density, it seems from the modelling that some sediment does get down to profound levels.
See also: Plank, T. & van Keken, P.E. 2008. The ups and downs of sediment. Nature Geoscience, v. 1, p. 17-18, especially their astonishing figure giving a graphic notion of the forms mantle convection might take (see Deep geothermal processes).

Moon formed from vapour cloud

January 2008

The Moon is generally believed to have formed from the debris ejected when a body (nicknamed Theia) about the size of Mars struck the partly formed Earth a glancing blow. That cataclysmic event can be considered to have marked the start of geochemical evolution of both Earth and Moon. From a purely mechanical standpoint, it seems almost inevitable that the Moon is made mainly from debris supplied by the offending small planet. Yet Earth and Moon have some profound geochemical similarities, the most remarkable being their now similar blend of oxygen isotopes. Meteorite studies suggest that oxygen isotopes varied widely in the early Solar System, probably differing according to distance from the Sun. That suggests that the Earth-Moon similarity is somewhat odd, unless the impacting planet formed in the same part of space as the Earth itself, i.e. in a very similar orbit. However, that is as mechanically unlikely as the Moon being a chunk of Earth flung off by the impact.

A new explanation for shared oxygen isotopes is based on a model for the collision that involves the vaporisation of most of the Earth and Theia (Pahlevan, K. & Stevenson, D.J. 2007. Equilibration in the aftermath of the lunar-forming giant impact. Earth and Planetary Science Letters. v. 262, p. 438–449). High temperature vapour would have involved sufficient turbulence for the geochemical signatures of both Earth and Theia to have been mixed efficiently.  The Moon would then have condensed from a disk of orbiting vapour of this mixed composition, most of the Earth re-accreting in a molten state too. Thus both bodies would have begun their evolution with deep magma oceans. The light-coloured, highland part of the Moon is thought to be a relic of the flotation of plagioclase crystals that floated to the top of its magma ocean as it began to cool; the lunar highlands are made of anorthosite and are at least 4.4Ga old. So far no tangible sign of such relics of early fractionation have appeared in the Earth’s geological record. Pahlevan and Stevenson’s model indicates that only between 100 to 1000 years would have elapsed from impact to appearance of the moon as a tangible body.

Another angle on the mysteries of the Hadean

January 2008

Geochemists will be celebrating the end of 2007 after a steady growth in knowledge about times before formation of the first real rocks, albeit of a proxy nature. The latest addition stems from the isotopes of the rare-earth element neodymium. Its heaviest isotope 144Nd is a direct product of nucleosynthesis in supernova star explosions The middleweight isotope 143Nd is well-known as the daughter product of the decay of one unstable isotope of a sister element, samarium (147Sm, half-life 1.06 x 105 Ma). The Sm-Nd dating method, based on this decay, has been an important means of dating ancient mafic and ultramafic rocks and examining the geochemistry of their source rocks in the mantle for over 20 years. The lightest isotope is also a daughter of radioactive decay but would have formed from short-lived 146Sm (108 Ma half life). Potentially, 142Nd in old rocks can be used to judge processes in the Hadean mantle as 146Sm would have declined rapidly in the early Solar System – none is detectable nowadays. In meteorites it reveals complexities in the early differentiation of their parental planetesimals, and lunar studies show that too was subject to fractionation. That something odd happened in the early Earth became apparent when it was discovered that modern crust and mantle had more radiogenic 142Nd than the chondritic meteorites thought to have been the building blocks for the Earth. A study of neodymium isotopes in the two largest old chunks of continental crust – the  Archaean gneisses of SW Greenland and Western Australia – revealed yet more (Bennett, V.C. et al. 2007. Coupled 142Nd-143Nd   isotopic evidence for Hadean mantle dynamics. Science, v. 318, p. 1907-1910). The two blocks are different as regards their neodymium, and this suggests that a fundamental chemical division of the Earth’s mantle took place during the Hadean, which lasted for the next billion years at least. Yet another long-held idea about the Earth’s origin seems condemned to the status of myth. It had been assumed that the early Earth was well-mixed as a result of its accretion from countless planetesimals – it doesn’t really matter if they included different varieties because accretion would have been such a chaotic process. Discovering whether the now-established mantle fractionation resulted during accretion or after a cataclysmic collision with another world formed the Earth-Moon system is set to be the next challenge for students of the Hadean. It will probably be argued that this requires yet more samples to be brought from the Moon…

The first whiffs of abundant oxygen

November 2007

Until recently, evidence for the first appearance of gaseous oxygen in the air came from the oldest terrestrial red beds, especially palaeosols. They go back to about 2200 Ma. A newer line of investigation is based on the oxidation of sulfide ions in sea water, to produce sulfate ions. Yet, sulfates such as gypsum or anhydrite are ephemeral minerals that are usually preserved in evaporites. However, the oxidative sulfur cycle selectively fractionates different sulfur isotopes in distinctive ways: a means of charting oxidation in considerable detail, for sulfur is ubiquitous in many sedimentary rocks. A couple of sequences from late Archaean BIF-rich formations in Australia and South Africa contain thick marine mudstones, ideal for producing intricate sulfur-isotope time sequences. Studies of sulfur and oxidation-reduction sensitive trace metals, such as molybdenum, show that oxygen had pervaded surface ocean waters right at the Archaean-Proterozoic boundary (Kaufman, A.J. and 9 others 2007. Late Archaean biospheric oxygenation and atmospheric evolution. Science, v.  317, p. 1900-1903. Anbar, A.D. and 10 others 2007. A whiff of oxygen before the Great Oxidation Event. Science, v.  317, p. 1903-1906). Almost certainly, the oxidation was produced by newly evolved metabolism based on sulfide-sulfate oxidation in prokaryotic organisms.

Another metabolic process is fuelled by the opposite reaction: sulfate-sulfide reduction, again exploited by some prokaryotes, such as those that help to precipitate metal sulfides around ocean-floor ‘black smokers’. This too is something that may be picked up by studies of sulfur isotopes, and previous work has suggested that peculiarities involving 34S and 32S in much older Archaean baryte (BaSO4) indicated the presence of such metabolism, and by inference that of oxidised sulfate ions in seawater. The baryte occurs in sulfide-rich cherts from the Pilbara Craton, associated with ultramafic lavas, dated at 3490 Ma. The rocks contain abundant signs of biofilms. New sulfur-isotope studies (Philippot, P. et al. 2007. Early Archaean microorganisms preferred elemental sulfur, not sulfate. Science, v.  317, p. 1534-1537) suggest that the microscopic sulfide particles in the cherts did not form by reduction of sulfate ions. Instead, it is probably due to prokaryote metabolism that exploits a peculiarity of elemental sulfur, which can act as both an electron donor and an electron acceptor; i.e. it can be an oxidizing and a reducing agent. Heterotrophic prokaryote fermenters, such as the delightfully named Desulfobulbus exploit such sulfur disproportionation. They consume organic debris in anaerobic waters, and part of their waste is sulfate ions, which would explain the baryte in the cherts (the oxygen comes from chemical dissociation of water, rather than dissolved oxygen).

See also: Thamdrup, B. 2007. New players in an ancient cycle. Science, v.  317, p. 1508-1509. Farquhar, J. et al. 2007. Isotopic evidence for Mesoarchaean anoxia and changing atmospheric sulfur chemistry. Nature, v. 449, p. 706-709.

Large-scale mantle melting and episodes of continental growth

November 2007

Ultimately, sialic crust came from the mantle, although the processes involved seem to have involved two stages: partial melting of mantle peridotite and then some process that either partially melted basaltic slabs or melting of mantle wedge material in association with subduction of oceanic lithosphere. Given the abundance of dateable materials, such as zircons, in continental crust, it has been plain sailing to establish when different chunks of the continents came into being. Primary mantle-melting events are not so easy to date. The long-known pulsating origin of the continents has been refined by zircon U-Pb dating to give three major peaks at around 1.2, 1.9 and 2.7 Ga, with lesser activity at other times since 4.0 Ga, for example at 3.3 Ga. Each time crust-forming processes shift significant amounts of ‘sial’ into the crust, the mantle becomes depleted in elements that favour continental materials. One of these is rhenium (Re) whose 187Re isotope is radioactive, decaying to 187Os (osmium). So, the 187Re-187Os decay system is a very useful means of monitoring past mantle depletion events, by dating mantle rocks as well as examining the osmium isotopes of basalts.

Recent work on the former shows a remarkable link between mantle events ond crust formation (Pearson, D.G. et al. 2007. Link between mantle melting events and continental growth seen in osmium isotopes. Nature, v. 449, p. 202-205). The group from Durham University, UK, found that rhenium-depletion events match the 1.2, 1.9 and 2.7 peaks of continental crust-formation. They link these to large mantle melting events, implying that they involved very large mantle plumes. Yet continental crust seems to be a product mainly of subduction processes; i.e. driven by plate tectonics. Plumes at present are less obviously connected with plates. Accelerated subduction demands increases in the rate of sea-floor spreading, and that might link with ‘superplume’ activity, as has been suggested by the coincidence of submarine Ontong Java Plateau flood basalts with signs of speeding up of subduction during the Cretaceous. Plumes have also been implicated in the break-up of supercontinents and the creation of new spreading oceans, continental flood basalts being associated with the start of Pangaea’s disintegration. Of the three coincident ages, none are known to link with CFBs or continental dyke swarms, and that at 1.2 Ga is about the time when Rodinia began to form.

Non-plume model for continental flood basalts

September 2007

The coincidence of the Ethiopian-Arabian continental flood basalts (CFBs) and later flood volcanism in the Afar triangle with seismic tomographic evidence for an underlying plume, and tectonic evidence that the Deccan CFBs were erupted as India passed over the Reunion plume of the Indian Ocean, a plume model for CFBs is popular.  However, the largest known is the Central Atlantic Magmatic Province, emplaced as Pangaea began to rift apart at the end of the Triassic (199 Ma), has become the focus for an alternative CFB model (Coltice, N. et al. 2007. Global warming of the mantle at the origin of flood basalts over supercontinents. Geology, v. 35, p. 391-394). The CAMP’s shape is elongated rather than roughly radial as would be expected had it formed above a plume. There is no sign of any elevated ocean floor produced by uplift that the collision of a plume head with deep crust would produce. Geochemical parameters of the basalts suggest a shallow mantle source that has been affected by subduction, rather than deep mantle associated with a plume.

Coltice and colleagues argue, as others have, that formation of a supercontinent acts as a thermal ‘lid’ that slows down removal of normal mantle heat flow. Their modelling shows that it is theoretically possible for regional warming of around 100°C to occur beneath large continental masses because of changes to convective flow in the mantle beneath.  Such heating would be sufficient to trigger partial melting of shallow mantle peridotites, especially if they were hydrated. The authors comment that mantle hydration would be most likely at the sites of continental accretion where subduction would have driven water from the descending slab into the overlying mantle wedge, as the CAMP geochemistry suggests. That might explain the development of rifting and magmatism along the line of Caledonian and Variscan collision to form the North Atlantic, but not the rifting of South America from Africa, which cuts across Precambrian collision zones. Interestingly, it can be demonstrated that no crustal uplift preceded emplacement of the Ethiopian-Arabian CFBs in the mid-Oligocene, although many authors assume such uplift without any evidence for it. Yet the basalts have geochemical features that strongly support their origin by partial melting of a mantle plume.

Pulsed formation of continental crust: support from helium

May 2007

Sometimes, playing around with masses of existing data throws up patterns that have major significance, hitherto overlooked. Plotting the frequencies of zircon ages from the continents against those of ⁴He / ³He ratios in ocean-island and mid-ocean ridge basalts, may seem an odd thing to do, but there is a rational reason for geochemists. Extraction of the material of continents from the mantle should leave a geochemical signature in the residual, depleted zones of mantle rock. A highly likely element to be lost from the mantle during crust formation is helium, in particular ³He which is a primordial remnant of the creation of elements by stellar processes. The other isotope ⁴He is added to by radioactive decay of naturally unstable uranium and thorium isotopes. Helium with a low is likely to be from mantle that has not been depleted, whereas a high value suggests the source of helium has been depleted by a melting event. So examining helium isotopes from oceanic basalts (i.e. unlikely to be contaminated by helium that might have come from the abundant uranium and thorium in continental crust) ought to say something about the melting history of their source mantle.

In fact there is a wide range of ⁴He / ³He ratios. If all the helium-isotope analyses are assembled in a histogram, the plot shows prominent peaks and troughs. This suggests zones of mantle that have been depleted in ³He at different times – 4He will have built up by decay of small amounts of U and Th in the zones, according to the age of the depletion event. That is a very broad and simplified view, but is a rationale for comparing helium histograms with those of zircon ages. This has been done by a geochemist at Durham University, UK, to give a compelling result (Parman, S.W. 2007. Helium isotope evidence for episodic mantle melting and crustal growth. Nature, v. 446, p. 900-903). A plot of continental zircon-age peaks against peaks of ⁴He / ³He ratio shows a remarkable linear relationship, with coincidences at 3.3, 2.7, 1.9, 1.2 and about 0.4 Ma ascribed to pulses of growth of the continents.

A majority of geoscientists consider the accumulation of crustal ages in clusters to represent episodic growth. There are other possibilities, for instance: most formed early and has been reworked in pulses; there has been steady growth at a constant rate and ages are biased by collections or variable surface exposure of crystalline crust. The empirical match with helium data leans heavily in favour of the consensus view. Moreover, it offers a possible mechanism – isolated, large (perhaps global) mantle-melting events. They would manifest themselves on Earth more as accelerations in plate tectonics than as planetary resurfacing as seems to have happened at least once on Venus. Flood basalt events (with a roughly 30 Ma period in the Mesozoic and Cenozoic) are far more common dwarfs of such upheavals.
See also: Porcelli, D. 2007. When crust is bred. Nature, v. 446, p. 863-864.

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Magmas from the mantle and recycled crust

May 2007

Two processes result in the Earth’s mantle consuming rocks that formed oceanic and continental crust: subduction of oceanic lithosphere, and delamination from the base of thickened continental crust. In both cases the rocks involved are likely to be dominantly basaltic in composition. Those from the continental crust probably include mafic layered complexes, representing magma chambers in which intermediate magmas had resulted from fractional crystallization of basalt magma, and undifferentiated mafic igneous rocks underplated to the crust. Highly fractionated materials from the upper continental crust may also make their way into the mantle in the sedimentary cover to subducted ocean-floor basaltic crust.

New magmas originate in various ways as partial melts of ultramafic mantle rocks. Yet 4 billion years worth or more of consumed masses of crustal rocks must increasingly become involved in the chemistry of mantle melting by adding to its heterogeneity. Mantle heterogeneity is well-established from several lines of evidence provided by isotopic and trace-element analyses of modern basaltic lavas erupted in different tectonic settings. Yet, judging the influence and role of recycled crust have so far been plagued by data that are ambiguous. One outcome of previous research is that some ocean island basalt magmas formed by partial melting of peridotite whose chemistry had been transformed previously by other melts that had flowed through it (see Herzberg, C. 2007. Food for a volcanic diet. Science, v. 316, p. 378-379).

A large team of geochemists, combining forces (and their data) from Russia, Germany, Australia, France, Taiwan, Eritrea, Britain, the USA, the Netherlands and Iceland, have sought to reduce the ambiguities by focussing on the chemistry of olivine phenocrysts found in basaltic lavas, rather than that of whole-rock samples (Sobolev, A.V. and 19 others 2007. The amount of recycled crust in sources of mantle-derived melts. Science, v. 316, p. 412-417). Dominantly basaltic crustal masses in the mantle would melt to form silica-rich magmas. Passing through mantle peridotite, such melts would transform parts of the mantle to olivine-free pyroxenite. Magmas derived by partial melting of pyroxenite in the upper mantle would be basaltic, but enriched in silicon and nickel, and depleted in magnesium, calcium and manganese partly retained in the residual pyroxenes. Olivines crystallising first from basalt magmas carry a chemical signature of the parental melt composition, and thus the source.

The olivine approach by Sobolev and colleagues provides evidence for recycled crust in products of all kinds of basaltic magmatism, ranging from a contribution of 5% in ocean-floor basalts formed at ridges to about 20% in within-plate basalts. The contribution of mantle transformed to pyroxenite by chemical interaction with melt from foundered crustal masses ranges from 10% in mid-ocean ridge basalts to 100% in within-plate basalts formed below thick continental lithosphere. These include the largest volcanic outpourings in Earth’s history, in the form of continental flood basalts. The largest of these, the Siberian Traps, accompanied the largest mass extinction of the Phanerozoic at the end of the Permian Period.

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Breakthrough in the origin of granites

March 2007

The term granite is specifically linked to plutonic igneous rocks that carry more than 20% of quartz and contain more alkali feldspar than plagioclase, but it is commonly used to cover a multitude of other quartz-rich rocks: tonalite; granodiorite; adamellite; trondhjemite. Detailed geochemistry of granitic rocks may suggest the magma formed by partial melting of crustal sedimentary rocks (S-type), as products of extreme fractional crystallisation of mantle-derived basaltic magma (M-type) or by partial melting of other igneous rocks in the deep continental crust (I-type). The most common are the I-type granitic rocks. Because they and their metamorphosed equivalents seem to form much of the continental crust, I-type granitic rocks hold a key to both the history of the continents and the evolution of the mantle. If they are descended from basalts, then their formation enlarges the continents and withdraws from the mantle those trace elements that are concentrated in granites. If they form by melting of older crustal rocks, from a geochemical standpoint they are products of crustal recycling that shunts the issue of continental origins and growth elsewhere.

Modern petrogenetic division of granites began with investigations in the crystalline terrains of eastern Australia , once suitable analytical techniques became available. Now that sophisticated mass spectrometry can address many isotopic systems, even at the level of individual mineral grains, it has only been a matter of time before they were directed at the long-running question of granite origins. A revealing focus is on the I-type granites of eastern Australia (Kemp, A.I.S. et al. 2007. Magmatic and crustal differentiation history of granitic rocks from Hf-O istopes in zircon. Science, v. 315, p. 980-983). The hafnium isotope ¹⁷⁶Hf forms by decay of the unstable lutetium isotope ¹⁷⁶Lu. As the mantle has a higher ratio of Lu/Hf than continental crust, it accumulates radiogenic ¹⁷⁶Hf faster than continental crust. So any magma formed by partial melting of the mantle has a relatively high content of radiogenic ¹⁷⁶Hf, whereas magmas formed by crustal melting should be depleted in this sense. Zircons date from the earliest crystallisation of granitic magmas, have easily measured concentrations of hafnium and also provide precise dates of formation. Both hafnium and oxygen isotopes ‘fingerprint' gross magmatic origins.

Interpretation of the results from upper-crustal granitic bodies is of the having-cake-and-eating-it variety, as signatures of mantle-derived melts and those of large-scale contamination by melting of older crustal rocks (probably metasediments) show up. In exposed deep and mid-crustal terrains, broadly granitic gneisses are usually intermixed with high-grade metasediments, and also relics of cumulates formed in magma chambers. The model of Kemp et al. might well apply nicely to this association, which does represent the bulk of crust. But quite likely, long histories of later deformation and reheating will have messed up the geochemistry of the zircons, so we may be stuck with a story that related directly only to the upper crust.

See also: Eiler, J.M. 2007. On the origins of granites. Science, v. 315, p. 951-952.

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Movement of partial melts

January 2007

The phenomenon of partial melting is one that chemists often find hard to grasp, since it take place inside rocks and involves contributions from several minerals according to the thermodynamic controls over multi-compound phase relations. It is a bulk process, with tiny contributions from throughout large volumes of solid hot rock. How it becomes magma baffles most geologists, because tiny amounts have to migrate and coalesce into large volumes before being able to move en masse, to intrude or be exuded at the Earth's surface. There is a popular model: that the melt fraction is channelled along mineral-grain boundaries and accumulates to migrate faster in interconnected cracks. The trouble is that any melts that remained in their parent rocks have generally crystallised before they were exhumed at the surface, obscuring all the fine detail. The other complicating aspect is that melting generally involves other fluids, mainly supercritical water and carbon dioxide, both as catalysts for and products of melting itself. They show up in fluid inclusions within minerals, but rarely relate to evidence of melting.

Occasionally igneous minerals contain glass that they trapped during crystallisation, for instance olivines sometimes include glass formed from their parental basaltic magma. Heating the minerals until the glass remelts is a simple way to look at the obverse of crystallization thereby mimicking the melting process (Schiana, P. et al. 2006. Transcrystalline melt migration and Earth's mantle. Science, v. 314, p. 970-974). Interestingly the melt does migrate through the solid mineral, towards higher temperatures but also taking on something of the crystal form of its enclosing mineral. It is slow, around 1 to 3 nm s^-1, but that is enough to get it to a grain boundary. Gas bubbles in the glass do the opposite, moving towards lower temperatures and separating from the melt to become trails of fluid inclusions in the mineral itself. Creep within the host mineral's lattice must permit this migration. This is all very interesting, and the authors ascribe portentous consequences of their observations in natural magmatic process. However, basaltic melts do not form by melting of olivine, but take up elements from pyroxenes and other minerals as well, according to the elements' different degrees of incompatibility with whatever minerals are left behind—mainly olivine in the case of mantle melting. To form the melt, the elements that comprise it first have to migrate themselves. Is there any reason why they should all meet up inside this or that mineral to form a melt of basaltic composition, rather than combine on common ground, along inter-crystal boundaries?

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Microbial alteration of oceanic crust

November 2006

The transformation of ocean-floor lavas from pristine assemblages of anhydrous minerals to cold, wet masses of hydrated silicates is of central importance to subduction processes that help to pull oceanic lithosphere apart and generate the hydrous arc magmas that can eventually become parts of continental crust. This geochemical heat exchanger is usually ascribed to hydrothermal circulation of seawater through hot new oceanic crust. When these fluids emerge as hydrothermal vents they sustain seething colonies of prokaryote and eukaryote life from the most minute Archaea to substantial metazoans. That this long-hidden part of the biosphere might play a role in plate-tectonic systems is beginning to seem possible. Evidence is emerging from the study of altered basaltic glass that the ocean-floor biosphere extends deep into the ocean floor (Staudigel, H. et al. 2006. Microbes and volcanoes: A tale from the oceans, ophiolites and greenstone belts. GSA Today, v. 16, October 2006 issue, p. 4-10). The US, Canadian and Norwegian authors review observations of unicellular organisms in the cracks that permeate modern volcanic glass when it forms by rapid cooling of sea-floor lava. They seem rapidly to colonise tiny cracks and to act as a medium through which water is more easily able to transform the sterile glass into complex clay assemblages known as palagonite. The bugs are everywhere, down to at least 300 m in modern oceanic crust. High-powered microscopy of ancient ophiolites, such as those of the Cretaceous Troodos Complex on Cyprus, reveals structures that appear exactly the same, including convincing evidence of the organisms themselves. Similar structures, but no irrefutable cell-like structures, occur in Archaean greenstone belt lavas too, as far back as 3400 Ma: among the oldest tangible signs of living processes.

From a cell-biology standpoint, hydration reactions in mafic to ultramafic lavas are potentially highly fertile, the formation of serpentine minerals by hydration being a well-known generator of hydrogen. Modern methanogens use the reaction of hydrogen with carbon dioxide as an energy source, with methane as a by-product. Other organisms exploit the oxidation of sulfides or the reduction of sulfates in a similar way. All these processes can go on inorganically, and the possibility that tiny cracks in volcanic glasses may have harboured the origin of life by providing a chemically simple energy source for metabolism is a possibility worth further exploration. If there is one process that has undoubtedly occurred since the Earth cooled sufficiently for liquid water to exist, it is the alteration of mantle-derived lavas.

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Oxygen in the atmosphere: why the delay?

November 2006

Several lines of evidence suggest that the Earth's atmosphere only accumulated sufficient oxygen for it to be significantly oxidising around 2400 Ma ago. Yet the much earlier emergence of blue-green bacteria, assumed to be the organisms that secreted the intricate biofilms that make up stromatolites, suggest that it was being generated by photosynthesis as a much as a billion years beforehand. Many geochemists now suggest that oxygen was readily mopped up in the oceans by the conversion of soluble iron(II) ions derived from sea-floor lavas to insoluble compounds of iron(III), through oxidation reactions. As the rate of production of oceanic lithosphere gradually slowed, there would come a point when all available iron(II) was precipitated leaving excess photosynthetic oxygen to accumulate and enter the atmosphere. But other factors would have been at work: burial of organic carbon produced by photosynthesisers also works to increase the rate at which oxygen remains uncombined (otherwise carbon combines with oxygen to reproduce carbon dioxide).

Complicating the chemistry of atmospheric oxygen is the way in which it may combine with biogenic methane through reactions catalysed by ultraviolet radiation. But UV penetration also falls as oxygen levels rise, because of the formation of ozone, which makes possible extremely complex systems of positive and negative feedback that would affect atmospheric oxygen concentrations. Assessing such mechanisms, three British environmental scientists suggest a kind of `flipping' from two possible states in the Archaean to Palaeoproterozoic atmosphere; one rich in oxygen the other forced to have low levels (Goldblatt, C. et al. 2006. Bistability of atmospheric oxygen and the Great Oxidation. Nature, v. 443, p. 683-686). Various permutations of the rates of carbon burial, methane and oxygen production might have locked the pre-2400 Ma atmosphere in a low-oxygen state. The authors estimate that just a 3% increase in organic carbon burial could have flipped the dynamics towards rapid oxygen accumulation that by generating ozone would be destined to persist. Their model helps resolve a number of awkward geochemical observations that an iron buffering model cannot explain.

See also: Kasting, J.F. 2006. Ups and downs of ancient oxygen. Nature, v. 443, p. 643-645.

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Confused by radiocarbon ages? Hopefully, not anymore

October 2006

When we come to the near past, signifying time that has elapsed becomes unclear. Most Christians divide the last four thousand years into AD and BC (with some confusion as to whether the division is at 1 or 0 AD), yet Muslims place their starting year differently, and so might many other faiths, if they so chose. The adoption of `Before the Common Era' and `the Common Era' (BCE and CE, which are the same as BC and AD) really doesn't help politically, being based on a now obvious fact: that the dominantly Christian US and EU dominate the planet. The only foolproof way to judge elapsed time in years is to have some continual and irrefutably annual events to count. Now, it is not always convenient to use the annual growth rings in a collection of enormous logs of a variety of ages to tell time, and the same goes for snow layers in polar ice caps and layered stalagmites. Using the decay of radioactive ¹⁴C in preserved carbon-containing materials revolutionised archaeology and the science of recent climate change. But it has a snag, for ¹⁴C, unlike many other geochronometers, is continually being formed, by cosmic ray bombardment of nitrogen in the upper atmosphere. Cosmic ray flux is not constant, so the proportion of ¹⁴C to stable carbon was different at any time in the past. Until recently nobody knew how that proportion had varied. Radiocarbon ages have to be calibrated in some way, so that they record events in a truly absolute time-frame. Without calibration, even the most precise age determinations give a warped view of history (see Rationalising radiocarbon dating in the February 2004 issue of EPN). For instance, the date when the Younger Dryas glacial pulse began was a thousand calendar years before its calibrated ¹⁴C age.

Despite heroic efforts to establish a link between radiocarbon ages and the true passage of years from long annual records in dateable materials, calibration gaps in the ~50 ka period achievable by using the quite short half-life of ¹⁴C have caused a problem. Many published and even some new dates are given without calibration, while others are in `years before present (BP)', i.e. before the start of above-ground atomic bomb tests in 1950, which uniformly contaminated all later atmospheric carbon with ¹⁴C produced by nuclear transformation. The confusion should soon be resolved as the effort to match productivity of ¹⁴C to real time nears completion (Balter, M. 2006. Radiocarbon dating's final frontier. Science, v. 313, p. 1560-1563). But some workers are impatient to give real ages using calibration curves for difficult periods, which have not yet been verified and are controversial. An interesting case relates to the possible overlap period, roughly around 35 to 30 ka ago, between fully modern humans and Neanderthals in Europe. That awkward era may soon be clarified with the unearthing of monstrous logs from New Zealand swamps, which may contain annual rings back to the 50 ka limit.

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Is the idea of Hadean continental crust bunkum?

October 2006

As these monthly jottings have noted several times, the geological record of the Hadean (before 4 Ga ago) could easily be lost through an ill-timed sneeze: it consists of a few minute zircon grains extracted from common or garden Archaean meta-sandstones in Western Australia. Milligram for milligram, these have become the heaviest punchers in the world of geochemical debate. They undoubtedly crystallized as long ago as 4.4 Ga. More controversially their detailed chemistry has been suggested to indicate that their crystallization was from granitic magma formed by partial melting of materials that interacted with water at around 700°C; materials that were not primarily of mantle composition (see Zircons and early continents no longer to be sneezed at in EPN February 2006 issue). If true, that would suggest low-density crust that found difficulty in being recycled into the mantle only a few tens of Ma after the Earth's formation. Either that crust was too thin to resist subduction by some kind of tectonic slicing and has gone for ever, or some of it is still out there waiting to be found…by those who become very excited by extremely aged rocks. There is a simple way of putting the early-granite hypothesis to the test—by seeing if zircons in basalts are any different from them (Coogan, L.A. & Hinton, R.W. 2006. Do the trace element compositions of detrital zircons require Hadean continental crust? Geology, v. 34, p. 633-636).

Coogan and Hinton, of the University of Waterloo, Canada and Edinburgh University respectively, show that Hadean zircons cannot be distinguished chemically from those found in gabbros that have differentiated from basaltic magmas at modern mid-ocean ridges. As if that were not sufficiently deflating, they also made crystallization-temperature estimates of the gabbro-derived zircons, using a geothermometer that uses the titanium content of zircon in equilibrium with rutile. Despite the fact that the real temperature of gabbro crystallization is well over 1000°C, these estimates came in at between 700 and 800°C. That is, about the same as those proposed as evidence for the crystallization temperature of Hadean zircons from a granitic magma. Coogan and Hinton were not content, and go on to offer an alternative explanation for the zircon's oxygen isotopes, used by others as evidence for the influence of water at shallow depths back to 4.4 Ga. The seemingly water-derived ¹⁸O excess in the zircons could well have come from carbonates recycled from surface weathering of basalt, to be assimilated by deep basaltic magma chambers.

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Getting to the matter of the root

May 2006

As well as by its own low density, continental crust may be prevented from subduction because of the strength and buoyancy of cold, thick mantle that forms a root beneath the oldest cratonic crust. Geophysics shows that such roots are there, and in the case of African cratons they merge with the deeper mantle without the intermediate, more ductile asthenosphere: in a sense Africa is `nailed' in place and barely moves. Except for xenoliths in some continental volcanic rocks and in kimberlite pipes, samples of the deep continental lithosphere are uncommon. One place where they are abundant at the surface is in the zone of ~400 Ma continent-continent collision in western Norway (Spengler, D. et al. 2006. Deep origin and hot melting of an Archaean orogenic peridotite massif in Norway. Nature, v. 440, p.913-917).

These rocks are Archaean (~3.3 Ga) in age, and contain tiny diamonds. Their more common metamorphic minerals indicate that the peridotites stabilised at depths of about 180 to 250 km. Yet they carry trace element and mineralogical evidence that they formed as residues of partial melting from a body of mantle that rose from almost 400 km down. Compositionally, they seem to represent an outcome of high degrees of partial melting, probably to release high-magnesium or komatiitic magmas that are only common in early Archaean greenstone belts. Most likely, this peridotitic root material continued to rise, eventually to underplate Archaean continental crust. Unable to melt any further, being depleted in incompatible elements, the root became a permanent and very rigid fixture once it had formed. Regarding the unending, but probably fruitless quest for crustal materials that predate 4.0 Ga, other than a snuff-pinch of tiny zircons, this well-supported model for cratonisation perhaps offers an explanation. No doubt in the higher heat-producing mantle of Hadean times komatiite magma was the norm for oceanic crust formation, and such depleted, high-pressure peridotite residues formed continually. Unless they rose to adhere to substantial low-density sialic crustal masses, they would be recycled back to deeper levels. Equally, without the support of such rigid underplates, any sialic material at the surface would have been unable to withstand deformation and would become subductible by tectonic mixing with more common, dense, mafic-ultramafic oceanic lithosphere. A great deal of Archaean tectonics suggests that continents then were not fully cratonised – Archaean crustal rocks seem to have been pervasively and repeated deformed, cratons of undeformed old rocks not appearing until the Proterozoic, when modern plate tectonics became established.

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Acasta gneiss and another old zircon

May 2006

Readers may by now be satiated with comment on geriatric zircons. Most of them – and they can be counted – are detrital grains that survived around a billion years of sedimentary processes to end up in an otherwise common-or-garden quartz-rich sandstone in Western Australia. Their number has been added to by one more grain, which might be cause for jollification in some quarters, because its host was a piece of deep continental crust of good provenance (Iizuka, T. et al. 2006. 4.2 Ga zurcon xenocryst in an Acasta gneiss from northwestern Canada: evidence for early continental crust. Geology, v. 34, p. 245-248).

The Acasta gneisses form the western flank of the Slave craton in northern Canada, and are the world's oldest rocks, having formed at 3.94-4.03 Ga as a series of plutonic rocks of tonalitic to dioritic composition. Archaean geochemists from various Japanese universities, and a lone Briton from Leicester University, understandable wished to confirm and refine the age of the Acasta gneisses as the earliest `golden spike' in the continental crust , and subjected many zircons extracted from gneiss samples to the latest mass spectrometric dating that uses the U-Pb scheme. Indeed they achieved excellent precision to the nearest few tens of Ma. Using an ion microprobe, they were able to date the zoned interiors of the zircons, revealing progressive crystallisation of the grains, mainly as the igneous precursors of the Acasta complex evolved. In a single grain, however, they came upon zircon in its core that was 200 Ma older. That tiny, trapped granule itself had engulfed even smaller particles of apatite, unlike the bulk of the whole grain.

Ion microprobes are wonderful pieces of kit, as they can give extremely precise and revealing trace element abundances in the mineral into which they burn a hole. In the case of the aged zircon core, such analyses revealed clearly that these few micrograms of zirconium silicate had formed from a magma with broadly granitic composition. Their conclusion: pre-4 Ga granitic crust was more widespread than previously thought. No, not the Acasta gneiss, but whatever material its igneous precursors had picked up while they were magma. In the previous comment in this section, I put forward the view that sial may well have formed before tangible continental material had stabilised as a permanent resident at the Earth's surface. Yet, for reasons that seem to be emerging, such crust would not have resisted subduction and ended up mixed back into the mantle. Since the Acasta gneisses were most certainly not formed before 4.0 Ga, then it is from their mantle source region that their igneous precursors must have picked up this tiny, alien xenocryst. Unless, that is, someone can show me a 2-5 kg lump of gneiss heaving with these blessed grains (preferably with signs of almost as old crustal deformation). There is an obvious prediction to make. Geochemists are fighting in a heap to acquire ion microprobes and inductively-coupled, laser-ablation, plasma-source mass spectrometers, and why ever not? Now they have something to aim for instead of trawling quartz sandstones for relics of Earth's Hadean past. My prediction is that every single mantle-sourced rock of granitic composition, whatever its age, will contain at least one pre-4.0 Ga zircon granule. Zirconium silicate is sturdy stuff.

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Arc-like andesites from the ocean floor

January 2006

To most geologists `andesite' spells subduction beneath island arcs and continental margins. Geochemically they share a universal sigNature: their depletion in the elements niobium and tantalum. Both find the aqueous fluids that rise from subducting slabs repellent and so they stay in the source of arc magmas, almost certainly in amphibole minerals. Negative Nb and Ta anomalies pervade the continental crust, suggesting that it owes its origin to subduction processes of some kind over maybe the whole of recorded geological time. The other dominant means of expelling magmas is through the adiabatic melting of drier upper mantle as it rises along oceanic rift zones. Theoretically and also in innumerable analyses of ocean-floor rocks Nb and Ta behave like other elements that favour melts over the minerals of mantle residues. That there are ocean-floor rocks that show evidence of incompatible behaviour of the two elements comes as quite a surprise. More surprising still is that they are of bulk andesitic to more silica-rich dacitic composition (Haase, K.M. et al., 2005. Nb-depleted andesites from the Pacific-Antarctic Rise as an analogue for early continental crust. Geology, v. 33, p. 921-924). The rocks analysed by the team from the Christian-Albrechts University of Kiel, Germany, occur close to a hotspot in the South Pacific and span about 130 km of the ridge system, along with basalts.

Modelling the geochemistry of the silicic lavas suggests a dominant role for fractional crystallization of magnetite and ilmenite from a basaltic parent magma that itself is enriched in iron and titanium. Yet, associated basalts do not show depleted Nb and Ta, so some other mechanism must be responsible for their occurrence in the andesites. One possibility is production of silicic magma by partial melting of amphibole-rich mafic oceanic crust, and then its mixing with fractionated basalt to form low-density magma that rises. Silicic lavas in Archaean greenstone belts are often associated with basalts that chemical affinities to those in modern oceanic settings. It is therefore possible that a substantial proportion of Archaean continental crust originated in ocean hotspot settings, rather than by some form of subduction process.

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Sulphides in the ocean

September 2004

About 2.3 billion years ago, ancient soils begin to reveal that Earth, or more precisely life upon it had developed an atmosphere that contained oxygen, albeit at quite low levels. One of the most interesting events during the Proterozoic Aeon was the world-wide disappearance of vast deposits of iron oxides known as banded iron formations or BIFs, at about 1.8 billion years. Many authorities view that as the time when sufficient oxygen was dissolved in seawater to have removed soluble Fe-2 at its source, on the ocean floor near hydrothermal vents – BIFs formed in shallow water, and that requires Fe-2 to have permeated the entire oceans. There is another possibility. The presence of atmospheric oxygen would have ensured the oxidation of iron sulphide exposed at the land surface, thereby adding sulphate ions to river water, and eventually seawater. Another line of evidence for atmospheric oxygen is the disappearance of detrital sulphide grains from sedimentary rocks younger than 2.3 billion years, so a build-up of sulphate ions in later seawater is quite plausible. Should deep-ocean chemistry have been reducing, it is possible that sulphide ions would form there. The insolubility of iron sulphides would then remove Fe-2 from seawater equally as efficiently as would oxygen. Danish and Canadian geochemists have investigated this possibility using data from sediments in Canada that mark the last phase of major BIF deposition around 1.8 billion years (Poulton, S.W. et al. 2004. The transition to a sulphidic ocean ~1.84 billion years ago. Nature, v. 431, p. 173-177). They found that conditions changed from one in which seawater contained dissolved Fe-2 at the time of the last BIF deposition to one dominated by sulphide ions, similar to that found in modern anoxic waters such as those in the Black Sea. That would have sequestered any available Fe-2 to pyrite in sediments, a feature typical of many later Proterozoic sediments. Since seawater during the Phanerozoic was dominated by sulphate ions, except in periods of ocean anoxia, it looks likely that late Precambrian sulphidic oceans gave way to more modern sulphur chemistry following a rapid rise in atmospheric oxygen at the end of the Proterozoic. One consequence of highly-reducing deep ocean water would have been very efficient burial of dead organic matter while it lasted, because anaerobic bacteria do not fully convert organic molecules back to water and carbon dioxide. During the Neoproterozoic δ¹³C in seawater underwent rapid swings from highly negative to highly positive, on which all kinds of connotations have been placed. Another explanation for the carbon hiccups might be that periodically there were short-lived increases in oxygenation of deep ocean water.

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A "Whoops" moment for geochemists?

February 2004

A great deal of effort and innumerable theses and papers have gone into modelling the derivation of magmas from their parent rocks, especially the mantle, over the last three decades. Most is based on the division of trace elements into "compatible" and "incompatible", the first being those which tend to remain in minerals that make up the residuum during magmagenesis, and the second those that favour melts. Most incompatible elements have large ionic radii. The modelling centres on the degree to which elements remain in solids, the appropriate parameter being an element's mineral-melt partition coefficient (K_D). Partition coefficients are usually deduced from an element's abundance in phenocrysts that are in contact (and supposed equilibrium) with an igneous rock's groundmass material, which is assumed to have formed from magma, and its concentration in that once liquid phase. Models for partial melting and fractional crystallisation, plus several variants, all involve K_Ds, for olivines, pyroxenes, feldspars, garnet, amphiboles and so on. For the generation of basaltic magmas, the first step is partial melting in the mantle itself, for which direct estimation of K_Ds is not possible. Instead they are assumed from mineral-melt chemistries in crustal igneous rocks, with some allowance for elevated temperatures and pressures and other conditions. Each mineral has its own distinctive suite of K_Ds for many elements, and the chemistry of an igneous rock has often been traced back to which suite of minerals was present in a residue, i.e. the source rock itself, as well as the degree to which one or other process proceeded. The 19 February 2004 issue of Nature included an ominous article (Hiraga, T, et al. 2004. Grain boundaries as reservoirs of incompatible elements in the Earth's mantle. Nature, v. 427, p. 699-703).

The study by geochemists at the University of Minnesota and Oak Ridge National Laboratory, USA, concentrated only on the mineral olivine, and a few elements present at trace levels in it. Their experiments simulated equilibrium conditions under mantle conditions. Results showed that incompatible elements in olivine, such as Ca and Al, tend to concentrate mainly at boundaries between grains where they are readily available to any melt that starts to form, rather than uniformly throughout the mineral grain. The finer the grain size of the rock, the greater the area of grain boundaries, and so the more incompatible elements tend to be concentrated at them The tendency is predictable on thermodynamic grounds, but has only been studied previously in alloys and other artificial materials. Geochemists have generally regarded grain boundaries as places where impurities in rocks gather. If the same rock is analysed with and without the crushed powder having been washed in acid, different trace element concentrations result. This has been attributed to secondary effects, such as the passage of hydrothermal fluids or groundwater. Since K_Ds that are used widely involve concentrations in whole mineral grains, the basis of geochemical modelling might be compromised. Melting begins at grain boundaries, so the low degrees involved in generating basalts could be biased by the effect. Moreover, vapour phases moving through the mantle (supercritical water and CO₂), will follow grain boundaries too, and so may easily pick up and transport incompatible elements. Their entry into the crust carrying mantle-derived incompatible elements, such as rare-earths, strontium and lead, would lead to metasomatic effects that could play havoc with interpretations of isotopic data based on these elements. Carbonatites, probably formed from mantle-derived carbonic fluids, are enriched in many incompatible elements. Similarly worrying data, such as estimates of the incompatible element partitioning into carbonic fluids, have emerged in the past, but so far have been notable only for the silence with which most geochemists greeted them.

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Mantle and core do not mix

January 2004

Given the growing controversy about whether or not plumes of mantle rock can rise from the core-mantle boundary to source large igneous provinces (see Geoscience consensus challenged in EPN January 2004) the hypothesis has been tested by seeking material in hot-spot lavas that may have crossed from the outer core into the deepest mantle. Some hot-spot lavas contain traces of Osmium-186 that may have formed by decay of an unstable platinum isotope (¹⁹⁰Pt) that is most likely to be enriched in the core, thereby supporting the hypothesis. Another isotopic approach is to look at tungsten (W) isotopes (Scherstén, A. et al. 2004. Tungsten isotope evidence that mantle plumes contain no contributions from the Earth's core. Nature, v. 427, p. 234-237). Tungsten, like osmium, has a strong affinity for iron, and the bulk of terrestrial W is likely to be present in the core. One isotope ¹⁸²W forms from the decay of an unstable isotope of hafnium ¹⁸²Hf, whose half life is geologically short (about 9 Ma). As a result all ¹⁸²W in the Earth must have been produced in the first 60 Ma of the planet's evolution. Moreover, hafnium is likely to favour the mantle far more than the core, so most ¹⁸²W seems likely to be present in the mantle and the core should be depleted in it. This is borne out by comparing values in primitive meteorites with those in mantle-derived lavas; the mantle is enriched by comparison. So, if there was significant chemical exchange between the core and mantle a lot of tungsten with very low ¹⁸²W should contaminate the lower mantle. If plumes did rise from the core-mantle boundary, then lavas derived from them ought to have anomalously low ¹⁸²W contents. Scherstén and colleagues from the University of Bristol and the Australian National University show that Hawaiian lavas (the same samples used to suggest a mantle-wide plume beneath Hawaii using osmium isotopes) and South African kimberlites do not show this signature, and argue convincingly that the osmium data must represent another source of contamination, probably recycled crustal rocks. However, that does not rule out a plume rising from the core-mantle boundary, just that the core did not play a significant geochemical role.

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Archaean sea-floor hydrothermal fluids

October 2003

The circulation of ocean water through new oceanic crust not only cools oceanic lithosphere sufficiently for it to droop and help drive sea-floor spreading. It also re-emerges as hot submarine springs that today host curious ecosystems, which depend entirely on energy and chemicals that spew out of these "smokers". The chemistry of life molecules, particularly the metals in them, reveals a blend that is surprisingly similar to that of hydrothermal fluids. This, along with other matters, such as the highly primitive genetics of thermophilic bacteria, make sea-floor hydrothermal vents or the crust beneath them excellent candidates for the cradle of life's origin. So getting samples of the very earliest such fluids has to be among the most exciting discoveries relevant to palaeobiology. Jacques Touret of the Free University of Amsterdam, one of the pioneers of fluid inclusion studies, believes that he has found some (Touret , J.L.R. 2003. Remnants of early Archaean hydrothermal methane and brines in pillow-breccia from the Isua Greenstone Belt, West Greenland. Precambrian Research, v. 126, p. 219-233). The host rock is an undeformed, but metamorphosed breccia made of basaltic pillows from the famous Isua greenstone belt of West Greenland, which formed about 2.8 billion years ago. Quartz crystals in amygdales and veins that cement the breccia together contain minute fluid inclusions. There is little of interest in that fact alone, for most igneous or metamorphic minerals trap samples of the fluids involved in the origin of the host rocks. What is intriguing abut the Isua fluids is their high content of methane and brine; just as expected from low temperature hydrothermal fluids. Their chemistry compares well with that of inclusions in altered basalts from modern oceanic crust, in which bacterial activity is implicated. Metamorphism generally results in carbon dioxide as the main carbon-containing gas in fluid inclusions. Formation of methane in sea-floor environments can be biologically controlled, but the hydration of deeper ultramafic rocks to serpentine can also generate enough hydrogen to reduce CO₂ to methane abiogenically. The full association at Isua suggests carbon-dominated hydrothermal activity, which today precipitates carbonates at vents, forming so-called "white smokers". ["Black smokers" are sulphur dominated, and take their name from the massive precipitation of metal sulphides when the fluids emerge at the seabed.] These create alkaline conditions that are well suited to bacterial growth. Touret does not claim that the inclusions indicate living processes, merely that the right conditions were around in the earliest Archaean for life to thrive. It would be an immense feat if he subsequently discovers bacterial fossils in the inclusions, but that is highly unlikely. However, the brines might provide proxy evidence, because living cells uniquely accumulate bromine from sea water. Anomalous ratios of chlorine to bromine might point strongly towards life having been around during Isua times.

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Geochemistry of the vanishingly tiny

June 2003

The British press has been awash with speculation that the Prince of Wales is worried about nanotechnology and the slim possibility that the next big threat after Osama and SARS might be minute, self-replicating robots that invade our bodily orifices. It stemmed from the Prince of Wales' having asked experts for a briefing, and that may well have been just HRH's curiosity about a changing world. There is rarely an issue of the weekly science journals without news of some discovery of phenomena that occur in nanotubes and minuscule cavities; the world at scales less than a micrometre is beginning to seem strange. Rocks are full of pore spaces and inter-grain boundaries with the dimensions on which new wings of the other sciences are emerging. So it is no surprise to learn that there will soon be "nanogeochemistry" (Wang, Y. et al. 2003. Nanogeochemistry: geochemical reactions and mass transfers in nanopores. Geology, v. 31, p. 387-390). The use of natural and artificial zeolites as ionic filters has been around for a long time, so this is a branch with a new name, rather than a fundamental breakthrough. But zeolites are profitable, and only now has "blue-skies" research turned up the magnification.

Typical nanopores and pathways are grain boundaries in crystalline rocks, cleavage planes in phyllosilicates and clay minerals, and pores in fine-grained sediments, such as diatomite and kaolin, and minerals that have been precipitated as amorphous masses rather than discrete crystals, a good example being the iron oxy-hydroxides in soils. To see these structures requires advanced transmission electron microscopy, and even with them the features are somewhat indistinct. Nanopores can make up to 40% of a material's porosity, and having such minute radii they contribute as much as 90% of the internal surface area that is exposed to chemical reactions. Artificial materials that show nanoporosity have internal surface areas as high as hundreds of square metres per gram. Clearly, such materials in nature must play a major, but largely uncharted role in geochemical change. Among the oddities discovered by Wang and colleagues at the Sandia National Laboratories and the University of New Mexico, are inclusions of native copper in weathered clay minerals and equally small particles of gold along microfractures in mylonites. Their experiments with artificial simulants of natural fine-grained materials focussed on two simple phenomena: the electrical charge on small surfaces in relation to acidty; and their ability to absorb trace elements. The paper is highly technical, but the conclusions are surprising . Nanopores develop unusually high surface-charge densities that should affect their ability to adsorb ions, and also exert controls on reactions that might seem unlikely in macro-scale simulations of geological conditions. Indeed, finely porous materials enrich trace elements by an order of magnitude compared with isolated small particles, and encourage precipitation or solution of different compounds when that would be unexpected in more open systems. As well as bearing on burial of toxic and radioactive wastes, and on mineralising processes, nano-scale processes are probably central to the whole process of weathering. Interestingly, such small scales exclude even the tiniest bacteria, so that the geochemical processes seem unlikely to impinge on life. However, spaces in rocks comprise a nested series of dimensions, and changing conditions may well flush material from one scale to another. In particular, bacteria of various kinds can control pH at the micro-scale, thereby creating the ambient conditions for nano-scale geochemistry.

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Potassium in the core

June 2003

It might seem impossible for planetary cores dominated by iron-nickel alloys to contain any source of heat generation. The main three elements (uranium, thorium and potassium) with long-lived radioactive isotopes and sufficient abundance to produce substantial heat energy are all highly concentrated in the Earth's crust. That is because they are incompatible with the minerals in mantle rocks, and so readily enter magmas that contribute to continental growth. However, the only natural materials that bear any resemblance to geoscientists' notions of core materials, metallic meteorites, contain abundant sulphur. Theoretically, potassium can enter sulphide minerals. So, since as long ago as the 1970s there has been debate about whether motion in the core was driven entirely by residual heat from Earth's accretion and the formation of the core, or that it contained its own heat source in the form of ⁴⁰K. If the first was true, then the self-exciting dynamo responsible for the Earth's magnetic field has been running down over geological time, because heat is transferred across the core-mantle boundary, eventually to reach the surface by convection. The existence of a solid inner core might result from such cooling, though its formation would release latent heat of crystallization and prolong inner motion. However, some calculations suggest that core motion and so geomagnetism ought to have vanished long ago, through loss of core heat to the surface. Substantial potassium in the core would demand considerable revision of ideas about the bulk evolution of the Earth, and other rocky planets. Experiments to prove that iron-sulphur alloys can contain abundant potassium have had a chequered history. Research at the University of Minnesota and the Carnegie Institute of Washington has discovered why there were such ambiguous results (Murthy, V.R. et al, 2003. Experimental evidence that potassium is a substantial radioactive heat source in planetary cores. Nature, v. 423, p. 163-165). The problem was in the preparation of samples for analysis. Rama Murthy and colleagues found that the oils used in polishing samples for electron-microprobe analysis actually leach potassium from the sulphides in them, nearly all disappearing in a few days of contact. With great care, they repeated experiments on mixtures of metallic iron, iron sulphide and potassium bearing glass held at high temperature under pressures between 5 and 10 % of those experienced in the core. Their results show that potassium can indeed enter core materials with high sulphur contents. The higher the temperature the more gets in, and their most extreme run saw almost 4 % K in the quenched sulphide. Plan are afoot to discover if uranium and thorium might also be in core materials.

Incidentally, in the week that the film The Matrix: Reloaded was premiered in the USA, a proposal to send a probe to the core-mantle boundary also appeared (Stephenson, D.J. 2003. Mission to Earth's core – a modest proposal. Nature, v. 423, p. 239). David Stephenson, of the California Institute of Technology, builds on the notion of the "China Syndrome", in which meltdown of the core of a nuclear reactor would lead to superdense molten uranium melting its way through the mantle. In his proposal, ruggedised instruments in a capsule the size of a grapefruit would make the journey, along with about 10 million tons of molten iron, by propagating a large crack started by a 10 Mt nuclear explosion. Data is to be transmitted by modulated acoustic signals in the kHz range. The article helps to demonstrate the delays in publication, even in a prestigious weekly journal; it should have appeared 6 weeks earlier….

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Silica in BIFs

March 2003

Following close on the heels of the hypothesis that iron in Precambrian banded iron formations was precipitated by bacteria (see BIFs and bacteria in February 2003 issue of Earth Pages News) is an account of the origin of silica that makes up roughly half the banding (Hamade, T. and 4 others 2003. Using Ge/Si ratios to decouple iron and silica fluxes in Precambrian banded iron formation. Geology, v. 31, p. 35-38). The rare-earth elements and Nd isotopes in the iron-rich layers suggests that they probably originate from ocean-floor hydrothermal activity. How their cherty layers formed has largely been overlooked. Before the Cambrian Explosion there were no organisms that secreted silica in their skeletons. Consequently, the dissolved silica content of Precambrian oceans was probably much higher than now. Because silica becomes highly soluble only under very alkaline conditions, it may have been close to saturation in Precambrian seawater. Quite small changes in seawater chemistry would result in its precipitation as fine-grained chert. But the main issue is where the dissolved silica came from.

Hamade et al. examined the amount of germanium in the cherts, because it is in the same group of elements and acts as if it were a heavy isotope of silicon. So it follows Si very closely in its distribution. Alteration of mafic rocks by sea-floor hydrothermal activity dissolves Si, and so does weathering of continental materials; there is a dual source of seawater Si. However, basalts have more than 10 times as much Ge as do granitic rocks, and the Ge/Si ratio is a good guide to the dominant source of Si. Cherts in the BIFs from the famous Hamersley basin in Western Australia have Ge/Si ratios that increase with the amount of iron. The most silica-rich BIFs seem to have formed from waters derived from continental areas, whereas the iron-rich varieties have a sea-floor hydrothermal signature. The authors conclude that these BIFs formed on a continental shelf subject to regular, periodic upwellings of deep ocean water.

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BIFs and bacteria

February 2003

The steel in your car almost certainly contains iron mined from a banded iron formation or BIF.  These Precambrian sediments are the largest repository of high-grade iron ore on the planet, and nearly all of them formed before about 2 billion years ago, when Earth's atmosphere and hydrosphere are reckoned by many to have contained very low amounts of free oxygen.  The enigma of BIFs is that, as well as vast amounts of iron, they contain equally large amounts of oxygen combined in hematite and magnetite.  However they formed, there must have been sufficient iron and oxygen in their environment to make these minerals in astounding quanties.  Iron is problematic, because in its Fe-3 form it is almost completely insoluble, and modern sea water contains very little because it is an oxidizing fluid now.  Nobody doubts that BIFs formed in a marine environment, and that would have had to contain plenty of soluble Fe-2.  So seawater before 2 Ga must have been a reducing fluid so that iron emanating from hydrothermal vents on the basaltic ocean floor could remain in solution and end up in near-surface water.  A popular explanation for the oxygen in BIFs is that it was released by the photosynthetic metabolism of blue-green bacteria, near to the basins where BIFs accumulated.  So BIFs mopped up any free oxygen that would otherwise have ended up in air or water and made both oxidising.  Eventually oxygen production outstripped that of soluble Fe-2 (perhaps by a gradual slowdown of sea-floor spreading) and thereby caused all hydrothermal iron to be precipitated near to ocean floor hydrothermal vents; the oceans became iron-poor after 2 Ga.

There is another plausible scenario for BIF formation, explored by a team from Canada, Britain, Australia and Denmark.  Some types of modern bacteria, chemolithoautotrophs and photosynthesisers that do not produce oxygen, are able to fix iron as Fe-3 hydroxides where there is very little oxygen or none at all.  The simple chemical equilibria that they exploit provide both energy and carbohydrate (Konhauser and 6 others 2002.  Could bacteria have formed the Precambrian banded iron formations?  Geology, v. 30, p. 1079-1082).  Evidence that such a process might have "grown" the massive BIFs comes from the famous Palaeoproterozoic Hamersley Group of Western Australia, the source of all the steel in cars produced in east Asia.  The Hamersley BIFs contain extraordinarily fine layers of iron oxides and silica, which may be annual or even daily records of biological cycles.  The key evidence lies in the relative concentrations of other elements in the deposit, phosphorus and trace metals (V, Mn, Co, Zn and Mo), which are close to the nutritional balance needed by the bacteria that Konhauser et al. suggest to have been involved.  Experiments with colonies modern bacteria of these kinds show that they are quite capable of depositing iron hydroxide at rates that would easily build vast thicknesses, given time.  Around 10²² individual cells could do the job at a rate that would have built the Hamersley BIFs—about 100 metres per million years.  That might seem to be an awful lot of bacteria, but it amounts to only about 40 thousand cells per cubic centimetre—far less than the number that build plaque on our teeth!

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Phanerozoic marine strontium record throws spanners in the works

February 2003

Jan Veizer of Ruhr University, Germany and the University of Ottawa is rightly known as "Dr Strontium". Almost single handedly he has created the record of strontium variation in seawater through geological time, by analysing carbonates that have extracted it along with calcium. Input of strontium to the oceans is through continental weathering and hydrothermal solutions from the oceanic crust, and it has proved tempting to use variations in the Sr/Ca ratio of carbonates as a proxy for the rates of both processes, particularly using Sr isotopes. It is not so simple however, as Thomas Steuber of Ruhr University and Veizer have shown (Steuber, T. & Veizer, J. 2002. Phanerozoic record of plate tectonic control of seawater chemistry and carbonate sedimentation. Geology, v. 30, p. 1123-1126). As in many geochemical cycles, the other important process is burial of strontium in marine sediments, and that depends very much on the type of carbonate that carries it from solution. Aragonite is between 8 and 4 times more efficient at mopping up dissolved strontium than the other common calcium carbonate, calcite. So, if aragonite is the main carbonate that is buried, seawater strontium is likely to fall more rapidly than with calcite burial. Which form dominates in sedimentation depends a great deal on the kind of animal that builds shells—most carbonate buried during the Phanerozoic has been of biogenic origin. Corals and carbonate-secreting algae use aragonite, whereas molluscs, brachiopods, coccoliths and forams have calcite shells.

Other workers have suggested that there have been periods dominated by deposition of one or other form of calcium carbonate, mainly calcite until the mid-Carboniferous, then aragonite up to the mid-Jurassic, calcite through the Cretaceous and most of the Tertiary, and a current tendency for more aragonite. Steuber and Veizer show how there is good correlation between changing ocean-crust formation and seawater Sr, and a negative correlation with the Mg/Ca ratio of seawater. Clearly there are linkages between the three variables, as follows: hydrothermal alteration of new ocean crust exchanges Mg for Ca, so the rate of sea-floor spreading modulates the seawater Mg/Ca ratio; magnesium inhibits the formation of calcite, thereby encouraging aragonite formation; periods of slow spreading therefore favour a higher rate of strontium removal from seawater. This has profound negative implications for the use of strontium isotopes in marine sediments to monitor the pace of continental weathering (the crux for some gross models of global climate change), and using the Mg/Ca ratio as a means of monitoring seawater temperature variations.

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Water recycling in the mantle

January 2003

The cold, dense oceanic lithosphere that descends subduction zones is also rich in water. These features result from the circulation of seawater through young basaltic crust, the exothermic hydration of originally anhydrous minerals in basalt and efficient convective cooling through hydrothermal processes. Because of this, it might seem as though subduction is a means of re-introducing water into the mantle, thereby enhancing the ability of rising mantle plumes to melt. The critical process that destines subducted lithosphere to sink inexorably is the conversion of oceanic crust to eclogite by high-pressure, low-temperature metamorphism in the subduction zone. Eclogite consists mainly of garnet and the pyroxene omphacite, which confer its higher density than mantle peridotite, and the reactions which form them involve dehydration. Rise of hydrous fluids from the descending slab is implicated in partial melting of the over-riding wedge of mantle to form the volatile-rich magmas that build volcanic arcs. The higher gas content of arc magmas, compared with those at constructive margins and above mantle plumes, makes them explosive and able to build volcanoes high above sea level. Most eclogites found at the Earth's surface are accompanied by still hydrous metamorphic rocks of basaltic composition—blueschists—and others that clearly formed from the sedimentary veneer of the oceanic crust. So, it might seem that blueschists and metasediments could carry a substantial amount of water into the mantle. Eventually, its recycling through the mantle could influence later magmatic processes.

Testing this seemingly reasonable extension of the hydrological cycle depends on assessing the water content of newly erupted magmas. This is virtually impossible for eruptions at the Earth's surface, because low pressure results in water escape within the higher parts of the volcanic plumbing system, before lavas can be sampled. However, eruptions onto the ocean floor deeper than a kilometre experience pressures high enough to keep gases in solution, which is why pillow lavas of true oceanic crust contain no signs of gas bubbles. Crystallised oceanic basalts soon react with percolating water, and their volatile contents are meaningless. Only the rapidly chilled margins are likely to retain their original composition, locked into quenched basaltic glass. Even then, a direct measurement of water content can be misleading. A cunning approach is to consider H₂O as if it behaved like a single element, based on its bulk distribution coefficient between melt and residual solid mantle. That is close to the values for light rare-earth elements, such as cerium. So a check for either degassing or contamination of basaltic glass with seawater is the glass's H₂O/Ce ratio (decreased by the first and increased by the second process). Jacqueline Dixon of the University of Miami, and co-workers from Harvard and the University of Rhode Island have used this method to assess the probable water content of the mantle source for mid-Atlantic Ridge basalts, whose lead and strontium isotopes suggest that their source was contaminated by older, recycled crust (Dixon, J.E. et al. 2002. Recycled dehydrated lithosphere observed in plume-influence mid-ocean-ridge basalt. Nature, v. 420, p. 385-389). The surprising conclusion of their work is that oceanic basalts formed from mantle with a recycled component have considerably less water in them than those formed by melting of pristine mantle. This suggests that subduction processes are extremely efficient (>92%) at removing volatiles from the subducted slab; lithosphere descending to depth is almost anhydrous.

Incidentally, the paper begins with an excellent explanation of the somewhat arcane distinctions between different mantle sources affected by lithosphere recycling and mixing.

See also:White, W.M. 2002. Through the wringer. Nature, v. 420, p. 366-367; and Tectonics section.

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Changing composition of seawater

December 2002

Using carbonate sediments and fossil shells to assess how the composition of seawaterhas changed is a long-standing technique in sedimentary geochemistry.Isotopes of strontium and oxygen have provided revolutionising windowson the pace of continental weathering and fluctuations in sea-surface temperatureand continental ice cover for over 30 years.The magnesium to calcium ratio in fossil shells has given insights intodeep-water temperatures for the Cenozoic, more recently. However, tracking changes in the bulk compositionof seawater through time, through analyses of carbonates, is plagued by the continualchemical interaction between rocks and the waters with which they are in contact.The Mg/Ca ratio of sea water is a potential proxy for the amount of hydrothermalactivity on the sea floor, and thus the rate of sea-floor spreading. This is not because oceanic basalts are magnesium rich comparedwith continental crust that provides much of the dissolved matter that entersthe oceans, but because hydrothermal reactions tend to mop up dissolved magnesiumand release calcium.. Unfortunately, magnesiumalso easily replaces calcium in carbonates during diagenetic processes, particularlydolomitisation. There are two means of overcoming this hindrance,by analysing seawater trapped as fluid inclusions in evaporite minerals and theshells of echinoderms that still contain minute structures formed in life andare unlikely to have been altered (Dickson, J.A.D. 2002.Fossil echinoderms as monitor of the Mg/Ca ratio of Phanerozoic oceans.Science, v. 298,p. 1222-1224). Early results seem to match a prediction thatwhile supercontinents existed, the length of mid-ocean ridges and therefore oceanfloor hydrothermal activity were at a minimum. Around the Precambrian boundary and duringthe Carboniferous to Jurassic periods, Mg/Ca was high at the time of the Vendianand Pangaea supercontinents. During majorbouts of continental break up—the Lower Palaeozoic and Mesozoic—the ratiois low. Oddly, the ratio has risen tounprecedented high levels during the Cenozoic Era, when clearly there is highhydrothermal activity.

Despite the fact that the Mg-Ca record of the oceans is limited to just a few short timespans in the 545 Ma record of the Phanerozoic, plenty of geochemists and palaeobiologistsare speculating about the possible consequences for evolution of changes in thebulk composition of seawater. There have been major swings in the proportionof calcite to dolomite in carbonate sediments throughout geological time (seeBacteria and dolomites, January 2001Earth Pages News). Discussion now centres on the possible effectof changing Mg/Ca ratios on the waxing and waning of important carbonate secretingorganisms, ranging from corals and molluscs that build reefs to the minute coccolithsthat formed the Cretaceous Chalk. Perhaps different groups responded differently to changing watercomposition, and maybe the Cambrian Explosion of shelly faunas was triggered somehowby a critical shift in the ratio.

See also: Kerr, R.A. 2002. Inconstant ancient seas and life's path. Science, v. 298, p. 1165-1166

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Deep carbon cycling, and gold mineralization

December 2002

One of the more speculative aspects of the carbon cycle concerns the fate of carbonatesediments that descend subduction zones. One popular hypothesis, with an acronym thatis likely to amuse colloquially inclined, British readers (the BLAG model namedafter its three originators Berner, Lasaga and Garrels) avowsthat such carbonates contribute to CO₂ emissions from volcanoes abovesubduction zones by reacting with silica. The presence in blueschists of abundant aragoniteassociated with silica suggests that if that does happen, not all carbonate isconsumed and a great deal enters very long-term storage in the mantle.Indeed, aragonite-magnesite associations are stable to pressures that areequivalent to depths of 240 km. Rocksformed under exceptionally high-pressure conditions, which might shed furtherlight on the deep part of the carbon cycle, are exceptionally rare.One such occurrence is the Kokchetav massif of Kazakhstan, in which dolomiticmarbles accompany eclogites. Notable forthe occurrence of metamorphic diamonds, Kokchetav rocks probably equilibrateddeeper than 250 km, so the carbonates are particularly interesting.Yongfeng Zhu and Yoshihide Ogasawara of Beijing University in China andWaseda University in Japan have found evidence for dissociation of dolomite inthem (Zhu, Y. & Ogasawara, Y. 2002. Carbonrecycled into deep Earth: Evidence from dolomite dissociation in subduction-zonerocks. Geology,v. 30, p. 947-950) during reactionsthat generate garnet and clinochlore. Themineral textures reveal equilibria that involve the production of carbon and oxygen,rather than CO₂, so it is quite possible that reflux of CO₂from subduction zones to the atmosphere may not be as significant as the "BLAGgers"suppose.

Interestingly, the same issue of Geology includes a paper on the geochemical conditions under whichgold and copper enter subduction-zone magmas to source major ore deposits (Mungall,J.E. 2002. Roasting the mantle: Slab meltingand the genesis of major Au and Au-rich Cu deposits. Geology, v. 30, p.915-918). Mungall focuseson the inability of chalcophile metals to enter magmas when sulphides are stablein the mantle. Under those condition Auand Cu tend to enter sulphide melts whose density and immiscibility separate themfrom silicate melts. Oxidation of sulphur is needed to overcome this tendency, and thatrequires high oxygen fugacity at the depths involved, suggested by him to accompanyabundant iron-3 in the subducted materials. That may be so, but release of molecular oxygen by high-pressurecarbonate dissociation, as described by Zhu and Ogasawara, seems an even morelikely means of freeing chalcophile metals to magmas.

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Empirical geochemistry points to continents' role in mantle dynamics

November 2002

Major-element chemistry of basalts provides proxies for key parameters involved in magmatism. Sodium content, normalized to an MgO content of 8%, relates to the degree of mantle melting, and similarly normalized iron content helps assess the depth of melt production. Such proxies help establish potential mantle temperatures—the temperature of magma that would erupt after rising adiabatically from different mantle depths. Low Na_8.0 suggests high potential temperature in a magma's source.

Vast repositories of basalt chemistry relate to every conceivable setting of magmatism, so Na_8.0 and Fe_8.0 numbers are useful in testing various hypotheses. One of these is that slabs of continental lithosphere affect mantle convection, by forming insulating "lids" that control surface heat flow. Eric Humler and Jean Besse, of the Université Denis Diderot in Paris, focus on the relationship between mantle potential temperature beneath ocean-ridge systems and their distance to passive continental margins (Humler, E. & Besse, J. 2002. A correlation between mid-ocean ridge basalt chemistry and distance to continents. Nature, v. 419, p, 607-609). Leaving out the complicating factors of continental margins that involve subduction and ridges affected by hot spots, they found that recent ridge basalts show higher potential temperatures when the ridge is close to continental lithosphere than for more distant ridges. This suggests that the mantle cools away from continents by between 0.05 to 0.1°C per kilometre. This matches the well-known increase in depth to ridges as they become further from continents. Rather than being inert passengers on modern plates, continents do play a role in the mantle's thermal structure.

The scope for synopsis of geochemical data is boosted by wider availability of existing data. How tedious it used to be, trawling paper journals for tables of analyses with which to compare ones own. It is still quite a task, but there is light on the horizon, because geochemists at the University of Mainz in Germany have made their compilations for ocean-island volcanic rocks and those from large igneous provinces (flood basalts) available on the web as the initial input to the GEOROC (Geochemistry of Rocks of the Oceans and Continents) database (http://georoc.mpch-mainz.gwdg.de). A similar database for ocean-floor basalts is PETDB at Columbia University in the USA (http://petdb.ldeo.columbia.edu/petdb/). Between them, the two web sites amass over 200 thousand analyses of major- and trace-elements, and isotopes, enough for even the most ardent user of MS Excel!

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Detrital platinum-group grains and "plum pudding" mantle heterogeneity

November 2002

Evidence for the degree and longevity of geochemical heterogeneities in the mantle has largely stemmed from studies of basalts derived by mantle melting. The great diversity of melting and fractionation processes involved in their genesis obviously complicates assessment of whether or not the mantle is a mixture of several chemical domains, even though it is suspected. Indeed it is only to be expected as a result of 4.5 billion years of mantle melting events and recycling of surface materials that find their way into subduction zones, unless, that is, long-term convection is an efficient means of mixing. A novel approach by a team from Stanford University, the University of Copenhagen and the US Geological Survey uses a combination of the rhenium-osmium radioactive decay scheme and the tendency for Re to enter melts, while Os is highly compatible to address this long-standing conundrum (Meibom, A. et al. 2002. Re-Os isotopic evidence for long-lived heterogeneity and equilibration processes in the Earth's upper mantle. Nature, v. 419, p. 705-708). The novelty lies in their use of detrital grains of platinoids in alluvium derived from the many ultramafic masses in the western USA, rather than individual basalts or peridotites themselves.

Measurements of ¹⁸⁷Os/¹⁸⁸Os in the grains span a wide range from extremely unradiogenic values to those signifying a high component of radiogenic ¹⁸⁷Os. The data occupy a bell-shaped (Gaussian) frequency distribution. While that probably reflects equilibration of old, unradiogenic material with radiogenic Os in melts derived from the mantle ultramafic rocks, and the destruction of any age information, it does point to mantle dotted with patches with different origins.

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Sea level fluctuations and large igneous provinces

October 2002

On a global scale, shifts in sea level recorded by stratigraphers and on seismic profiles stem from one of two main processes: changes in land-ice volume and the volume of the ocean basins. The latter most often results from changing rates of sea-floor spreading, so that when it is rapid a greater volume of the lithosphere near spreading centres retains sufficient buoyancy to displace the oceans onto continental margins. During slow spreading, cooling of the lithosphere and an increase in its density enlarges the deep abyssal plains, so that the oceans withdraw to low levels. The mid-Cretaceous saw vast outpourings of plume-related lavas onto the floor of the West Pacific. So large, that they reduced the volume of the Pacific basin enough to result in continental flooding that was unprecedented in the Phanerozoic Eon.

On a local scale, changes in sea level recorded by the stratigraphic record include those due to local processes, generally ascribed to tectonic events at continental margins, which involved rising continental lithosphere. However, one of the greatest forces for local change in the continental freeboard is changing density of the lithosphere due to thermal effects. Anywhere once affected by major igneous events should record relative falls in sea level during the acme of magmatism, and rises when activity waned. The British Tertiary Igneous Province, a precursor to the eventual rifting of the North Atlantic under the influence of the Iceland plume is a good candidate for charting magma-sea level connections. The central volcanic complexes of the Hebrides, and their enveloping flood basalt piles formed at the start of the Palaeocene (~60 Ma). Around that time, much of the British Isles underwent several kilometres of vertical uplift and exhumation, whose effects remain today. In the surrounding marine basins, this event is recorded by Palaeogene sandstone bodies, presumable derived by erosion of the uplifted crust. Yet local Palaeogene sediments also record episodes of rising sea level. John Maclennan and Brian Lovell of the French Institut de Physique du Globe and Cambridge University have modelled the likely effect on sea levels around the British Isles by crustal underplating of magmas formed during the BTIP magmatism (Maclennan, J. & Lovell, B. 2002. Control of regional sea level by surface uplift and subsidence caused by magmatic underplating of the Earth's crust. Geology, v. 30, p. 675-678).

Up to 8 km of mafic igneous rocks seem to have ponded at the base of the British Isles' crust while the BTIP was active. This estimate stems from the fact that the lavas of the province evidence high-pressure fractional crystallization. Calculations of the percentage of cumulates needed to generate the bulk chemistry of the BTIP lavas suggest that their volume far outweighs that of the volcanic part of the province. Given estimates of the volume of underplated cumulates, modelling boils down to examining the consequences for lithospheric density of initial heating and its subsequent relaxation. The Palaeogene sedimentary record provides good support for the model, with massive uplift from 60-56 Ma (the period when the BTIP was forming). Sudden sea-level rise at the end of this period never reached the level prior to magmatism; in fact it amounts to one half the estimated uplift. That is precisely in line with the underplating model.

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Flood basalts of Siberian Traps doubled at a stroke

August 2002

Erupted at the time of the Palaeozoic-Mesozoic boundary, and coinciding with the largest mass extinction during the Phanerozoic, the Siberian Traps are by far the biggest example of flood-basalt volcanism known. They blanket a huge area of the Siberian Platform. To the east of their outcrops is a large extensional downwarp, known as the West Siberian Basin, where recent deep drilling has cut through up to 1 km of flood basalts. Dating samples from 15 boreholes proves that these too are members of the Siberian Trap suite (Reichow, M.K. et al. 2002. ⁴⁰Ar/³⁹Ar dates from the West Siberian Basin: Siberian flood basalt province doubled. Science, v. 296, p. 1846-1849). Combined, the two zones of Siberian Traps represent eruption of around 2.3 million km³ of plume-derived magma at around 250 Ma ago, possibly within 2 or 3 Ma. Gas release from such a stupendous event is implicated in the Permian-Triassic mass extinction, either through climate change associated with CO₂ and SO₂, or toxic effects of hydrofluoric acid. Unlike the end-Triassic and K-T extinctions, no clear evidence has emerged for coincident flood volcanism and major impact at the end of the Palaeozoic Era. However, the use of tungsten isotopes as "fingerprints" for extraterrestrial debris in boundary sediments may help resolve the issue of whether an impact accompanied the Siberian Traps (see Tungsten and Archaean heavy bombardment, this issue)

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Earth's earliest events

The Earth has a core made, probably, of alloyed iron, nickel and sulphur. Much evidence points to the core having formed very early in our planet's history, probably in its first 100 million years. Core formation explains the depletion in iron of mantle rocks and magmas derived from them, compared with iron's abundance in the cosmos. Because some rarer elements have a 10000 times greater tendency to partition into melts containing metallic iron than into silicates, such siderophile ('iron-loving') metals are also highly depleted in the outer Earth. That is one of the reasons why gold and the platinum-group metals are so rare and highly prized at the Earth's surface. In fact, such noble metals are a lot more abundant than the presence of a metallic core could have allowed; they should be at vanishingly low abundances.

One solution to this paradox is that the 'extra' gold and PGEs arrived after core-formation had finished, the agency of delivery being continual bombardment by meteoritic debris in the first half billion years of the Solar System's history. The other is that somehow, the affinity of such metals for iron drops off at extremely high pressures. German, Canadian and Australian geochemists (Holzheid, A. et al., 2000. Evidence for a late chondritic veneer in the Earth's mantle from high-pressure partitioning of palladium and platinum. Nature, v. 406, p. 396-399) have shown experimentally that such a decrease doesn't occur, at least in the outermost 500 km of the Earth. This points strongly to impacts having seeded the upper mantle with noble metals, and therefore, perhaps, with lots more besides. This re-opens the old controversy between homo- and heterogeneous accretion of the Earth, tempered by the fact that more common siderophile metals, such as nickel and cobalt do not show mantle abundances that are in disequilibrium with core formation. The distinction is not trivial, for much of Earth's evolution has been driven by its internal composition, most especially its content of radioactive isotopes and water.

The Moon seems to have formed as a result of a gigantic impact of a Mars-sized body with the early Earth. Since the Moon has neither a core nor its full cosmic complement of iron, such a catastrophic beginning (effectively 'Year Zero' for the geochemistry of both bodies) must have taken place after core formation in the Earth. Because lunar rocks are so little changed by later events, its age is known with considerable accuracy—the Lunar Highlands are about 4450 million years old. It would be interesting to compare gold and PGE abundances between Earth and its Moon, for that might reveal the period during which bombardment delivered siderophile elements. Up to 3.8 billion years ago, both bodies received lots of visitors, culminating in a bout of huge impacts between 4.0 and 3.8 billion years ago that formed the huge lunar craters, that early astronomers termed maria or 'seas'.

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Extraterrestrial Gases in Buckyballs

A new geochemical tool to identify meteorite impact events is reported by Becker, Poreda & Bunch in the March 28 Issue of Proceeding of the National Academy of Sciences. Carbon in the form of fullerene molecules occurs in meteorite samples examined, as well as in samples from the clay layer thought to be associated with the 65 MYA end-Cretaceous impact event. These fullerene molecules contain within their lattices trapped noble gas molecules with isotopic ratios that can only be interpreted as extraterrestrial in origin.

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