Tag: slab pull

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