Category Archives: Geochemistry, mineralogy, petrology and volcanology

The late-Ordovician mass extinction: volcanic connections

The dominant feature of Phanerozoic stratigraphy is surely the way that many of the formally named major time boundaries in the Stratigraphic Column coincide with sudden shifts in the abundance and diversity of fossil organisms. That is hardly surprising since all the globally recognised boundaries between Eras, Periods and lesser divisions in relative time were, and remain, based on palaeontology. Two boundaries between Eras – the Palaeozoic-Mesozoic (Permian-Triassic) at 252 Ma and Mesozoic-Cenozoic (Cretaceous-Palaeogene) at 66 Ma – and a boundary between Periods – Triassic-Jurassic at 201 Ma – coincide with enormous declines in biological diversity. They are defined by mass extinctions involving the loss of up to 95 % of all species living immediately before the events. Two other extinction events that match up to such awesome statistics do not define commensurately important stratigraphic boundaries. The Frasnian Stage of the late-Devonian closed at 372 Ma with a prolonged series of extinctions (~20 Ma) that eliminated  at least 70% of all species that were alive before it happened. The last 10 Ma of the Ordovician period witnessed two extinction events that snuffed out about the same number of species. The Cambrian Period is marked by 3 separate events that in percentage terms look even more extreme than those at the end of the Ordovician, but there are a great many less genera known from Cambrian times than formed fossils during the Ordovician.

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Faunal extinctions during the Phanerozoic in relation to the Stratigraphic Column.

Empirical coincidences between the precise timing of several mass extinctions with that of large igneous events – mainly flood basalts – suggest a repeated volcanic connection with deterioration of conditions for life. That is the case for four of the Famous Five, the end-Ordovician die-off having been ascribed to other causes; global cooling that resulted in south-polar glaciation of the Gondwana supercontinent and/or an extra-solar gamma-ray burst (predicated on the preferential extinction of Ordovician near-surface, planktonic fauna such as some trilobite families). Neither explanation is entirely satisfactory, but new evidence has emerged that may support a volcanic trigger (Jones, D.S. et al. 2017. A volcanic trigger for the Late Ordovician mass extinction? Mercury data from south China and Laurentia. Geology, v. 45, p. 631-634; doi:10.1130/G38940.1). David Jones and his US-Japan colleagues base their hypothesis on several very strong mercury concentrations in thin sequences in the western US and southern China of late Ordovician marine sediments that precede, but do not exactly coincide with, extinction pulses. They ascribe these to large igneous events that had global effects, on the basis of similar Hg anomalies associated with extinction-related LIPs. Yet no such volcanic provinces have been recorded from that time-range of the Ordovician, although rift-related volcanism of roughly that age has been reported from Korea. That does not rule out the possibility as LIPs, such as the Ontong Java Plateau, are known from parts of the modern ocean floor that formed in the Mesozoic and Cenozoic. Ordovician ocean floor was subducted long ago.

The earlier Hg pulses coincide with evidence for late Ordovician glaciations over what is now Africa and eastern South America. The authors suggest that massive volcanism may then have increased the Earth’s albedo by blasting sulfates into the stratosphere. A similar effect may have resulted from chemical weathering of widely exposed flood basalts which draws down atmospheric CO2. The later pulses coincide with the end of Gondwanan glaciation, which may signify massive emanation of volcanic CO2 into the atmosphere and global warming. Despite being somewhat speculative, in the absence of evidence, a common link between the Big Five plus several other major extinctions and LIP volcanism would quieten their popular association with major asteroid and/or comet impacts currently being reinvigorated by drilling results from the K-Pg Chicxulub crater offshore of Mexico’s Yucatan Peninsula.

Steam-bath Earth

The Earth’s mantle probably contained a significant amount of water from the start. Its earliest history was one of intense bombardment, including the impact that formed the Moon. Together with the conversion of gravitational potential energy to heat while the core was settling out from the mantle, impacts would have kept its overall temperature high enough to prevent water vapour from condensing on the surface until such heat input ceased and heat loss by radiation allowed the surface rapidly to cool. The atmosphere would have been rich in water vapour. Evidence from zircons that are the earliest tangible materials yet recovered hint at the formation of Zr-rich magmas – probably granitic in the broad sense – about 100 Ma after the Moon-forming event (see EPN July 2001: Zircons’ window on the Hadean). Yet no trace of substantial granitic rocks that old have ever been found.

Don Baker and Kassandra Sofonio of McGill University in Montreal, Canada have considered processes other than partial melting or fractional crystallisation that may have been possible during the earliest Hadean. In particular they have looked at one thought once to be a contender in the genesis of granite and latterly sidelined (Baker, D.R. & Sofonio, K. 2017. A metasomatic mechanism for the formation of Earth’s earliest evolved crust. Earth and Planetary Science Letters, v. 463, p. 48-55; http://dx.doi.org/10.1016/j.epsl.2017.01.022 ). They heated powdered artificial samples that chemically resembled the Earth’s original silicate mantle in sealed double capsules – an inner part containing the silicate powder and an outer one containing water. The capsules were held at around 727°C for a time and then quenched. The outer part of each capsule was found to be a glass of roughly granite composition. The experimental design ensured that superheated water diffused across the inner-outer capsule wall. So the ‘granite’ must have formed by a metasomatic process – essentially preferential solution of its component elements in supercritical water – the experimental temperature being insufficient to partially melt the ultramafic charge in the inner capsule.

Baker and Sofonio conclude that degassing of this metasomatic fluid – silicate-rich ‘steam’ – may have produced substantial masses of sialic crust on the Earth’s surface. Removal of material produced in such a manner would also have extracted trace elements with an affinity for granite from the early mantle – so-called incompatible elements. The subsequent recycling of such granitic blobs back into the mantle may explain geochemical signs in >500 Ma younger Archaean crust – produced by ‘normal’ igneous processes – of incompatible-element enriched reservoirs in the Early mantle.

Archaean continents derived from Hadean oceanic crust

As DNA is to tracing  human evolution and migration, so various isotope systems are to the evolution of the Earth. One of the most fruitful is the samarium-neodymium (Sm-Nd) system. The decay of 147Sm to 143Nd is used in dating rocks across the full range of Earth history, given coeval rocks with a suitable range of Sm/Nd ratios, because the decay has a long half life (1.06 x 1011 years). However, samarium has another radioactive isotope 147Sm with a half life that is a thousand times shorter (1.06 x 108 years). So it remains only as a minute proportion of the total Sm in rocks, most having decayed since it was formed in a pre-Solar System supernova. But its daughter isotope 142Nd is present in easily measurable quantities, having accumulated from 147Sm decay over the first few hundred million years of Earth’s history; i.e. during the Hadean and earliest Archaean Eons. It is this fact that allows geochemists to get an indirect ‘handle’ on events that took place in the Earth’s earliest, largely vanished history. The principle behind this approach is that when an ancient rock undergoes partial melting to produce a younger magma the rock that crystallizes from it inherits the relative proportions of Nd isotopes of its source and thereby carries a record of the earlier history.

English: An outcrop of metamorphosed volcanose...

Metamorphosed volcanosedimentary rocks from the Porpoise Cove locality, Nuvvuagittuq supracrustal belt, Canada. Possibly the oldest rocks on Earth. (credit: Wikipedia)

The eastern shore of Hudson Bay in Canada hosts the oldest tangible geology known, in form of some metamorphosed basaltic rocks dated at 4200 Ma old known as the Nuvvuagittuq Greenstone Belt – the only known Hadean rocks. They occur in a tiny (20 km2) patch associated with gneisses of tonalite-trondjhemits-granodiorite composition that are dated between 3760 and 3350 Ma. Engulfing both are younger (2800 to 2500 Ma) Archaean plutonic igneous rocks of felsic composition. Jonathan O’Neil and Richard Carlson of the University of Ottawa, Canada and the Carnegie Institution for Science, Washington DC, USA respectively, measured proportions of Nd isotopes in both sets of felsic igneous rocks (O’Neil, J. & Carlson, R.W. 2017. Building Archean cratons from Hadean mafic crust. Science, v. 355, p. 1199-1202; doi:10.1126/science.aah3823).

The oldest gneisses contained relative proportions of 142Nd commensurate with them having been formed by partial melting of the Hadean mafic rocks about a few hundred million years after they had been erupted to form the oldest known crust; no surprise there. However, the dominant components of the local continental crust that are about a billion years younger also contain about the same relative proportions of 142Nd. A reasonable conclusion is that the Archaean continental crust of NE Canada formed by repeated melting of mafic crust of Hadean age over a period of 1.5 billion years. The modern Earth continually replenishes its oceanic crust over about 200 Ma due to plate tectonics. During the Archaean mantle dynamics would have been driven faster by much higher internal heat production. Had this involved simply faster plate tectonics the outermost skin of mafic crust would have been resorbed into the mantle even faster. By the end of the Archaean (2500 Ma) barely any Hadean crust should have been available to produce felsic magmas. But clearly at least some did linger, adding more weight to the idea that plate tectonics did not operate during the Hadean and Archaean Eons. See Formation of continents without subduction below.

Formation of continents without subduction

The formulation of the theory of plate tectonics provided plausible explanations for the growth of continental crust over time, among many other fundamental Earth processes. Briefly expressed, once basalt capped oceanic lithosphere is forced downwards at plate boundaries where plates move towards one another, beyond a certain penetration cool, moist basalt undergoes a pressure-controlled change of state. Its chemical constituents reassemble into minerals more stable under elevated pressure. In doing so, one outcome involves dehydration reactions the other being that the bulk composition is recast mainly in the form of high-pressure pyroxene and the mineral garnet: the rock eclogite. The density of the basaltic cap increases above that of the mantle. Gravity acts to pull the subducting slab downwards, this slab-pull force being the main driver of plate motions globally. Water vapour and other fluids shed by dehydration reactions rise from the subducted slab into the wedge of overlying mantle to change its conditions of partial melting and the composition of the magma so produced. This is the source of arc magmatism that persists at the destructive plate margin to increase the volcanic pile’s thickness over time. When magma is able to pond at the base of the new crust its fractional crystallisation produces dense cumulates of high-temperature mafic silicates and residual melt that is both lighter and more enriched in silica. Residual magma rises to add to the middle and upper crust while the cumulate-rich lower crust becomes less gravitationally stable, eventually to spall downwards by delamination. Such a process helps to explain the bulk low density of continental crust built up over time together with the freeboard of continents relative to the ocean floor: a unique feature of the Earth compared with all other bodies in the Solar System. It also accounts for the vast bulk of continental crust having remained at the surface since it formed: it rarely gets subducted, if at all.

One suggested model for pre-plate tectonic continent formation (credit, Robert J Stern https://speakingofgeoscience.org/2013/04/28/when-did-plate-tectonics-begin-on-earth-and-what-came-before/)

Tangible signs that such subduction was taking place in the past – eclogites and other high-pressure, low-temperature metamorphosed basalts or blueschists – are only found after 800 Ma ago. Before that time evidence for plate tectonics is circumstantial. Some geologists have argued for a different style of subduction in earlier times, plates under riding others at low angles. Others have argued for a totally different style of tectonics in Earth early history, marked by changes in bulk chemical composition of the continental crust at the Archaean-Proterozoic boundary. A new twist comes from evidence in the Archaean Pilbara Craton of Western Australia (Johnson, T.E. et al. 2017. Earth’s first stable continents did not form by subduction. Nature, v. 543, p. 239-242; doi:10.1038/nature21383). The authors found that basalts dated at about 3.5 Ga have trace-element geochemistry with affinities to the primitive basalts of island arcs. That makes them a plausible source for slightly younger felsic plutonic rocks with a tonalite-trondhjemite-granodiorite (TTG) compositional range (characteristic of Archaean continental crust). If the basalts were partially melted to yield 30% of their mass as new magma the melt composition would match that of the TTG crust. This would be feasible at only 30 km depth given a temperature increase with depth of at least 25° C per kilometre; more than the average continent geothermal gradient today but quite plausible with the then higher heat production by less decayed radioactive isotopes of uranium, thorium and potassium 3.5 Ga ago. This would have required the basalts to have formed a 30 km thick crust. However, the basalts’ geochemistry requires their generation by partial melting of earlier more mafic basalts rather than directly from the mantle. That early Archaean mantle melting probably did generate vast amounts such primary magma is generally acknowledged and confirmed by the common occurrence of komatiitic lavas with much higher magnesium content than common basalts of modern constructive margins. In essence, Johnson et al. favour thermal reworking of primitive Archaean crust, rather than reworking in a plate tectonic cycle.

More on continental growth and plate tectonics

See also

When did Plate Tectonics begin on Earth, and what came before?

A ‘recipe’ for Earth’s accretion, without water

The Earth continues to collect meteorites, the vast majority of which are about as old as our planet; indeed many are slightly older. So it has long been thought that Earth originally formed by gravitational accretion when the parental bodies of meteorites were much more abundant and evenly distributed. Meteorites fall in several classes, metallic (irons) and several kinds that contain silicate minerals, some with a metallic component (stony irons) others without, some with blebs or chondrules of once molten material (chondrites) and others that do not (achondrites), and more subtle divisions among these general groups. In the latter half of the 20th century geochemists and cosmochemists became able to compare the chemical characteristics of different meteorite classes with that of the Sun –from its radiation spectrum – and those of different terrestrial rocks – from direct analysis. The relative proportions of elements in chondrites turned out to match those in the Sun – inherited from the gas nebula from which it formed – better than did other classes. The best match with this primitive composition turned out to be the chemistry of carbonaceous chondrites that contain volatile organic molecules and water as well as silicates and sulfides. The average chemistry of one sub-class of carbonaceous chondrites (C1) has been chosen as a ‘standard of standards’ against which the composition of terrestrial rocks are compared in order that they can be assessed in terms of their formative processes relative to one another. For a while carbonaceous chondrites were reckoned to have formed the bulk of the Earth through homogeneous accretion: that is until analyses became more precise at increasingly lower concentrations. This view has shifted …

Geochemistry is a complex business(!), bearing in mind that rocks that can be analysed today predominantly come from the tiny proportion of Earth that constitutes the crust. The igneous rocks at the centre of wrangling how the whole Earth has evolved formed through a host of processes in the mantle and deep crust, which have operated since the Earth formed as a chemical system. To work out the composition of the primary source of crustal igneous rocks, the mantle, involves complex back calculations and modelling. It turns out that there may be several different kinds of mantle. To make matters worse, those mantle processes have probably changed considerably from time to time. To work back to the original formative processes for the planet itself faces the more recent discovery that different meteorite classes formed in different ways, different distances from the Sun and at different times in the early evolution of the pre-Solar nebula. Thankfully, some generalities about chemical evolution and the origin of the Earth can be traced using different isotopes of a growing suite of elements. For instance, lead isotopes have revealed when the Moon formed from Earth by a giant impact, and tungsten isotopes narrow-down the period when the Earth first accreted. Incidentally, the latest ideas on accretion involve a series of ‘embryo’ planets between the Moon and Mars in size.

An example of an E-type Chondrite (from the Ab...

An example of an enstatite chondrite (from the Abee fall) in the Gallery of Minerals at the Royal Ontario Museum. (Photo credit: Wikipedia)

Calculating from a compendium of isotopic data from various types of meteorite and terrestrial materials, Nicolas Dauphas of the University of Chicago has convincingly returned attention to a model of heterogeneous accretion of protoplanetary materials from different regions of the pre-Solar nebula (Dauphas, N. 2017. The isotopic nature of the Earth’s accreting material through time. Nature, v. 541, p. 521-524; doi:10.1038/nature20830). His work suggests that the first 60% of Earth’s accretion involved materials that were a mixture of meteorite types, half being a type known as enstatite chondrites. These meteorites are dry and contain grains of metallic iron-nickel alloy and iron sulfides set in predominant MgSiO3 the pyroxene enstatite. The Earth’s remaining bulk accumulated almost purely from enstatite-chondrite material. A second paper in the same issue of Nature (Fischer-Gödde, M. & Kleine, T. 2017. Ruthenium isotopic evidence for an inner Solar System origin of the late veneer. Nature, v. 541, p. 525-527; doi:10.1038/nature21045) reinforces the notion that the final addition was purely enstatite chondrite.

This is likely to cause quite a stir: surface rocks are nothing like enstatite chondrite and nor are rocks brought up from the upper mantle by volcanic activity or whose composition has been back-calculated from that of surface lavas; and where did the Earth’s water at the surface and in the mantle come from? It is difficult to escape the implication of a mantle dominated by enstatite chondrite From Dauphas’s analysis, for lots of other evidence from Earth materials seem to rule it out. One ‘escape route’ is that the enstatite chondrites that survived planetary accretion, which only make up 2% of museum collections, have somehow been changed during later times.  The dryness of enstatite chondrites and the lack of evidence for a late veneer of ‘moist’ carbonaceous chondrite in these analyses cuts down the options for delivery of water, the most vital component of the bulk Earth and its surface.  Could moister meteorites have contributed to the first 60% of accretion, or was  post-accretion cometary delivery to the surface able to be mixed in to the deep mantle? Nature’s News & Views reviewer, Richard Carlson of the Carnegie Institution for Science in Washington DC, offers what may be a grim outlook for professional meteoriticists: that perhaps “the meteorites in our collection are not particularly good examples of Earth’s building blocks” (Carlson, R.W. 2017. Earth’s building blocks. Nature, v. 541, p. 468-470; doi:10.1038/541468a).

Animation of how the Solar System may have formed.

Oceans of magma, Moon formation and Earth’s ‘Year Zero’

That the Moon formed and Earth’s geochemistry was reset by our planet’s collision with another, now vanished world, has become pretty much part of the geoscientific canon. It was but one of some unimaginably catastrophic events that possibly characterised the early Solar System and those around other stars. Since the mantle geochemistry of the Earth’s precursor was fundamentally transformed to that which underpinned all later geological events, notwithstanding the formation of the protoEarth about 4.57 Ga ago, I now think of the Moon-forming event as our homeworld’s ‘Year Zero’. It was the ‘beginning’ of which James Hutton reckoned there was ‘no vestige’. Any modern geochemist might comment, ‘Well, there must be some kind of signature!’, but what that might be and when it happened are elusive, to say the least. One way of looking for answers is, as with so many thorny issues these days, to make a mathematical model. James Connelly and Martin Bizzarro of the University of Copenhagen, Denmark, have designed one based on the fact that one of the volatile elements that must have been partially ‘blown off’ by such a collision is lead and, of course, that is an element with several isotopes that are daughters of long-term decay of radioactive uranium and thorium (Connelly, J.N. & Bizzarro, M. 2016. Lead isotope evidence for a young formation age of the Earth–Moon system. Earth and Planetary Science Letters, v. 452, p. 36-43. doi:10.1016/j.epsl.2016.07.010).

Artist’s impression of the impact of a roughly Mars-size planet with the proto-Earth to form an incandescent cloud, from part of which the Moon formed.

Artist’s impression of the impact of a roughly Mars-size planet with the proto-Earth to form an incandescent cloud, from part of which the Moon formed. A NASA animation of lunar history can be viewed here.

Loss of volatile daughter isotopes of Pb produced by the decay schemes of highly refractory isotopes of U and Th would have reset the U-Pb and Th-Pb isotopic systems and therefore the radiogenic ‘clocks’ that depend on them in the same way as melting or high-temperature metamorphism resets the simpler 87Rb-87Sr decay scheme. Each radioactive U isotope has a different decay rate that produces a different Pb isotope daughter (235U Þ 207Pb; 238U Þ 206Pb, so it is possible to devise means of using present-day values of ratios between Pb isotopes, such as 207Pb/206Pb, 206Pb/204Pb and 207Pb/204Pb, to work back to such a ‘closure’ time. In short, that is the approach used by Connelly and Bizzarro. The most complicated bit of that geochemical ruse is estimating values of the ratios for the Earth’s modern mantle and for the Solar system in general – a procedure based on what we can actually measure: lots of mantle-derived basalts and lots of meteorites. Cutting out some important caveats, the result of their model is quite a surprise: ‘Year Zero’ on their account was between 4426 and 4417 Ma years ago, which is astonishingly precise. And it is pretty close to the measured age of the of lunar Highland anorthosites – products of fractional crystallisation of the Moon’s early magma ocean – and also to that of the oldest zircons on Earth. But is also about 60 Ma later than previous estimates

The Connelly and Bizzarro paper follows hard on the heels of another with much the same objective  (Snape, J.F. and 8 others 2016. Lunar basalt chronology, mantle differentiation and implications for  determining the age of the Moon. Earth and Planetary Science Letters, v. 451, p. 149-158. doi.org/10.1016/j.epsl.2016.07.026). Once again omitting a great deal of argument, Snape and colleagues end up with an age for the isotopic resetting of the lunar mantle of 4376 Ma to the nearest 18 Ma; i.e. an age significantly different from that arrived at by Connelly and Bizzarro. So the answer to the question, ‘When was there a vestige of a beginning?’ is, ‘It depends on the model’… Thankfully, neither estimate for ‘Year Zero’ has much bearing on the big, practical questions, such as, ‘When did life form?’, ‘Has there always been plate tectonics?’

More on the origin of the early Solar System and formation of the Earth-Moon system

Salt and Earth’s atmosphere

It is widely known that glacial ice contains a record of Earth’s changing atmospheric composition in the form of bubbles trapped when the ice formed. That is fine for investigations going back about a million years, in particular those that deal with past climate change. Obviously going back to the composition of air tens or hundreds of million years ago cannot use such a handy, direct source of data, but has relied on a range of indirect proxies. These include the number of pores or stomata on fossil plant leaves for CO2, variations in sulfur isotopes for oxygen content and so on. Variation over time of the atmosphere’s content of oxygen has vexed geoscientists a great deal, partly because it has probably been tied to biological evolution: forming by some kind of oxygenic photosynthesis and being essential for the rise to dominance of eukaryotic animals such as ourselves. Its presence or absence also has had a large bearing on weathering and the associated dissolution or precipitation of a variety of elements, predominantly iron. Despite progressively more clever proxies to indicate the presence of oxygen, and intricate geochemical theory through which its former concentration can be modelled, the lack of an opportunity to calibrate any of the models has been a source of deep frustration and acrimony among researchers.

Yet as is often said, there are more ways of getting rid of cats than drowning them in butter. The search has been on for materials that trap air in much the same way as does ice, and one popular, if elusive target has been the bubbles in crystals of evaporite minerals. The trouble is that most halite deposits formed by precipitation of NaCl from highly concentrated brines in evaporating lakes or restricted marine inlets. As a result the bubbles contain liquids that do a grand job of preserving aqueous geochemistry but leave a lot of doubt as regards the provenance of gases trapped within them. For that to be a sample of air rather than gases once dissolved in trapped liquid, the salt needs to have crystallized above the water surface. That may be possible if salt forms from brines so dense that crystals are able to float, or perhaps where minerals such as gypsum form as soil moisture is drawn upwards by capillary action to form ‘desert roses’. A multinational team, led by Nigel Blamey of Brock University in Canada, has published results from Neoproterozoic halite whose chevron-like crystals suggest subaerial formation (Blamey, N.J.F. and 7 others, 2016. Paradigm shift in determining Neoproterozoic atmospheric oxygen. Geology, v. 44, p. 651-654). Multiple analyses of five halite samples from an ~815 Ma-old horizon in a drill core from the Neoproterozoic Canning Basin of Western Australia contained about 11% by volume of oxygen, compared with 25% from Cretaceous salt from China, 20% of late-Miocene age from Italy, and 19 to 22% from samples modern salt of the same type.

Salar de Atacama salt flat in the Chilean puna

Evaporite salts in the Salar de Atacama Chile (credit: Wikipedia)

Although the Neoproterozoic result is only about half that present in modern air, it contradicts results that stem from proxy approaches, which suggest a significant rise in atmospheric oxygenation from 2 to about 18% during the younger Cryogenian and Ediacaran Periods of the Neoproterozoic, when marine animal life made explosive developments at the time of repeated Snowball Earth events. Whether or not this approach can be extended back to the Great Oxygenation Event at around 2.3 Ga ago and before depends on finding evaporite minerals that fit stringent criteria for having formed at the surface: older deposits are known even from the Archaean.

Bury the beast in basalt

Global warming cannot simply be reversed by turning off the tap of fossil fuel burning. Two centuries’ worth of accumulated anthropogenic carbon dioxide would continue to trap solar energy, even supposing that an immediate shutdown of emissions was feasible; a pure fantasy for any kind of society hooked on coal, oil and gas. It takes too long for natural processes to download CO2 from the atmosphere into oceans, living organic matter or, ultimately, back once more into geological storage. In the carbon cycle, it has been estimated that an individual molecule of the gas returns to one of these ‘sinks’ in about 30 to 95 years. But that is going on all the time for both natural and anthropogenic emissions. Despite the fact that annual human emissions are at present only about 4.5 % of the amount emitted by natural processes, clearly the drawdown processes in the carbon cycle are incapable of balancing them, at present. Currently the anthropogenic excess of CO2 over that in the pre-industrial atmosphere is more than 100 parts per million achieved in only 250 years or so. The record of natural CO2 levels measured in cores through polar ice caps suggests that natural processes would take between 5 to 20 thousand years to achieve a reduction of that amount.
Whatever happens as regards international pledges to reduce emissions, such as those reported by the Paris Agreement, so called ‘net-zero emissions’ leave the planet still a lot warmer than it would be in the ‘natural course of things’. This is why actively attempting to reduce atmospheric carbon dioxide may be the most important thing on the real agenda. The means of carbon sequestration that is most widely touted is pumping emissions from fossil fuel burning into deep geological storage (carbon capture and storage or CCS), but oddly that did not figure in the Paris Agreement, as I mentioned in EPN December 2015. In that post I noted that CCS promised by the actual emitters was not making much progress: a cost of US$50 to 100 per tonne sequestered makes most fossil fuel power stations unprofitable. Last week CCS hit the worlds headlines through reports that an Icelandic initiative to explore a permanent, leak-proof approach had made what appears to be a major breakthrough (Matter, J.M. and 17 others, 2016. Rapid carbon mineralization for permanent disposal of anthropogenic carbon dioxide emissions. Science, v. 352, p. 1312-1314). EPN January 2009 discussed the method that has now been tested in Iceland. It stems from the common observation that some of the minerals in mafic and ultramafic igneous rocks tend to breakdown in the presence of carbon dioxide dissolved in slightly acid water. The minerals are olivine ([Fe,Mg]2SiO4)] and pyroxene ([Fe,Mg]CaSi2O6), from whose breakdown the elements calcium and magnesium combine with CO2 to form carbonates.
Iceland is not short of basalts, being on the axial ridge of the North Atlantic. Surprisingly for a country that uses geothermal power to generate electricity it is not short of carbon dioxide either, as the hot steam contains large quantities of it. In 2012 the CarbFix experiment began to inject a 2 km deep basalt flow with 220 t of geothermal CO2 ‘spiked’ with 14C to check where the gas had ended up This was in two phases, each about 3 months long. After 18 months the pump that extracted groundwater directly from the lave flow for continuous monitoring of changes in the tracer and pH broke down. The fault was due to a build up of carbonate – a cause for astonishment and rapid evaluation of the data gathered. In just 18 months 95% of the 14C in the injected CO2 had been taken up by carbonation reactions. A similar injection experiment into the Snake River flood basalts in Washington State, USA, is said to have achieved similar results (not yet published). A test would be to drill core from the target flow to see if any carbonates containing the radioactive tracer filled either vesicles of cracks in the rock – some press reports have shown Icelandic basalt cores that contain carbonates, but no evidence that they contain the tracer .
Although this seems a much more beneficial use of well-injection than fracking, the problem is essentially the same as reinjection of carbon dioxide into old oil and gas fields; the high cost. Alternatives might be to spread basaltic or ultramafic gravel over large areas so that it reacts with CO2 dissolved in rainwater or to lay bear fresh rocks of that kind by removal of soil cover.

Kintisch, E., 2016. Underground injections turn carbon dioxide to stone. Science, v. 352, p. 1262-1263.

In a first, Iceland power plant turns carbon emissions to stone. Phys.org

Tungsten isotopes provide a ‘vestige of a beginning’

Apart from ancient detrital zircons no dated materials from the Earth’s crust come anywhere near the age when our home world formed, which incidentally was derived by indirect means. Hutton’s famous saying towards the close of the 18th century, ‘The result, therefore, of our present enquiry is, that we find no vestige of a beginning, – no prospect of an end’ seems irrefutable. Hardly surprising, you might think, considering the frantic pace of events that have reworked the geological record for four billion years and convincing evidence that not long after accretion the Moon-forming collision may have melted most of the early mantle. But there is a way of peering beyond even that definitive catastrophe. The metal tungsten, as anyone from the steel town of Rotherham will tell you, alloys very nicely with iron and makes it harder, stronger and more temperature resistant. Most of the Earth’s original complement of tungsten probably ended up in the core; it is a siderophile element. But traces can be detected in virtually any rock and, of course, in W-rich ore bodies. Its interest to modern-day geochemists lies in its naturally occurring isotopes, particularly 182W, a proportion of which forms by decay of a radioactive isotope of hafnium (182Hf). Or rather it did, for 182Hf has a half-life of about 9 million years. Only a vanishingly small amount from a nearby supernova that may have triggered  formation of the solar system remains undecayed.

Artistic impression of the early Earth before Moon formation. (Source: Creative Commons)

Artistic impression of the early Earth before Moon formation. (Source: Creative Commons)

A sign of the former presence of 182Hf in the early Earth comes from higher amounts of its daughter isotope 182W in some Archaean rocks (3.96 Ga) than in younger rocks. That excess is probably from undecayed  182Hf  in asteroidal masses that bombarded the Earth between 4.1 and 3.8 Ga. Now it turns out that some much younger flood basalts from the Ontong Java Plateau on the floor of the West Pacific Ocean (~120 Ma) and Baffin Island in northern Canada (~60 Ma) also contain anomalously high 182W/184W ratios (Rizo, H. et al. 2016. Preservation of Earth-forming events in the tungsten isotopic composition of modern flood basalts. Science, v. 352, p. 809-812; see also: Dahl, T.W. 2016. Identifying remnants of early Earth. Science, v. 352, p. 768-769). A different explanation is required for these occurrences. The flood basalts must have melted from chemically anomalous mantle, which originally contained undecayed 182Hf. The researchers have worked out that this heterogeneity stems from a silicate-rich planetesimal that had formed in the first 50 Ma of the solar system’s history, and was accreted to the Earth before the Moon-forming event – lunar rocks formed after 182Hf became extinct. That catastrophe and the succeeding 4.51 Ga of mantle convection failed to mix the ancient anomaly with the rest of the Earth.

So, when did plate tectonics start up?

Tiny, 4.4 billion year old zircon grains extracted from much younger sandstones in Western Australia are the oldest known relics of the Earth system. But they don’t say much about early tectonic processes. For that, substantial exposures of rock are needed, of which the undisputedly oldest are the Acasta gneisses 300 km north of Yellowknife in Canada’s North West Territories, which have an age of slightly more than 4 Ga. The ‘world’s oldest rock’ has been something of a grail for geologists and isotope geochemists who have combed the ancient Archaean cratons for 5 decades. But since the discovery of metasediments with an age of 3.8 Ga in West Greenland during the 1970s they haven’t made much headway into the huge time gap between Earth’s accretion at 4.54 Ga and the oldest known rocks (the Hadean Eon).

The Deccan Traps shown as dark purple spot on ...

Continental cratons (orange) where very-old rocks are likely to lurk. (credit: Wikipedia)

There have been more vibrant research themes about the Archaean Earth system, specifically the issue of when our planet settled into its modern plate tectonic phase A sprinkling of work on reconstructing the deep structural framework of Archaean relics has convinced some that opposed motion of rigid, brittle plates was responsible for their geological architecture, whereas others have claimed signs of a more plastic and chaotic kind of deformation of the outer Earth. More effort has been devoted to using the geochemistry of all the dominant rocks found in the ancient cratons, seeking similarities with and differences from those of more recent vintage. There can be little doubt that the earliest processes did form crust whose density prevented or delayed it from being absorbed into the mantle. Even the 4.4 Ga zircons probably crystallized from magma that was felsic in composition. Once trapped by buoyancy at the surface and subsequently wrapped around by similarly low density materials continental crust formed as a more or less permanent rider on the Earth’s deeper dynamics. But did it all form by the same kinds of process that we know to be operating today?

Plate tectonics involves the perpetual creation of rigid slabs of basalt-capped oceanic lithosphere at oceanic rift systems and their motions and interactions, including those with continental crust. Ocean floor cools as it ages and becomes hydrated by seawater that enters it. The bulk of it is destined eventually to oppose, head-to-head, the motions of other such plates and to deform in some way. The main driving force for global tectonics begins when an old, cold plate does deform, breaks, bends and drives downwards. Increasing pressure on its cold, wet basaltic top transforms it into a denser form: from a wet basaltic mineralogy (feldspar+pyroxene+amphibole) to one consisting of anhydrous pyroxene and garnet (eclogite) from which watery fluid is expelled upwards. Eclogite’s density exceeds that of mantle peridotite and compels the whole slab of oceanic lithosphere to sink or subduct into the mantle, dragging the younger parts with it. This gravity-induced ‘slab pull’ sustains the sum total of all tectonic motion. The water rising from it induces the wedge of upper mantle above to melt partially, the resulting magma evolves to produce new felsic crust in island arcs whose destiny is to be plastered on to and enlarge older continental masses.

Relics of eclogites and other high-pressure, low-temperature versions of hydrated basalts incorporated into continents bear direct and unchallengeable witness to plate tectonics having operated back to about 800 Ma ago. Before that, evidence for plate tectonics is circumstantial and in need of special pleading. Adversarial to-ing and fro-ing seems to be perpetual, between geoscientists who see no reason to doubt that Earth has always behaved in this general fashion and others who see room for very different scenarios in the distant past. The non-Huttonian tendency suggests an early, more ductile phase when greater radioactive heat production in the mantle produced oceanic crust so fast that when it interacted with other slabs it was hot enough to resist metamorphic densification wherever it was forced down. Faster production of magma by the mantle without slab-pull could have produced a variety of ‘recycling’ turnover mechanisms that were not plate-tectonic.

One thing that geochemists have discovered is that the composition of Archaean continental crust is very different from that produced in later times. In 1985 Ross Taylor and Scott McLennan, then of the Australian National University, hit on the idea of using shales of different ages as proxies for the preceding continental crust from which they had been derived by long erosion. Archaean and younger shales differed in such a way that suggests that after 2.5 Ga (the end of the Archaean) vast amounts of feldspar were extracted from the continent-forming magmas. This left the later Precambrian and Phanerozoic upper crust depleted in the rare-earth element europium, which ended up in a mafic, feldspar-rich lower crust. On the other hand, no such mass fractionation had left such a signature before 2.5 Ga. Another ANU geochemist, now at the University of Maryland, Roberta Rudnick has subsequently carried this approach further, culminating in a recent paper (Tang, M., Chen, K and Rudnick, R.L. 2016. Archean upper crust transition from mafic to felsic marks the onset of plate tectonics. Science, v. 351, p. 372-375). This uses nickel, chromium and zinc concentrations in ancient igneous and sedimentary rocks to track the contribution of magnesium (the ‘ma’ in ‘mafic’) to the early continents. The authors found that between 3.0 to 2.5 Ga continental additions shifted from a dominant more mafic composition to one similar to that of later times by the end of the Archaean. Moreover, this accompanied a fivefold increase in the pace of continental growth. Such a spurt has long been suspected and widely suggested to mark to start of true plate tectonics: but an hypothesis bereft of evidence.

A better clue, in my opinion, came 30 years ago from a study of the geochemistry of actual crustal rocks that formed before and after 2.5 Ga (Martin, H. 1986. Effect of steeper Archean geothermal gradient on geochemistry of subduction-zone magmas. Geology, v. 14, p. 753-756). Martin showed that plutonic Archaean and post-Archaean felsic rocks of the continental crust lie in distinctly different fields on plots of their rare-earth element (REE) abundances. Archaean felsic plutonic rocks show a distinct trend of enrichment in light REE relative to heavy REE as measures of the degree of partial melting decreases, whereas the younger crustal rocks show almost constant, low values of heavy REE/light REE whatever the degree of melting. The conclusion he reached was that while in the post Archaean the source was consistent with modern subduction processes – i.e. partial melting of hydrated peridotite in the mantle wedge above subduction zones – but during the Archaean the source was hydrated, garnet-bearing amphibolite of basaltic composition, in the descending slab of subducted oceanic crust. Together with Taylor and McLennan’s lack of evidence for any fractional crystallization in Archaean continental growth, in contrast to that implicated in Post-Archaean times.

The geochemistry forces geologists to accept that a fundamental change took place in the generation and speed of continental growth at the end of the Archaean, marking a shift from a dominance of melting of oceanic, mafic crust to one where the upper mantle was the main source of felsic, low-density magmas. Yet, no matter how much we might speculate on indirect evidence, whether or not subduction, slab-pull and therefore plate tectonics dominated the Archaean remains an open question.

More on continental growth and plate tectonics