Tag: Subduction

A new explanation for the Neoproterozoic Snowball Earth episodes

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The Cryogenian Period that lasted from 860 to 635 million years ago is aptly named, for it encompassed two maybe three episodes of glaciation. Each left a mark on every modern continent and extended from the poles to the Equator. In some way, this series of long, frigid catastrophes seems to have been instrumental in a decisive change in Earth’s biology that emerged as fossils during the following Ediacaran Period (635 to 541 Ma). That saw the sudden appearance of multicelled organisms whose macrofossil remains – enigmatic bag-like, quilted and ribbed animals – are found in sedimentary rocks in Australia, eastern Canada and NW Europe. Their type locality is in the Ediacara Hills of South Australia, and there can be little doubt that they were the ultimate ancestors of all succeeding animal phyla. Indeed one of them Helminthoidichnites, a stubby worm-like animal, is a candidate for the first bilaterian animal and thus our own ultimate ancestor. Using the index for Palaeobiology or the Search Earth-logs pane you can discover more about them in 12 posts from 2006 to 2023. The issue here concerns the question: Why did Snowball Earth conditions develop? Again, refresh your knowledge of them, if you wish, using the index for Palaeoclimatology or Search Earth-logs. From 2000 onwards you will find 18 posts: the most for any specific topic covered by Earth-logs. The most recent are Kicking-off planetary Snowball conditions (August 2020) and Signs of Milankovich Effect during Snowball Earth episodes (July 2021): see also: Chapter 17 in Stepping Stones.

One reason why Snowball Earths are so enigmatic is that CO2 concentrations in the Neoproterozoic atmospheric were far higher than they are at present. In fact since the Hadean Earth has largely been prevented from being perpetually frozen over by a powerful atmospheric greenhouse effect. Four Ga ago solar heating was about 70 % less intense than today, because of the ‘Faint Young Sun’ paradox. There was a long episode of glaciation (from 2.5 to 2.2 Ga) at the start of the Palaeoproterozoic Era during which the Great Oxygenation Event (GOE) occurred once photosynthesis by oxygenic bacteria became far more common than those that produced methane. This resulted in wholesale oxidation to carbon dioxide of atmospheric methane whose loss drove down the early greenhouse effect – perhaps a narrow escape from the fate of Venus. There followed the ‘boring billion years’ of the Mesoproterozoic during which tectonic processes seem to have been less active. in that geologically tedious episode important proxies (carbon and sulfur isotopes) that relate to the surface part of the Earth System ‘flat-lined’.  The plethora of research centred on the Cryogenian glacial events seems to have stemmed from the by-then greater complexity of the Precambrian Earth System.

Since the GOE the main drivers of Earth’s climate have been the emission of CO2 and SO2 by volcanism, the sedimentary burial of carbonates and organic carbon in the deep oceans, and weathering. Volcanism in the context of climate is a two-edged sword: CO2 emission results in greenhouse warming, and SO2 that enters the stratosphere helps reflect solar radiation away leading to cooling. Silicate minerals in rocks are attacked by hydrogen ions (H+) produced by the solution of CO2 in rain water to form a weak acid (H2CO3: carbonic acid). A very simple example of such chemical weathering is the breakdown of calcium silicate:

CaSiO3  +  2CO2  + 3H2O  =  Ca2+  +  2HCO3–  +  H4SiO4  

The reaction results in calcium and bicarbonate ions being dissolved in water, eventually to enter the oceans where they are recombined in the shells of planktonic organisms as calcium carbonate. On death, their shells sink and end up in ocean-floor sediments along with unoxidised organic carbon compounds. The net result of this part of the carbon cycle is reduction in atmospheric CO2 and a decreased greenhouse effect: increased silicate weathering cools down the climate. Overall, internal processes – particularly volcanism – and surface processes – weathering and carbonate burial – interact. During the ‘boring billion’ they seem to have been in balance. The two processes lie at the core of attempts to model global climate behaviour in the past, along with what is known about developments in plate tectonics – continental break-up, seafloor spreading and orogenies – and large igneous events resulting from mantle plumes. A group of geoscientists from the Universities of Sydney and Adelaide, Australia have evaluated the tectonic factors that may have contributed to the first and longest Snowball Earth of the Neoproterozoic: the Sturtian glaciation (717 to 661 Ma) (Dutkiewicz, A. et al. 2024. Duration of Sturtian “Snowball Earth” glaciation linked to exceptionally low mid-ocean ridge outgassing. Geology, v. 52, online early publication; DOI: 10.1130/G51669.1).

Palaeogeographic reconstructions (Robinson projection) during the early part of the Sturtian global glaciation: LEFT based on geological data from Neoproterozoic terrains on modern continents; RIGHT based on palaeomagnetic pole positions from those terrains. Acronyms refer to each terrains, e.g. Am is Amazonia, WAC is the West African Craton. Orange lines are ocean ridges, those with teeth are subduction zone. (Credit: Dutkiewicz et al., parts of Fig. 1)

Shortly before the Sturtian began there was a major flood volcanism event, forming the Franklin large igneous province, remains of which are in Arctic Canada. The Franklin LIP is a subject of interest for triggering the Sturtian, by way of a ‘volcanic winter’ effect from SO2 emissions or as a sink for CO2­ through its weathering. But both can be ruled out as no subsequent LIP is associated with global cooling and the later, equally intense Marinoan global glaciation (655 to 632 Ma) was bereft of a preceding LIP. Moreover, a world of growing frigidity probably could not sustain the degree of chemical weathering to launch a massive depletion in atmospheric CO2. In search of an alternative, Adriana Dutkiewicz and colleagues turned to the plate movements of the early Neoproterozoic. Since 2020 there have been two notable developments in modelling global tectonics of that time, which was dominated by the evolution of the Rodinia supercontinent. One is based largely on geological data from the surviving remnants of Rodinia (download animation), the other uses palaeomagnetic pole positions to fix their relative positions: the results are very different (download animation).

Variations in ocean ridge lengths, spreading rates and oceanic crust production during the Neoproterozoic estimated from the geological (orange) and palaeomagnetic (blue) models. Credit: Dutkiewicz et al., parts of Fig. 2)

The geology-based model has Rodinia beginning to break up around 800 Ma ago with a lengthening of global constructive plate margins during disassembly. The resulting continental drift involved an increase in the rate of oceanic crust formation from 3.5 to 5.0 km2 yr-1. Around 760 Ma new crust production more than halved and continued at a much slowed rate throughout the Cryogenian and the early part of the Ediacaran Period.  The palaeomagnetic model delays breakup of the Rodinia supercontinent until 750 Ma, and instead of the rate of crust production declining through the Cryogenian it more than doubles and remains higher than in the geological model until the late Ediacaran. The production of new oceanic crust is likely to govern the rate at which CO2 is out-gassed from the mantle to the atmosphere. The geology-based model suggests that from 750 to 580 Ma annual CO2 additions could have been significantly below what occurred during the Pleistocene ice ages since 2.5 Ma ago. Taking into account the lower solar heat emission, such a drop is a plausible explanation for the recurrent Snowball Earths of the Neoproterozoic. On the other hand, the model based on palaeomagnetic data suggests significant warming during the Cryogenian contrary to a mass of geological evidence for the opposite.

A prolonged decrease in tectonic activity thus seems to be a plausible trigger for global glaciation. Moreover, reconstruction of Precambrian global tectonics using available palaeomagnetic data seems to be flawed, perhaps fatally. One may ask, given the trends in tectonic data: How did the Earth repeatedly emerge from Snowball episodes? The authors suggest that the slowing or shut-down of silicate weathering during glaciations allowed atmospheric CO2 to gradually build up as a result of on-land volcanism associated with subduction zones that are a quintessential part of any tectonic scenario.

This kind of explanation for recovery of a planet and its biosphere locked in glaciation is in fact not new. From the outset of the Snowball Earth hypothesis much the same escape mechanisms were speculated and endlessly discussed. Adriana Dutkiewicz and colleagues have fleshed out such ideas quite nicely, stressing a central role for tectonics. But the glaring disparities between the two models show that geoscientists remain ‘not quite there’. For one thing, carbon isotope data from the Cryogenian and Ediacaran Periods went haywire: living processes almost certainly played a major role in the Neoproterozoic climatic dialectic.

News about when subduction began

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Tangible signs of past subduction take the form of rocks whose mineralogy shows that they have been metamorphosed under conditions of high pressure and low temperature, and then returned to the surface somehow. Ocean-crust basaltic rocks become blueschist and eclogite. The latter is denser than mantle peridotite so that oceanic lithosphere can sink and be recycled. That provides the slab-pull force, which is the major driver of plate tectonics. Unfortunately, neither blueschists nor eclogites are found in metamorphic complexes older than about 800 Ma. This absence of direct proof of subduction and thus modern style plate tectonics has resulted in lively discussion and research seeking indirect evidence for when it did begin, the progress of which since 2000 you can follow through the index for annual logs about tectonics. An interesting new approach emerged in 2017 that sought a general theory for the evolution of silicate planets, which involves the concept of ‘lid tectonics’. A planet in a stagnant-lid phase has a lithosphere that is weak as a result of high temperatures: indeed so weak and warm that subduction was impossible. Stagnant-lid tectonics does not recycle crustal material back to its source in the mantle and it simply builds up the lithosphere. Once planetary heat production wanes below a threshold level that permits a rigid lithosphere, parts of the lid can be driven into the mantle. The beginnings of this mobile-lid phase and thus plate tectonics of some kind involves surface materials in mantle convection: the may be recycled.

Cartoon of possible Hadean stagnant lid tectonics, dominated by mantle plumes. (Credit: Bédard, J.H. 2018, Fig 3B, DOI: 10.1016/j.gsf.2017.01.005)

A group of geochemists from China, Canada and Australia have sought evidence for recycled crustal rocks from silicon and oxygen isotopes in the oldest large Archaean terrane, the  4.0 Ga old Acasta Gneiss Complex in northern Canada (Zhang, Q. and 10 others 2023. No evidence of supracrustal recycling in Si-O isotopes of Earth’s oldest rocks 4 Ga ago. Science Advances, v.9, article eadf0693; DOI: 10.1126/sciadv.adf0693). Silicon has three stable isotopes 28Si, 29Si, and 30Si. As happens with a number of elements, various geochemical processes are able to selectively change the relative proportions of such isotopes: a process known as isotope fractionation. As regards silicon isotopes used to chart lithosphere recycling, the basic steps are as follows: Organisms that now remove silicon from solution in seawater to form their hard parts and accumulate in death as fine sediments like flint had not evolved in the Archaean. Because of that reasonable supposition it has been suggested that seawater during the Archaean contained far more dissolved silicon than it does now. Such a rich source of Si would have entered Archaean oceanic crust and ocean-floor sediments to precipitate silica ‘cement’. The heaviest isotope 30Si would have left solution more easily than the lighter two. Should such silicified lithosphere have descended to depths in the mantle where it could partially melt the anomalously high 30Si would be transferred to the resulting magmas.

Proportions of 30Si in zircons, quartz and whole rock for Acasta gneisses (coloured), other Archaean areas (grey) and Jack Hills zircons (open circles. Vertical lines are error bars. (Credit: simplified from Zhang et al. Fig 1)

Stable-isotope analyses by Zhang et al. revealed that zircon and quartz grains and bulk rock samples from the Acasta gneisses, with undisturbed U-Pb ages, contain 30Si in about the same proportions relative to silicon’s other stable isotopes as do samples of the mantle. So it seems that the dominant trondhjemite-tonalite-granodiorite (TTG) rocks that make up the oldest Acasta gneisses were formed by partial melting of a source that did not contain rocks from the ocean crust. Yet the Acasta Gneiss Complex also contains younger granitic rocks (3.75 to 3.50 Ga) and they are significantly more enriched in 30Si, as expected from a deep source that contained formerly oceanic rocks. A similar ‘heavy’ silicon-isotope signature is also found in samples from other Archaean terranes that are less than 3.8 Ga old. Thus a major shift from stagnant-lid tectonics to the mobile-lid form may have occurred at the end of the Hadean. But apart from the Acasta Gneiss Complex only one other, much smaller Hadean terrane has been discovered, the 4.2 Ga Nuvvuagittuq Greenstone Belt. It occupies a mere 20 km2 on the eastern shore of Hudson Bay in Canada, and appears to be a sample of Hadean oceanic crust. It does include TTG gneisses, but they are about 3.8 Ga old and contain isotopically heavy silicon. So it seems unlikely that testing this hypothesis with silicon-isotope data from other Hadean gneissic terranes will be possible for quite a while, if at all.

Did Precambrian BIFs ‘fall’ into the mantle to trigger mantle plumes?

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How the Earth has been shaped has depended to a large extent on a very simple variable among rocks: their density. Contrasts in density between vast rock masses are expressed when gravity attempts to maintain a balance of forces. The abrupt difference in elevation of the solid surface at the boundaries of oceans and continents – the Earth’s hypsometry – stems from the contrasted densities of continental and oceanic crust: the one dominated by granitic rocks (~2.8 t m-3) the other by those of basaltic composition (~ 3.0 t m-3). Astronomers have estimated that Earth’s overall density is about 5.5 t m-3 – it is the densest planet in the Solar System. The underlying mantle makes up 68% of Earth’s mass, with a density that increases with depth from 3.3 to 5.4 t m-3 in a stepwise fashion, at a number of discontinuities, because mantle minerals undergo changes induced by pressure. The remaining one third of Earth’s mass resides in the iron-nickel core at densities between 9.5 to 14.5 t m-3. Such density layering is by no means completely stable. Locally increased temperatures in mantle rocks reduce their density sufficiently for masses to rise convectively to be replaced by cooler ones, albeit slowly. By far the most important form of convection affecting the lithosphere involves the resorption of oceanic lithosphere plates at destructive margins, which results in subduction. This is thought to be due to old, cold oceanic basalts undergoing metamorphism as pressure increases during subduction. They are transformed at depth to a mineral assemblage (eclogite) that is denser (3.4 to 3.5 t m-3) than the enveloping upper mantle. That density contrast is sufficient for gravity to pull slabs of oceanic lithosphere downwards. This slab-pull force is transmitted through oceanic lithosphere that remains at the surface to become the dominant driver of modern plate tectonics. As a result, extension of the surface oceanic lithosphere at constructive margins draws mantle upwards to partially melt at reduced pressure, thus adding new basaltic crust at mid-ocean rift systems to maintain a form of mantle convection. Seismic tomography shows that active subducted slabs become ductile about 660 km beneath the surface and below that no earthquakes are detected. Quite possibly, the density of the reconstituted lithospheric slab becomes less than that of the mantle below the 660 km discontinuity. So the subducted slab continues by moving sideways and buckling in response to the ‘push’ from its rigid upper parts above. But it has been suggested that some subducted slabs do finally sink to the core-mantle boundary, but that is somewhat conjectural.

Typical banded iron formation

There are sedimentary rocks whose density at the surface exceeds that of the upper mantle: banded iron formations (BIFs) that contain up to 60% iron oxides (mainly Fe2O3) and have an average density at the surface of around 3.5 t m-3. BIFs formed mainly in the late Archaean and early Proterozoic Eons  (3.2 to 1.0 Ga) and none are known from the last 400 Ma. They formed when soluble iron-2 (Fe2+) – being added to ocean water by submarine hydrothermal activity –was precipitated as Fe3+ in the form of iron oxide (Fe2O3) where oxygen was present in ocean water. With little doubt this happened only in shallow marine basins where cyanobacteria that appeared about 3.5 Ga ago had sufficient sunlight to photosynthesise. Until about 2.4 Ga the atmosphere and thus the bulk of ocean water contained very little oxygen so the oceans were pervaded by soluble iron so that BIFs were able to form wherever such biological activity was going on. Conceivably (but not proven), that BIF-forming biochemical reaction may even have operated far from land in ocean surface water, slowly to deposit Fe2O3 on the deep ocean floor. After 2.4 Ga oxygen began to build in the atmosphere after the Great Oxidation Event had begon. That time was also when the greatest production of BIFs took place. Strangely, the amount of BIF in the geological record fell during the next 600 Ma to rise again to a very high peak at 1.8 Ga. Since there must have been sufficient soluble iron and an increasing amount of available oxygen for BIFs to form throughout that ‘lean’ period the drop in BIF formation is paradoxical. After 1.0 Ga BIFs more or less disappear. By then so much oxygen was present in the atmosphere and from top to bottom in ocean water that soluble iron was mostly precipitated at its hydrothermal source on the ocean floor. Incidentally, modern ocean surface water far from land contains so little dissolved iron that little microbiological activity goes on there: iron is an essential nutrient so the surface waters of remote oceans are effectively ‘wet deserts’.

Plots of probability of LIPs and BIFs forming at the Earth’s surface during Precambrian times, based on actual occurrences (Credit: Keller, et al., modified Fig 1A)

Spurred by the fact that if a sea-floor slab dominated by BIFs was subducted it wouldn’t need eclogite formation to sink into the mantle, Duncan Keller of Rice University in Texas and other US and Canadian colleagues have published a ‘thought experiment’ using time-series data on LIPs and BIFs compiled by other geoscientists (Keller, D.S. et al. 2023. Links between large igneous province volcanism and subducted iron formations. Nature Geoscience, v. 16, article; DOI: 10.1038/s41561-023-01188-1.). Their approach involves comparing the occurrences of 54 BIFs through time with signs of activity in the mantle during the Palaeo- and Mesoproterozoic Eras, as marked by large igneous provinces (LIPs) during that time span. To do this they calculated the degree of correlation in time between BIFs and LIPs. The authors chose a minimum area for LIPs of 400 thousand km2 – giving a total of 66 well-dated examples. Because the bulk of Precambrian flood-basalt provinces, such as occurred during the Phanerozoic, have been eroded away, most of their examples are huge, well-dated dyke swarms that almost certainly fed such plateau basalts. Rather than a direct time-correlation, what emerged was a match-up that covered 74% of the LIPs with BIFs that had formed about 241 Ma earlier. They also found a less precise correlation between LIPs associated with 241 Ma older BIFs and protracted periods of stable geomagnetic field, known as ‘superchrons’. These are thought by geophysicists to be influenced by heat flow through the core-mantle boundary (CMB).

The high bulk density of BIFs at the surface would be likely to remain about 15 % greater than that of peridotite as pressure increased with depth in the mantle. Such slabs could therefore penetrate the 660 mantle discontinuity. Their subduction would probably result in their eventually ‘piling up’ in the vicinity of the CMB. The high iron content of BIFs may also have changed the way that the core loses heat, thereby triggering mantle plumes. Certainly, there is a complex zone of ultra-low seismic velocities (ULVZ) that signifies hot, ductile material extending above the CMB. Because BIFs’ high iron-content makes them thermally highly conductive compared with basalts and other sediments, they may be responsible. Clearly, Keller et al’s hypothesis is likely to be controversial and they hope that other geoscientists will test it with new or re-analysed geophysical data. But the possibility of BIFs falling to the base of the mantle spectacularly extends the influence of surface biological processes to the entire planet. And, indeed, it may have shaped the later part of its tectonic history having changed the composition of the deep mantle. The interconnectedness of the Earth system also demands that the consequences – plumes and large igneous provinces – would have fed back to the Precambrian biosphere. See also: Iron-rich rocks unlock new insights into Earth’s planetary history, Science Daily, 2 June 2023

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

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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.   

See also: https://www.sci.news/othersciences/geoscience/land-plants-continental-crust-composition-11151.html;  https://www.eurekalert.org/news-releases/963296

Evidence for an early Archaean transition to subduction

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Modern plate tectonics is largely driven by slab-pull: a consequence of high-pressure, low-temperature metamorphism of the oceanic crust far from its origin at an oceanic ridge. As it ages, basaltic crust cools, become increasingly hydrated by hydrothermal circulation of seawater through it and its density increases. That is why the abyssal plains of the ocean floor are so deep relative to the shallower oceanic ridges where it formed. Due to the decrease in the Earth’s internal heat production by decay of radioactive isotopes, once oceanic lithosphere breaks and begins to descend high-P low-T metamorphism transforms the basaltic crust to a denser form: eclogite, in which the dense, anhydrous minerals garnet and sodium-rich pyroxene (omphacite) form. Depending on local heat flow, the entire oceanic slab may then exceed the density of the upper mantle to drag the plate downwards under gravity. Metamorphic reactions of any P-T regime creates minerals less capable of holding water and drive H2O-rich fluids upwards into the overriding lithosphere, thus inducing it to partially melt. Magmas produced by this create volcanism at the surface, either at oceanic island arcs or near to continental margins, depending on the initial position of the plate subduction.

A direct proof of active subduction in the geological record is the presence of eclogite and related blueschists. Such rocks are unknown before 2100 Ma ago (mid-Palaeoproterozoic of the Democratic Republic of Congo) but there are geochemical means of ‘sensing’ plate tectonic control over arc magmatism (See: So, when did plate tectonics start up? February 2016).  The relative proportions of rare-earth elements in ancient magmatic rocks that make up the bulk of continental crust once seemed to suggest that plate tectonics started at the end of the Archaean Eon (~2500 Ma). That method, however, was quite crude and has been superseded by looking in great detail at the geochemistry of the Earth’s most durable mineral: zircon (ZrSiO4), which began more than two decades ago. Minute grains of that mineral most famously have pushed back the geological record into what was long believed to be half a billion years with no suggestion of a history: the Hadean. Zircon grains extracted from a variety of ancient sediments have yielded U-Pb ages of their crystallisation from igneous magma that extend back 4.4 billion years (Ga) (see: Pushing back the “vestige of a beginning”;January 2001).  

Though simple in their basic chemical formula, zircons sponge-up a large range of other trace elements from their parent magma. So, in a sense, each tiny grain is a capsule of their geochemical environment at the time they crystallised. In 2020 Australian geochemists presented the trace-element geochemistry of 32 zircons extracted from a 3.3 Ga old sedimentary conglomerate in the Jack Hills of Western Australia, which lie within an ancient continental nucleus or craton. They concluded that those zircons mainly reveal that they formed in andesitic magmas, little different from the volcanic rocks that are erupted today above subduction zones. From those data it might seem that some form of plate tectonics has been present since shortly after the Earth’s formation. Oxygen-isotope data from zircons are useful in checking whether zircons had formed in magmas derived directly from partial melting of mantle rocks or by recycling of crustal magmatic rocks through subduction. Such a study in 2012 (see: Charting the growth of continental crust; March 2012) that used a very much larger number of detrital zircon grains from Australia, Eurasia, North America, and South America seemed, in retrospect, to contradict a subduction-since-the-start view of Earth dynamics and crust formation. Instead it suggested that recycling of crust, and thus plate-tectonic subduction, first showed itself in zircon geochemistry at about 3 Ga ago.

Detailed chemical and isotopic analysis of zircons using a variety of instruments has steadily become faster and cheaper. Actually finding the grains is much easier than doing interesting things with them. It is a matter of crushing the host rock to ‘liberate’ the grains. Sedimentary hosts that have not been strongly metamorphosed are much more tractable than igneous rocks. Being denser than quartz, the dominant sedimentary mineral, zircon can be separated from it along with other dense, trace minerals, and from them in turn by various methods based on magnetic and electrical properties. Zircons can then be picked out manually because of their distinctive colours and shapes. A tedious process, but there are now several thousand fully analysed zircons aged between 3.0 to 4.4 Ga, from eleven cratons that underpin Australia, North America, India, Greenland and southern Africa. The latest come from a sandstone bed laid down about 3.31 Ga ago in the Barberton area of South Africa (Drabon, N. et al. 2022. Destabilization of Long‐Lived Hadean Protocrust and the Onset of Pervasive Hydrous Melting at 3.8 Ga. AGU Advances, v. 3, article e2021AV000520; DOI: 10.1029/2021AV000520). The authors measured lutetium (Lu), hafnium (Hf) and oxygen isotopes, and concentrations of a suite of trace element in 329 zircons from Barberton dated between 3.3 to 4.15 Ga.

A schematic model of transition from Hadean-Eoarchaean lid tectonics to a type of plate tectonics that subsequently evolved to its current form, based on hafnium isotope data in ancient zircons (credit: Bauer et al. 2020; Fig 3)

The Hf isotopes show two main groups relative to the values for chondritic meteorites (assumed to reflect the composition of the bulk Earth). Zircons dated between 3.8 and 4.15 Ga all show values below that expected for the whole Earth. Those between 3.3 and 3.8 Ga show a broader range of values that extend above chondritic levels. The transition in data at around 3.8 Ga is also present in age plots of uranium relative to niobium and scandium relative to ytterbium, and to a lesser extent in the oxygen isotope data. On the basis of these data, something fundamentally changed in the way the Earth worked at around 3.8 Ga. Nadja Drabon and colleagues ascribe the chemical features of Hadean and Eoarchaean zircons to an early protocrust formed by melting of chemically undepleted mantle. This gradually built up and remained more or less stable for more than 600 Ma, without being substantially remelted through recycling back to mantle depths. After 3.8 billion years ago, geochemical signatures of the zircons start showing similarities to those of zircons derived from modern subduction zones. Hf isotopes and trace-element geochemistry in 3.6 to 3.8 Ga-old  detrital zircons from other cratons are consistent with a 200 Ma transition from ‘lid’ tectonics (see: Lid tectonics on Earth; December 2017) to the familiar tectonics of rigid plates whose basalt-capped lithosphere ultimately returns to the mantle to be involved in formation of new magmas from which continental crust stems. Parts of plates bolstered by this new, low density crust largely remain at the surface.

While Drabon et al. do provide new data from South Africa’s Kaapvaal craton, their conclusions are similar to earlier work by other geochemists based on data from other area (e.g. Bauer, A.M. et al. 2020. Hafnium isotopes in zircons document the gradual onset of mobile-lid tectonics. Geochemical Perspectives Letters, v. 14; DOI: 10.7185/geochemlet.2015), which the accompanying figure illustrates.

See also: Earliest geochemical evidence of plate tectonics found in 3.8-billion-year-old crystal. Science Daily, 21 April 2022. 3.8-Billion-Year-Old Zircons Offer Clues to When Earth’s Plate Tectonics Began. SciNews, 26 April 2022

New ideas on how subduction works

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Nowadays, plate tectonics is thought mainly to be driven by the sinking of old, relatively cold and dense oceanic lithosphere at subduction zones: slab-pull force dominates the current behaviour of the outermost Earth. At the eastern edge of Eurasia subduction beneath Japan has yet to consume Pacific Ocean lithosphere younger than 180 Ma (Middle Jurassic). The Pacific Plate extends eastwards from there for over 7000 km to its source at the East Pacific Rise. That spreading axis has disappeared quite recently beneath the North American Plate between Baha California and northern California. It has been subducted. Since, to a first approximation, sea-floor spreading is at the same pace either side of mid-ocean constructive plate margins, subduction at the western edge of the North America has consumed at least 7000 km of old ocean lithosphere. Slab-pull force there has been sustained for probably more than 250 Ma. As a result several former island arcs have been plastered onto the leading edge of the North American Plate to create the geological complexity of its western states. If at any time the weight of the subducting slab had caused it leading edge literally to snap and fall independently wouldn’t that have decreased slab-pull force or shut it off, and spreading at the East Pacific Rise, altogether? No, says the vast expanse of the West Pacific plate

That dichotomy once encouraged scientists of the plate-tectonic era to assume that a subducted slab remains as strong as rigid plates at the surface. They believed that subduction merely bends a plate so that it can slide into the mantle. The use of seismic waves (seismic tomography) to peer into the mantle has revealed a far more complex situation. Beneath North America traces of subducted slabs are highly deformed and must have lost their rigidity, yet they still maintain slab-pull force. Three geoscientists from the Swiss Federal Institute of Technology Zurich, Switzerland, and the University of Texas at Austin, USA (Gerya T. V., Becovici, D. & Becker, T.W. 2021. Dynamic slab segmentation due to brittle–ductile damage in the outer rise. Nature, v. 599, p 245-250; DOI: 10.1038/s41586-021-03937-x) used computer-generated models of how various forces and temperature conditions at small and large scales bear on the behaviour of slabs being subducted. Where a plate bends into a subduction zone its rigidity results in cracking and faulting of its no convex upper surface, while the base is compressed. Seismic anomalies in the descending slab reflect the formation of pulled-apart segments, similar to those in a bar of chocolate (for a possible example from an exhumed subduction zone see: A drop off the old block? May 2008). Thermo-mechanical modelling suggests that the slab becomes distinctly weakened through brittle damage and by reduction in grain size because of ductile deformation, yet each segment maintains a high viscosity relative to the surrounding mantle rocks. Under present conditions and those extrapolated back into the Proterozoic, where the slab is thinned between segments it remains sufficiently viscous to avoid segments detaching to sink independently of one another. Such delamination would reduce slab-pull force. Another process operates in the surrounding mantle. The occurrence of earthquakes in a subducted slab down to a depth of about 660 km – the level of a major discontinuity in the mantle where pressure induces a change in its mineralogy and density – confirms that a modern slab maintains some rigidity and deforms in a brittle fashion. But at this depth it cannot continue to descend steeply and travels horizontally along the discontinuity, pushed by the more shallow subduction. It can now become buckled as the mantle resists its lateral motion.

Left: the subduction zone beneath Japan defined by seismic tomography (yellow to red = lower seismic wave speeds – more ductile; yellow to blue = higher speeds – more rigid). Right: modelled evolution of viscosity in a similar subduction zone under modern conditions showing slab segmentation (blue to brown = increasing viscosity). (Credit: Gerya et al., Figs 4c & 1a-e)

Rather than trying to mimic the chaos beneath North America the authors compared their results with seismic tomography of the younger system of westward subduction beneath Japan. This allowed them to ‘calibrate’ their modelling against actual deep structure well-defined by seismic tomography. The tectonic jumble beneath North America probably resulted from a much longer history of eastwards subduction. The complexity there may be explained by successive foundering of deformed slabs into the deeper mantle looking a bit like a sheet of still viscous pie pastry dropped on its edge. This happened, perhaps, as island arcs that had formed in the eastern Pacific sporadically accreted to the continent as the intervening oceanic lithosphere was subducted.   

There is ample evidence that modern-style subduction was widespread back as far as the Palaeoproterozoic. But in the Archaean the evidence is fitful: some hints of subduction, but plenty of contrary evidence.  Gerya and co-workers suggest that higher heat production from radioactive decay mantle earlier in Earth’s history would have reduced plate strength and mantle resistance to slab penetration. Subduction may have occurred but was interrupted repeatedly by foundering/delamination of individual detached segments at much shallower depths. That implies weaker as well as intermittent slab pull, or even further back its complete absence, so that planetary recycling would then have required other mechanisms, such as ‘drip tectonics’.

See also: Crushed resistance: Tectonic plate sinking into a subduction zone and Fate of sinking tectonic plates is revealed, Science Daily, 11 November 2021

Nappe tectonics at the end of the Archaean

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The beginning of modern-style plate tectonics is still debated in the absence of definite evidence. Because Earth’s mantle generates heat through radioactive decay and still contains heat left over from planetary accretion and core formation it must always have maintained some kind of heat transfer through some kind of circulatory motion involving the mantle and lithosphere. That must always too have involved partial melting and chemical differentiation that created materials whose density was lower than that of the mantle; e.g. continental crust. Since continental materials date back to more than 4 billion years ago and some may have been generated earlier in the Hadean, only to be lrgely resorbed, a generalised circulation and chemical differentiation have been Earth’s main characteristics from the start. One view is that early circulation was a form of vertical tectonics without subduction via a sort of ‘dripping’ or delamination of particularly dense crustal materials back into the mantle. A sophisticated model of how the hotter early Earth worked in this way has been called ‘lid tectonics’, from which plate tectonics evolved as the Earth cooled and developed a thicker, more rigid lithosphere. Such an outer layer would be capable of self-generating the slab pull that largely drives lateral motions of lithospheric plates. That process occurs once a slab of oceanic lithosphere becomes cool and dense enough to be subducted (see: How does subduction start?; August 2018).

The most convincing evidence for early plate tectonics would therefore be tangible signs of both subduction and large horizontal movements of lithospheric plates: common enough in the Neoproterozoic and Phanerozoic records, but not glaringly obvious in the earlier Archaean Eon. These unequivocal hallmarks have now emerged from studies of Archaean rocks in the Precambrian basement that underpins northern China and North Korea. The North China Craton has two main Archaean components: an Eastern Block of gneisses dated between 3.8 and 3.0 Ga and a Western Block of younger (2.6 to 2.5 Ga) gneisses, metavolcanics and metasediments. They are separated by a zone of high deformation. A key area for understanding the nature of the deformed Central Orogenic Belt is the Zanhuan Complex near the city of Kingtai (Zhong, YL. et al. 2021. Alpine-style nappes thrust over ancient North China continental margin demonstrate large Archean horizontal plate motions. Nature  Communications, v. 12, article6172, DOI: 10.1038/s41467-021-26474-7).

Schematic cross sections through the Zanhuan Complex of northern China, showing early and final development of the Central Orogenic Belt in the North China Block . (Credit: Zhong, YL. et al.;Figs 10b and c)

This small, complex area reveals that the older Eastern Block is unconformably overlain by Neoarchaean sediments, above which has been thrust a stacked series of nappes similar in size and form to those of the much younger Alpine orogenic belt of southern Europe. Though highly complex, the rocks involved having been folded and stretched by ductile processes, they are still recognisable as having originally been at the surface. Metavolcanics in the nappes can be assigned from their geochemistry to a late-Archaean fore-arc, through comparison with that of modern igneous rocks formed at such a setting in the Western Pacific. Thrust over the nappe complex is a jumble or mélange of highly deformed metasediments containing blocks of metabasalts and occasional ultramafic igneous rocks that geochemically resemble oceanic crust formed at a mid-ocean ridge. Some of them contain high-pressure minerals formed at depth in the mantle, indicating that they had once been subducted. The whole complex is cut by undeformed dykes of granitic composition dated at 2.5 Ga, confirming that the older rocks and the structures within them are Archaean in age. Thrust over the melange and tectonically underlying nappe complex are less-deformed volcanic rocks and granitic intrusions that closely resemble what is generally found in modern island arcs.

Orogenic belts bear witness to enormous crustal shortening caused by horizontal compressive forces. Assuming the average rate of modern subduction (2 cm yr-1) the 178 Ma history of the Zanhuan Complex implies more than 3,500 km of lateral transport. 2.5 billion years ago, higher radioactive heat production in the mantle would have made tectonic overturning considerably faster  The unconformity at the base of the complex suggests that it was driven over the equivalent of a modern passive, continental margin. So the complex provides direct evidence of horizontal plate tectonics and associated subduction during the latter stages of the Archaean that ranks in scale with that of many Phanerozoic orogenic belts, such as that of the European Alps. The Zanhuan Complex is a result of arc accretion that played a major role in many later orogens. The North China craton itself is reminiscent of continent-continent collision, as required in the formation of supercontinents.

Subduction and continental collision in the Himalaya

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The Indian subcontinent after it separated from Madagascar in the Late Cretaceous to move northwards to its destined collision with Eurasia and the formation of the Himalaya. (Credit: Frame from an animation ©Christopher Scotese)

During the Early Cretaceous (~140 Ma ago) India, Madagascar, Antarctica and Australia parted company with Africa after 400 Ma of unity as components of the Gondwana supercontinent. By 120 Ma Antarctica and Australia split from India and Madagascar, and the Indian Ocean began to form. India moved northwards , leaving Madagascar in its wake after about 70 Ma ago. By 50 Ma the subcontinent began to collide with Eurasia, its northward motion driving before it crustal materials that eventually formed the Himalaya. This highly complex process is wonderfully documented in an animation made in 2015 by Christopher Scotese, Emeritus Professor in the Department of Earth and Environmental Sciences, Northwestern University, USA. At the start of its journey India moved northwards at a slow rate of about 5 cm per year. After 80 Ma it speeded up dramatically to 15 cm per year, about twice as fast as any modern continental drift and a pace that lasted for over 30 Ma until collision began. How could that, in a geological sense, sudden and sustained acceleration have been induced? It would have required a change in the slab-pull force that is the primary driver of plate tectonics, suggesting an increase in the amount of subduction in the Tethys Ocean that formerly lay between India and Eurasia, probably at two, now hidden destructive plate margins.

A group of geoscientists from Canada, the US and Pakistan has documented that collision in terms of the record of metamorphism experienced beneath the Himalaya as slab after slab of once near-surface rocks were driven beneath the rising orogen (Soret, M. et al. 2021. How Himalayan collision stems from subduction. Geology, v. 49, p. 894-898; DOI: 10.1130/G48803.1). The Western Himalaya has trapped a deformed and tilted magmatic rock sequence of an island arc – the Kohistan Arc – between  the Eurasian plate and a zone of crustal thickening and shortening that was thrust southward over the ancient metamorphic basement of India itself. That crust was mantled by a variety of younger sediments deposited on the Tethyan continental shelf of the northern Indian plate which became involved in the process of crustal thickening. The Kohistan Arc probably formed above one of the destructive margins that consumed the oceanic lithosphere of the now vanished Tethys Ocean. Two distinct types of rock make up the slabs stacked-up by thrusting.

The uppermost, which also forms the highest part of the Western Himalaya in the form of Nanga Parbat (at 8,126 metres the world’s ninth highest mountain) comprises rocks thought to represent Tethyan oceanic lithosphere subducted perhaps at the second destructive margin. Their mineral assemblages, especially those of eclogites, indicate that they have been metamorphosed under pressures corresponding to depths of up to 100 km, but at low temperatures along a geothermal gradient of about 7°C km-1, i.e. in a low heat-flow environment. These ultra-high pressure (UHP) metamorphic rocks formed at the start of the India-Eurasia collision. The sequence of sedimentary slabs now overridden by the UHP slab were metamorphosed at around the same time, but under very different conditions. Their burial reached only about 35 km – the normal thickness of the continental crust – and a temperature of about 600°C on a 30°C km-1 geothermal gradient. Detailed mineralogy of the UHP slab reveals that as it was driven over the metasediments it evolved to the same geothermal conditions.

Matthew Soret and his colleagues explain how this marked metamorphic duality may have arisen in rocks that are now part of the same huge thrust complex. Their results are consistent with slicing together of oceanic lithosphere in a subduction zone to form a tectonic wedge of UHP mineral assemblages at the same time as continental shelf sediments were metamorphosed under more normal geothermal conditions. This was happening just as India came into contact with Eurasia. When crustal thickening began in earnest through the inter-slicing of the two assemblages, pressure on the UHP rocks fell rapidly as a result of their being thrust over the dominantly metasedimentary shelf sequence. It also moved into a zone of normal heat flow, first heating up equally quickly and then following a path of decreasing pressure and temperature as erosion pared away the newly thickened crust. Both assemblages now became part of the same metamorphic regime. In this way a subduction system evolved to become incorporated in an orogenic zone as two continents collided; a complex process that finds parallels in other orogens such as the Alps.

The subduction pulley: a new feature of plate tectonics

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Geological map of part of the Italian Alps. The Sesia-Lanzo Zone is 6 in the Key: a – highly deformed gneisses; b – metasedimentary schists with granite intrusions; c – mafic rocks; d – mixed mantle and crystalline basement rocks. (Credit: M. Assanelli, Universita degli Studi di Milano)

To a first approximation, as they say, the basis of plate tectonics is that the lithosphere is divided up into discrete, rigid plates that are bounded by lines of divergent, convergent and sideways relative motions: constructive, destructive and conservative plate margins. These are characterised by zones of earthquakes whose senses of motion roughly correspond to the nature of each boundary: normal, reverse and strike-slip, respectively. The seismicity is mainly confined to the lithosphere in the cases of constructive and conservative boundaries (i.e. shallow) but extends as deep as 700 km into the mantle at destructive margins, thereby defining the subduction of lithosphere that remains cool enough to retain its rigidity. Although the definition assumes that there is no deformation within plates, in practice that does occur for a wide variety of reasons in the form of intra-plate seismicity, mainly within continental lithosphere. Oceanic plate interiors are much stronger and largely ‘follow the rules’; they are generally seismically quiet.

One important feature of plate tectonics is the creation of new subduction zones when an earlier one eventually ceases to function. Where these form in an oceanic setting volcanism in the overriding plate creates island arcs. They create precursors of new continental crust because the density of magmas forming the new lithosphere confers sufficient buoyancy for them to be more difficult to subduct. Eventually island arcs become accreted onto continental margins through subduction of the intervening oceanic lithosphere. Joining them in such ‘docking’ are microcontinents, small fragments spalled from much older continents because of the formation of new constructive plate margins within them. It might seem that arcs and microcontinents behave like passive rafts to form the complex assemblages of terranes that characterise continental mountain belts, such as those of western North America, the Himalaya and the Alps. Yet evidence has emerged that such docking is much more complicated (Gün, E. et al. 2021. Pre-collisional extension of microcontinental terranes by a subduction pulley. Nature Geoscience, v. 14, online publication; DOI: 10.1038/s41561-021-00746-9).

Erkan Gün and colleagues from the University of Toronto and Istanbul Technical University examined one of the terranes in the Italian Alps – the Sesia-Lanzo Zone (SLZ) – thought to have been a late-Carboniferous microcontinental fragment in the ocean that once separated Africa from Europe. When it accreted the SLZ was forced downwards to depths of up to 70 km and then popped up in the latter stages of the Alpine orogeny. It is now a high-pressure, low-temperature metamorphic complex, having reached eclogite facies during its evolution. Yet its original components, including granites that contain the high-pressure mineral jadeite instead of feldspar, are still recognisable. Decades of geological mapping have revealed that the SLZ sequence shows signs of large-scale extensional tectonics. Clearly that cannot have occurred after its incorporation into southern Europe, and must therefore have taken place prior to its docking. Similar features are present within the accreted microcontinental and island-arc terranes of Eastern Anatolia in Turkey. In fact, most large orogenic belts comprise hosts of accreted terranes that have been amalgamated into older continents.

An ‘engineering’ simplification of the subduction pulley. Different elements represent slab weight (slab pull force) transmitted through a pulley at the trench to a weak microcontinent and a strong oceanic lithosphere. (Credit: Gün et al., Fig. 4)

Lithospheric extension associated with convergent plate margins has been deduced widely in the form of back-arc basins. But these form in the plate being underidden by a subduction zone. Extension of the SLZ, however, must have taken place in the plate destined to be subducted. Gün et al. modelled the forces, lithospheric structure, deformation and tectonic consequences that may have operated to form the SLZ, for a variety of microcontinent sizes. The pull exerted by the subduction of oceanic lithosphere (slab pull) would exert extensional forces on the lithosphere as it approached the destructive plate boundary. Oceanic lithosphere is very strong and would remain intact, simply transmitting slab-pull force to the weaker continental lithosphere, which ultimately would be extended. This is what the authors call a subduction ‘pulley’ system. At some stage the microcontinent fails mechanically, part of it being detached to continue with the now broken slab down the subduction zone. The rest would become a terrane accreted to the overriding plate. Subduction at this site would stop because the linkage to the plate has broken. It may continue by being transferred to a new destructive margin ‘behind’ the accreted microcontinent. This would allow other weak continental and island-arc ‘passengers’ further out on the oceanic plate eventually to undergo much the same process.

The observed complexity of tectonic terranes in other vast assemblies of them, such as the northern Pacific coast of North America and in many more ancient orogenic belts, is probably as much a result of extension before accretion as the compressional deformation suffered afterwards. The theoretical work by Erkan Gün and colleagues will surely spur tectonicians to re-evaluate earlier models of orogenesis.

Note: Figure 2 in the paper by Gün et al. shows how the width (perpendicular to the subduction zone) affects the outcomes of the subduction pulley. View an animation of a subduction pulley

Diamonds and the deep carbon cycle

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When considering the fate of the element carbon and CO2, together with all their climatic connotations, it is easy to forget that they may end up back in the Earth’s mantle from which they once escaped to the surface. In fact all geochemical cycles involve rock, so that elements may find their way into the deep Earth through subduction, and they could eventually come out again: the ‘logic’ of plate tectonics. Teasing out the various routes by which carbon might get to mantle is not so easily achieved. Yet one of the ways it escapes is through the strange magma that once produced kimberlite intrusions, in the form of pure-carbon crystals of diamond that kimberlites contain. A variety of petrological and geochemical techniques, some hinging on other minerals that occur as inclusions, has allowed mineralogists to figure out that diamonds may form at depths greater than about 150 km. Most diamonds of gem quality formed in unusually thick lithosphere beneath the stable, and relatively cool blocks of ancient continental crust known as cratons, which extends to about 250 km. But there are a few that reflect formation depths as great as 800 km that span two major discontinuities in the mantle (at 410 and 660 km depth). These transition zones are marked by sudden changes in seismic speed due to pressure-induced transformations in the structure and density of the main mantle mineral, olivine.

Diamond crystal containing a garnet and other inclusions (Credit: Stephen Richardson, University of Cape Town, South Africa)

Carbon-rich rocks that may be subducted are not restricted to limestones and carbon-rich mudstones. Far greater in mass are the basalts of oceanic crust. Not especially rich in carbon when they crystallised as igneous rocks, their progress away from oceanic spreading centres exposes them to infiltration by ocean water. Once heated, aqueous fluids cause basalts to be hydrothermally altered. Anhydrous feldspars, pyroxenes and olivines react with the fluids to break down to hydrated-silicate clays and dissolved metals. Dissolved carbon dioxide combines with released calcium and magnesium to form pervasive carbonate minerals, often occupying networks of veins. So there has been considerable dispute as to whether subducted sediments or igneous rocks of the oceanic crust are the main source of diamonds. Diamonds with gem potential form only a small proportion of recovered diamonds. Most are only saleable for industrial uses as the ultimate natural abrasive and so are cheaply available for research. This now centres on the isotopic chemistry of carbon and nitrogen in the diamonds themselves and the various depth-indicating silicate minerals that occur in them as minute inclusions, most useful being various types of garnet.

The depletion of diamonds in ‘heavy’ 13C once seemed to match that of carbonaceous shales and the carbonates in fossil shells, but recent data from carbonates in oceanic basalts reveals similar carbon, giving three possibilities. Yet, when their nitrogen-isotope characteristics are taken into account, even diamonds that formed at lithospheric depths do not support a sedimentary source (Regier, M.E. et al. 2020. The lithospheric-to-lower-mantle carbon cycle recorded in superdeep diamonds. Nature, v. 585, p. 234–238; DOI: 10.1038/s41586-020-2676-z). That leaves secondary carbonates in subducted oceanic basalts as the most likely option, the nitrogen isotopes more reminiscent of clays formed from igneous minerals by hydrothermal processes than those created by weathering and sedimentary deposition. However, diamonds with the deepest origins – below the 660 km mantle transition zone – suggest yet another possibility, from the oxygen isotopes of their inclusions combined with those of C and N in the diamonds. All three have tightly constrained values that most resemble those from pristine mantle that has had no interaction with crustal rocks. At such depths, unaltered mantle probably contains carbon in the form of metal alloys and carbides. Regier and colleagues suggest that subducted slabs reaching this environment – the lower mantle – may release watery fluids that mobilise carbon from such alloys to form diamonds. So, I suppose, such ultra-deep diamonds may be formed from the original stellar stuff that accreted to form the Earth and never since saw the ‘light of day’.