Continental crust – Earth-logs

The Earth System in action: land plants affected composition of continental crust

Published on September 8, 2022September 15, 2022 by zooks777Leave a comment

The essence of the Earth System is that all processes upon, above and beneath the surface interact in a bewildering set of connections. Matter and energy in all their forms are continually being exchanged, deployed and moved through complex cycles: involving rocks and sediments; water in its various forms; gases in the atmosphere; magmas; moving tectonic plates and much else besides. The central and massively dominant role of plate tectonics connects surface processes with those of our planet’s interior: the lithosphere, mantle and, arguably, the core. Interactions between the Earth System’s components impose changes in the dynamics and chemical processes through which it operates. Living processes have been a part of this for at least 3.5 billion years ago, in part through their role in the carbon cycle and thus the Earth’s climatic evolution. During the Silurian Period life became a pervasive component of the continental surface, first in the form of plants, to be followed by animals during the Devonian Period. Those novel changes have remained in place since about 430 Ma ago, plants being the dominant base of continental ecosystems and food chains.

Schematic diagram showing changes in river systems and their alluvium before and after the development of land plants. (Credit: Based on Spencer et al. 2022, Fig 4)

Land plants exude a variety of chemicals from their roots that break down rock to yield nutrient elements. So they play a dominant role in the formation of soil and are an important means of rock weathering and the production of clay minerals from igneous and metamorphic minerals. Plant root systems bind near-surface sediments thus increasing their resistance to erosion by wind and water, and to mass movement under gravity. This binding and plant canopies efficiently reduce dust transport, slow water flow on slopes and decrease the sediment load of flowing water. Plants and their roots also stabilise channels systems. There is much evidence that before the Devonian most rivers comprised continually migrating braided channels in which mainly coarse sands and gravels were rapidly deposited while silts and muds in suspension were shifted to the sea. Thereafter flow became dominated by larger and fewer channels meandering across wide tracts on which fine sediment could accumulate as alluvium on flood plains when channels broke their banks. Land plants more efficiently extract CO₂ from the atmosphere through photosynthesis and the new regime of floodplains could store dead plant debris in the muds and also in thick peat deposits. As a result, greenhouse warming had dwindled by the Carboniferous, encouraging global cooling and glaciation.

Judging the wider influence of the ‘greening of the land’ on other parts of the Earth system, particularly those that depend on internal magmatic processes, relies on detecting geochemical changes in minerals formed as direct outcomes of plate tectonics. Christopher Spencer of Queen’s University in Kingston, Canada and co-workers at the Universities of Southampton, Cambridge and Aberdeen in the UK, and the China University of Geosciences in Wuhan set out to find and assess such a geochemical signal (Spencer, C., Davies, N., Gernon, T. et al. 2022. Composition of continental crust altered by the emergence of land plants. Nature Geoscience, v. 15 online publication; DOI: 10.1038/s41561-022-00995-2). Achieving that required analyses of a common mineral formed when magmas crystallise: one that can be precisely dated, contains diverse trace elements and whose chemistry remains little changed by later geological events. Readers of Earth-logs might have guessed that would be zircon (ZrSiO₄). Being chemically unreactive and hard, small zircon grains resist weathering and the abrasion of transport to become common minor minerals in sediments. Thousands of detrital zircon grains teased out from sediments have been dated and analysed in the last few decades. They span almost the entirety of geological history. Spencer et al. compiled a database of over 5,000 zircon analyses from igneous rocks formed at subduction zones over the last 720 Ma, from 183 publications by a variety of laboratories.

The approach considered two measures: the varying percentages of mudrocks in continental sedimentary sequences since 600 Ma ago; aspects of the hafnium- (Hf) and oxygen-isotope proportions measured in the zircons using mass spectrometry and their changes over the same time. Before ~430 Ma the proportion of mudrocks in continental sedimentary sequences is consistently much lower than it is in post post-Silurian, suggesting a link with the rise of continental plant cover (see second paragraph). The deviation of the ¹⁷⁶Hf/¹⁷⁷Hf ratio in an igneous mineral from that of chondritic meteorites (the mineral’s εHf value) is a guide to the source of the magma, negative values indicating a crustal source, whereas positive values suggest a mantle origin. The relative proportions of two oxygen isotopes ¹⁸O and ¹⁶O in zircons, expressed as δ¹⁸O, indicates the proportion of products of weathering, such as clay minerals, involved in magma production – ¹⁸O selectively moves from groundwater to clay minerals when they form, increasing their δ¹⁸O.

While the two geochemical parameters express very different geological processes, the authors noticed that before ~430 Ma the two showed low correlation between their values in zircons. Yet, surprisingly, the parameters showed a considerable and consistent increase in their correlation in younger zircons, directly paralleling the ‘step change’ in the proportions of mudstones after the Silurian. Complex as their arguments are, based on several statistical tests, Spencer et al. conclude that the geologically sudden change in zircon geochemistry ultimately stems from land plants’ stabilisation of river systems. As a result more clay minerals formed by protracted weathering, increasing the δ¹⁸O in soils when they were eroded and transported. When the resulting marine mudrocks were subducted they transferred their oxygen-isotope proportions to magmas when they were partially melted.

That bolsters the case for dramatic geological consequences of the ‘greening of the land’. But did its effect on arc magmatism fundamentally change the bulk composition of post-Silurian additions to the continental crust? To be convinced of that I would like to see if other geochemical parameters in subduction-related magmas changed after 430 Ma. Many other elements and isotopes in broadly granitic rocks have been monitored since the emergence of high-precision rock-analysing technologies around 50 years ago. There has been no mention, to my knowledge, that the late-Silurian involved a magmatic game-changer to match that which occurred in the Archaean, also revealed by hafnium and oxygen isotopes in much more ancient zircons.

Did giant impacts trigger formation of the bulk of continental crust?

Published on August 18, 2022September 15, 2022 by zooks777Leave a comment

Earth is the only one of the rocky Inner Planets that has substantial continental crust, the rest being largely basaltic worlds. That explains a lot. For a start, it means that almost 30 percent of its surface area stands well above the average level of the basaltic ocean basins – more than 5 km – because of the difference in density between continental and oceanic lithosphere. Without continents and the inability of subduction to draw them back into the mantle Earth would remain a water-world as it is thought to have been during the Hadean and early Archaean Eons. The complex processes involved in geochemical differentiation and the repeated reworking of the continents through continual tectonic and sedimentary processes has further enriched parts of them in all manner of useful elements and chemical compounds. And, of course, the land has had a huge biosphere since the Devonian period that subsequently helped to draw down CO₂ well as evolving us.

It has been estimated that during the Archaean (4.0 to 2.5 Ga) around 75% of continental crust formed. Much of this Archaean crust is made up of sodium-rich granitoids: grey tonalite-trondhjemite-granodiorite (TTG) gneisses in the main. Their patterns of trace elements strongly suggest that their parent magmas formed by partial melting at shallow depths (25 to 50 km). Their source was probably basalts altered by hydrothermal fluids to amphibolites, unlike the post-Archaean dominance of melting associated with subducted slabs of lithosphere. Yet most of the discourse on early continents has centred on when plate tectonics began and when they became strong enough to avoid disruption into subductible ‘chunks’. Yet 10 years ago geochemists at the University of St Andrews in Scotland used hafnium and oxygen isotopes in Archaean zircons to suggest that the first continents grew very quickly in the Hadean and early Archaean at around 3.0 km³ yr^-1, slowing to an average of 0.8 km³ yr^-1 after 3.5 Ga. In 2017 Geochemists working on one of the oldest cratons in the Pilbara region of Western Australia developed a new, multistage model for early crust formation that did not have a subduction component. They proposed that high degrees of mantle melting first produced a mafic-ultramafic crust of komatiites, which became the source for a 3.5 Ga mafic magma with a geochemistry similar to those of modern island-arc basalts. If a crust of that composition attained a thickness greater than 25 km and was itself partially melted at its base, theoretically it could have generated TTG magma and Archaean continental crust. Three members of that team from Curtin University, Western Australia, and others have now contributed to formulating a new possibility for early continent formation (Johnson, T.E. et al. 2022. Giant impacts and the origin and evolution of continents. Nature, v. 608, p. 330–335; DOI: 10.1038/s41586-022-04956-y).

The distinctive Archaean granite-greenstone terrain of the Pilbara craton of Western Australia. TTG granites are shown in reds in the form of domes, which are enveloped by metamorphosed sediments and mafic-ultramafic volcanics in khaki and emerald green. Other colours signify post Archaean rocks. (Credit: Warren B. Hamilton; Earth’s first two billion years. GSA, 2007)

Tim Johnson and colleagues base their views on oxygen isotopes in Archaean zircon grains from the Pilbara. The zircons’ O-isotopes fall into three kinds of cluster: low ¹⁸O that indicate a hydrothermally altered source; intermediate ¹⁸O suggesting a mantle source; high ¹⁸O signifying contamination by metasedimentary and volcanic rocks. The first two alternate in the 3.6 to 3.4 Ga period; 4 clusters with mantle connotations occupy the 3.4 to 3.0 Ga range; a cluster with supracrustal contamination follows 3.0 Ga. This record can be reconciled agreeably with the geological and broad geochemical history of the Pilbara craton. But there is another connection: the Late Heavy Bombardment (LHB) recognised on most rocky bodies in the Solar System.

Bodies with much more sluggish internal processes than the Earth have preserved much of their earliest surfaces and the damage they have suffered since the Hadean. The Moon is the best example. Its earliest rocks in the lunar Highlands record a vast number of impact craters. Their relative ages, deduced from older ones being affected by later ones, backed up by radiometric ages of materials produced by impacts, such as melt spherules and basaltic magmas that flooded the lunar maria, revealed the time span of the LHB. The maria formed between 4.2 and 3.2 billion years ago and the damage done then is shown starkly by the dark maria that make up the ‘face’ of the Man in the Moon. The lunar bombardment was at a maximum between 4.1 and 3.8 Ga but continued until 3.5 Ga, dropping off sharply from its maximum effects. Earth preserves no tangible sign of the LHB, but because it is larger and more massive than the Moon, and both have always been in much the same orbit around the Sun, it must have been subject to impacts on a far grander scale. Projectiles carry kinetic energy that enables them to do geological work when they impact: 1/2 x mass x speed². The minimum speed of an impact is the same as the target’s escape velocity – 2.4 km s^-1 for the Moon and 11.2 km s^-1 for the Earth. So the energy of an object hitting the Earth would be 20 times more than if it struck the lunar surface. Taking into account the Earth’s larger cross sectional area, the amount of geological work done here by the LHB would have been as much as 300 times greater than that on Earth’s battered satellite.

The Earth’s early geological history was rarely seen in that context before the 21^st century, but that is the framework plausibly adopted by Johnson and colleagues. Archaean sediments in South Africa contain several beds of impact spherules older than 3.2 Ga, as do those of the Pilbara. The LHB also left a geochemical imprint on Earth in the form of anomalous isotope proportions of tungsten in 3.8 Ga gneisses from West Greenland (See: Tungsten and Archaean heavy bombardment and Evidence builds for major impacts in Early Archaean; respectively, July and August 2002). Johnson et al. suggest a 3-stage process for the evolution of the Pilbara craton: First a giant impact akin to the lunar Maria that formed a nucleus of mafic-ultramafic crust from shallow melting of the mantle; its chemical fractionation to produce low-magnesium basalts; and in turn their melting to form TTG magmas and thus a continental nucleus. They conclude:

‘The search for evidence of the Late Heavy Bombardment on Earth has been a long one. However, all along it seems that the evidence was right beneath our feet.’

I agree wholeheartedly, but would add that, until quite recently, many scientists who referred to extraterrestrial influences over Earth history were either pilloried or lampooned by their peers as purveyors of ‘whizz-bang’ science. So, many ‘kept their powder dry’. The weight of evidence and a reversal of wider opinion over the last couple of decades has made such hypotheses acceptable. But it has also opened the door to less plausible notions, such as an impact cause for sudden climate change and even for mythological catastrophes such as the destruction of Sodom and Gomorrah!

See also: Timmer, J. 2022. Did giant impacts start plate tectonics? arsTechnica 11 August 2022.

Recycling of continental crust through time

Published on December 4, 2017 by zooks777Leave a comment

Because continental crust is so light – an average density of 2700 kg m^-3 compared with the mantles’ value of 3300 – it has been widely believed that continents cannot be subducted en masse. Yet it is conceivable that sial can be ‘shaved’ from below during subduction and from above by erosion and added to subductable sediment on the ocean floor. Certainly, there is overwhelming evidence for the net growth of continents through time and plenty for periods of increased and dwindling growth in the past. In some ancient orogens there are substantial slabs of continental composition whose mineralogy bears witness to ultra-high pressure metamorphism at depths greater than that of the base of continents. These slabs had been caught-up in subduction but never reached sufficiently high density to be retained by the mantle; they eventually ‘bobbed up’ again. On the other hand, if early continents were less silica rich through incorporation of substantial proportions of rock with basaltic composition parts of them could founder if subjected to high-pressure, low-temperature metamorphism. But not all crustal recycling to the mantle is through subduction. Some abnormally highly elevated parts of the continents that rose quickly in geological terms, such as the Tibetan Plateau, may have formed by lower crustal slabs becoming detached or delaminated from their base. Again modelling can help assess the past magnitude of continental recycling (Chowdhury, P. et al. 2017. Emergence of silicic continents as the lower crust peels off on a hot plate-tectonic Earth. Nature Geoscience, v. 10, p. 698-703; DOI: 10.1038/NGEO3010).

Various lines of evidence suggest that between 65 to 70% of the present continental volume existed by 3 billion years ago, yet that does not manifest itself in the rock record; perhaps a sign that some has returned to the mantle. It is also widely suggested that plate tectonics in the modern style began at about that time. Pryadarshi Chowdhury and colleagues simulate what may happen at depth in continent-continent collision zones – the classic site of orogenies –at different times in the past. Under the hotter conditions in the early Archaean mantle delamination would have been more likely than it has been during the Phanerozoic; i.e. the peeling off and sinking of the denser, more mafic lower crust and the attached upper mantle. The authors show that increased mantle temperature further back in time increases the likelihood and extent of such delamination. It also encourages partial melting of the descending continental material so creating rising bodies of more silicic magma that add to the remaining continent at the surface. Together with the lower crust’s attachment of to a mantle slab, this ensures that the peeled off material is able to descend under its own load. Once below a depth of 250 km felsic rocks are doomed to further descent. Waning of radiogenic mantle heat production encourages descending slabs to fail and break from the connection with lithosphere at higher levels so that a smaller proportion of the lower crust becomes detached and recycled. This evolution suggests that less and less continental crust is recycled with time. This broadly fits with current geochemical ideas based on the record of radiogenic Nd-, Sr- and Pb-isotopes in rocks ranging from early Archaean to Phanerozoic age.

Steam-bath Earth

Published on July 3, 2017 by zooks777Leave a comment

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.

Formation of continents without subduction

Published on March 12, 2017 by zooks777Leave a comment

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

How far has geochemistry led geology?

Published on July 16, 2015 by zooks7774 Comments

In the Solar System the Earth is unique in having a surface split into two distinct categories according to their relative elevation; one covered by water, the other not. More than 60% of its surface – the ocean basins – falls between 2 to 11 km below sea level with a mean around 4 to 5 km deep. A bit less than 40% – land and the continental shelves – stands higher than 1 km below sea level up to almost 9 km above, with a mean around 1 km high. Between 1 and 2 km below sea level is represented by only around 3 % of the surface area. This combined hypsography and wetness is reckoned to have had a massive bearing on the course of climate and biological evolution, as far as allowing our own emergence. The Earth’s bimodal elevation stems from the near-surface rock beneath each division having different densities: continental crust is less dense than its oceanic counterpart, and there is very little crustal rock with an intermediate density. Gravitational equilibrium ensures that continents rise higher than oceans. That continents were underpinned mainly by rocks of granitic composition and density, roughly speaking, was well known by geologists at the close of the 19^th century. What lay beneath the oceans didn’t fully emerge until after the advent of plate tectonics and the notion of simple basaltic magmas pouring out as plates became detached.

In 1915 Canadian geologist Norman Levi Bowen resolved previously acquired knowledge of the field relations, mineralogy and, to a much lesser extent, the chemistry of igneous rocks, predominantly those on the continents in a theory to account for the origin of continents. This involved a process of distillation or fractionation in which the high-temperature crystallisation of mafic (magnesium- and iron-rich) minerals from basaltic magma left a residual melt with lower Mg and Fe, higher amounts of alkalis and alkaline earth elements and especially enriched in SiO₂ (silica). A basalt with ~50% silica could give rise to rocks of roughly granitic composition (~60% SiO₂) – the ‘light’ rocks that buoy-up the continental surface – through Bowen’s hypothetical fractional crystallisation. Later authors in the 1930s, including Bowen’s teacher Reginald Aldworth Daly, came up with the idea that granites may form by basalt magma digesting older SiO₂-rich rocks or by partially melting older crustal rocks as suggested by British geologist Herbert Harold Read. But, of course, this merely shifted the formation of silica-rich crust further back in time

A great deal of field, microscope and, more recently, geochemical lab time has been spent since on to-ing and fro-ing between these hypotheses, as well as on the petrology of basaltic magmas since the arrival of plate theory and the discovery of the predominance of basalt beneath ocean floors. By the 1990s one of the main flaws seen in Bowen’s hypothesis was removed, seemingly at a stroke. Surely, if a basalt magma split into a dense Fe- Mg-rich cumulate in the lower crust and a less dense, SiO₂-rich residual magma in the upper continental crust the bulk density of that crust ought to remain the same as the original basalt. But if the dense part somehow fell back into the mantle what remained would be more able to float proud. Although a neat idea, outside of proxy indications that such delamination had taken place, it could not be proved.

Since the 1960s geochemical analysis has became steadily easier, quicker and cheaper, using predominantly X-ray fluorescence and mass-spectrometric techniques. So geochemical data steadily caught up with traditional analysis of thin sections of rock using petrological microscopes. Beginning in the late 1960s igneous geochemistry became almost a cottage industry and millions of rocks have been analysed. Recently, about 850 thousand multi-element analyses of igneous rocks have been archived with US NSF funding in the EarthChem library. A group from the US universities of Princeton, California – Los Angeles and Wisconsin – Madison extracted 123 thousand plutonic and 172 thousand volcanic igneous rocks of continental affinities from EarthChem to ‘sledgehammer’ the issue of continent formation into a unified theory (Keller, C.B. et al. 2015. Volcanic-plutonic parity and the differentiation of the continental crust. Nature, v. 523, p. 301-307).

In a nutshell, the authors compared the two divisions in this vast data bank; the superficial volcanic with the deep-crustal plutonic kinds of continental igneous rock. The gist of their approach is a means of comparative igneous geochemistry with an even longer pedigree, which was devised in 1909 by British geologist Alfred Harker. The Harker Diagram plots all other elements against the proportionally most variable major component of igneous rocks, SiO₂. If the dominant process involved mixing of basalt magma with or partial melting of older silica-rich rocks such simple plots should approximate straight lines. It turns out – and this is not news to most igneous geochemists with far smaller data sets – that the plots deviate considerably from straight lines. So it seems that old Bowen was right all along, the differing deviations from linearity stemming from subtleties in the process of initial melting of mantle to form basalt and then its fractionation at crustal depths. Keller and colleagues found an unexpected similarity between the plutonic rocks of subduction-related volcanic arcs and those in zones of continental rifting. Both record the influence of water in the process, which lowers the crystallisation temperature of granitic magma so that it freezes before the bulk can migrate to the surface and extrude as lava. Previously. rift-related magmas had been thought to be drier than those formed in arcs so that silica-rich magma should tend to be extruded.

But there is a snag, the EarthChem archive hosts only data from igneous rocks formed in the Phanerozoic, most being less than 100 Ma old. It has long been known that continental crust had formed as far back as 4 billion years ago, and many geologists believe that most of the continental crust was in place by the end of the Precambrian about half a billion years ago. Some even reckon that igneous process may have been fundamentally different before 3 billion years ago(see: Dhuime, B., Wuestefeld, A. & Hawkesworth, C. J. 2015. Emergence of modern continental crust about 3 billion years ago. Nature Geoscience, v. 8, p.552–555). So big-science data mining may flatter to deceive and leave some novel questions unanswered .

Plate tectonics monitored by diamonds

Published on August 2, 2011 by zooks777Leave a comment

eclogite — Norwegian Eclogite. Image by kevinzim via Flickr

For more than 30 years a debate has raged about the antiquity of plate tectonics: some claim it has always operated since the Earth first acquired a rigid carapace not long after a molten state following formation of the Moon; others look to the earliest occurrences of island-arc volcanism, oceanic crust thrust onto continents as ophiolite complexes, and to high-pressure, low-temperature metamorphic rocks. The earliest evidence of this kind has been cited from as far apart in time as the oldest Archaean rocks of Greenland (3.9 Ga) and the Neoproterozoic (1 Ga to 542 Ma). A key feature produced by plate interactions that can be preserved are high-P, low-T rocks formed where old, cool oceanic lithosphere is pulled by its own increasing density into the mantle at subduction zones to form eclogites and blueschists. In the accessible crust, both rock types are unstable as well as rare and can be retrogressed to different metamorphic mineral assemblages by high-temperature events at lower pressures than those at which they formed. Relics dating back to the earliest subduction may be in the mantle, but that seems inaccessible. Yet, from time to time explosive magmatism from very deep sources brings mantle-depth materials to the surface in kimberlite pipes that are most commonly found in stabilised blocks of ancient continental crust or cratons. Again there is the problem of mineral stability when solids enter different physical conditions, but there is one mineral that preserves characteristics of its deep origins – diamond. Steven Shirer and Stephen Richardson of the Carnegie Institution of Washington and the University of Cape Town have shed light on early subduction by exploiting the relative ease of dating diamonds and their capacity for preserving other minerals captured within them (Shirey, S.B. & Richardson, S.H. 2011. Start of the Wilson cycle at 3 Ga shown by diamonds from the subcontinental mantle. Science, v. 333, p. 434-436). Their study used data from over four thousand silicate inclusions in previously dated large diamonds, made almost worthless as gemstones by their contaminants. It is these inclusions that are amenable to dating, principally by the Sm-Nd method. Adrift in the mantle high temperature would result in daughter isotopes diffusing from the minerals. Once locked within diamond that isotopic loss would be stopped by the strength of the diamond structure, so building up with time to yield an age of entrapment when sampled. The collection spans five cratons in Australia, Africa, Asia and North America, and has an age spectrum from 1.0 to 3.5 Ga. Note that diamonds are not formed by subduction but grow as a result of reduction of carbonates or oxidation of methane in the mantle at depths between 125 to 175 km. In growing they may envelop fragments of their surroundings that formed by other processes.

A notable feature of the inclusions is that before 3.2 Ga only mantle peridotites (olivine and pyroxene) are trapped, whereas in diamonds younger than 3.0 Ga the inclusions are dominated by eclogite minerals (garnet and Na-, Al-rich omphacite pyroxenes). This dichotomy is paralleled by the rhenium and osmium isotope composition of sulfide mineral inclusions. To the authors these consistent features point to an absence of steep-angled subduction, characteristic of modern plate tectonics, from the Earth system before 3 Ga. But does that rule out plate tectonics in earlier times and cast doubt on structural and other evidence for it? Not entirely, because consumption of spreading oceanic lithosphere by the mantle can take place if basaltic rock is not converted to eclogite by high-P, low-T metamorphism when the consumed lithosphere is warmer than it generally is nowadays – this happens beneath a large stretch of the Central Andes where subduction is at a shallow angle. What Shirey and Richardson have conveyed is a sense that the dominant force of modern plate tectonics – slab-pull that is driven by increased density of eclogitised basalt – did not operate in the first 1.5 Ga of Earth history. Eclogite can also form, under the right physical conditions, when chunks of basaltic material (perhaps underplated magmatically to the base of continents) founder and fall into the mantle. The absence of eclogite inclusions seems also to rule out such delamination from the early Earth system. So whatever tectonic activity and mantle convection did take place upon and within the pre-3 Ga Earth it was probably simpler than modern geodynamics. The other matter is that the shift to dominant eclogite inclusions appears quite abrupt from the data, perhaps suggesting major upheavals around 3 Ga. The Archaean cratons do provide some evidence for a major transformation in the rate of growth of continental crust around 3 Ga; about 30-40 percent of modern continental material was generated in the following 500 Ma to reach a total of 60% of the current amount, the remaining 40% taking 2.5 Ga to form through modern plate tectonics

Deep Diamonds Shed Light on Ancient Continental Movements (news.sciencemag.org)
van Kranendonk, M.J. 2011. Onset of plate tectonics. Science, v. 333, p. 413-414.

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