Category: Geochemistry, mineralogy, petrology and volcanology

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.

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.

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

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.

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…

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).

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

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 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 pull oceanic lithosphere apart and generate the hydrous arc magmas that can eventually become parts of the continents. This geochemical heat engine 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 biosphere does extend 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 team reviews observations of modern unicellular organisms in the cracks that permeate volcanic glass when it forms by rapid cooling of lava erupted into seawater. 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 ocean floor. 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 3.4 Ga: possibly 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, as well as thriving ‘ecosystems’, 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.

Oxygen in the atmosphere: why the delay?

Several lines of evidence suggest that the Earth’s atmosphere only accumulated sufficient oxygen for it to be significantly oxidising around 2.4 Ga 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 it combines with oxygen to reproduce carbon dioxide). Complicating the geochemistry of atmospheric oxygen is the way in which it may combine with biogenic methane by reactions catalysed by ultraviolet radiation. Since UV penetration also falls as oxygen levels rise, because of the formation of ozone. That makes possible extremely complex systems of positive and negative feedback. Assessing such mechanisms, three British environmental scientists suggest a kind of potential ‘flipping’ from two possible states for 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-2.4 Ga atmosphere in a low-oxygen state. The authors estimate that just a 3% increase in organic carbon burial could have flipped the dynamics towards a state of 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.

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.

Is the idea of Hadean continental crust bunkum?

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.

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.

Acasta gneiss and another old zircon

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.

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.