Tag: Hadean

Modelling the effects of Hadean impacts

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The Hadean Eon (~4.6 to 4.0 Ga) is short on rocks that represent it. In fact geologists only know of a single 20 km2 outcrop within that age span (~4.3 Ga): the Nuvvuagittuq Greenstone Belt (~4.3 Ga)on the eastern shore of Hudson Bay. Even that age remains disputed. But a few, tiny detrital zircon grains extracted from much younger sandstones yield an age range up to 4.4 Ga: barely enough to refute James Hutton’s ‘No vestige of a beginning’. So, the Hadean is long on speculation, most based on less than 3% of all the detrital zircon grains that have been dated. What zircons can tell us is based on their isotopic geochemistry, their trace-element content and even tinier granules of a few other minerals that they encapsulate. The data from them suggest the presence of some kind of felsic magma production that crystallised at low temperatures (~700° C) and was exposed to watery fluids. All very vague compared with what can be gleaned with confidence from post-4.0 Ga rock exposures. But there is a sound astronomical context and a theory based on geophysical and geochemical processes known from experiment and observation of later geology, that can shed a little light.

The planetary system began to form by gravitational accretion of material in a protoplanetary disc of pre-solar gas and dust. The first step would have been gravitational sticking together of dust particles. Fast when this cloud was dense but slowing as the available starting material was depleted by growing planetesimals. This early accretion would easily have radiated away the heat generated by the gravitational potential energy that was released. But that became less effective as the accreting bodies grew to sizes of tens to hundreds of kilometres. Studies of meteorites, formed by collisions of larger planetesimals, show that they became hot enough to melt their contents and even to undergo internal, geochemical differentiation. The current view of the next step is that gravitational perturbations associated with Jupiter drove bodies ranging from asteroidal to Mars size into chaotic motion through the Solar System. Assembly of protoplanets thereafter was dominated by collisions. In the case of the proto-Earth this involved its collision with another, Mars-sized body, to result in the formation of the Moon and the early Earth, each initially enveloped by magma oceans. This event can be considered to be the starting point for all subsequent geological processes on both bodies. But that did not ‘calm down’ planetary bombardment. Plenty of large asteroids were still around: their size range can be judged roughly from those that remain in the Asteroid Belt, that are up to 940 km across in the case of the dwarf planet Ceres. This repository of Hadean objects is what motivated Tim Johnson of Curtin University, Western Australia and three Australian colleagues to ponder on the influence on the Hadean Earth of far more bodies, large and small, hurtling around the early Solar System (Johnson, T.E. et al. 2026. Impact heating and the hidden Hadean. Science, v. 392, p. 1408-1412; DOI: 10.1126/science.aeb5402. PDF requests to tim.johnson@curtin.edu.au).

The impact history of the Earth has largely been expunged by tectonics, erosion and sedimentary burial. Johnson et al. assumed an early impact flux from the almost pristine ‘stratigraphy’ of lunar cratering scaled up to the roughly 13 times greater gravitational pull of the Earth. They calculated that energy being released by impacts and partly incorporated into the Earth during the Hadean outweighed that being generated by internal radioactive decay by several orders of magnitude. Hadean tectonics was thus thermally dominated by impact energy, whose supply probably fluctuated wildly because of different sizes of impacting bodies. By far the largest crater on the Moon – the South Pole-Aitken basin – is 2500 km across. It formed about 4.3 Ga ago when a body 200 km wide struck the lunar surface. Being larger and having a greater gravitational pull, Earth would have suffered up to ten collisions of this magnitude.

In Archaean and later times tectonics became the main means of shedding ‘smoothly’ generated internal radiogenic heating. Dated lunar rock samples strongly suggest that such awesome bombardment lasted until the early Archaean, around 3.8 Ga ago. Traces of this Late Heavy Bombardment are anomalous tungsten isotopes in gneisses of that age from West Greenland (see: Tungsten and Archaean heavy bombardment; July 2002). Internal heating now governs the physical behaviour of rock: whether it is ductile or brittle. Modern-style lithosphere is brittle, hence plate tectonics. The mantle beneath, in the long term, behaves in a ductile fashion, hence convection. As thermal energy built up with each massive impact neither thermal conduction nor bulk convection in the deeper mantle – i.e. the general state of Earth’s present thermal balance – would have been sufficient to check its effects. Rock would need to melt and magma move rapidly in vast amounts to the surface to dissipate energy by radiation into space: by far the most efficient planet-cooling process. The authors also modelled the geotherm – the variation of temperature with depth – established by conductive heat loss and radiation from the surface under Hadean conditions. This is shown in the figure and explained below.

Melting conditions in an early Hadean basaltic crust. Credit: Johnson et al., Fig 4

The thick white line is the modelled conductive geotherm for the ‘coolest’ impact-heating scenario; a usually safe scientific approach. The thin white line shows beginning of melting of hydrous basaltic crust: the ‘mafic solidus’ – the blue area to its left remains solid. The dark to light green shading towards the right marks increasing percentages of basalt melting in 10% steps (dashed white lines). The palest area at right represents a completely molten crust, beyond the ‘mafic liquidus’. The dashed purple line is the liquidus of mantle peridotite. Moving leftwards, the solid purple, pink and orange lines represent the beginning of melting (solidus) for peridotite, anhydrous basalt and sodium-rich granite respectively

The modelled Hadean geotherm shows very rapid temperature increase down to about 7.5 km. It passes across the solidi of granite, hydrous basalt, anhydrous basalt and mantle peridotite: everything begins to melt. Clearly, whatever its composition, the uppermost Hadean crust would have been in a partially molten condition below about 3.5 km. At depths of 10 km or more, between 40 to 70 % of basalt would be molten. The distinction between brittle and ductile becomes meaningless in the light of Johnson et al.’s analysis of Hadean impact heating. Not only does the modelling rule out any rigid lithosphere and plate tectonics during the Hadean, it also explains the almost complete absence today of tangible Hadean rock. In particular, continental crust dominated by granitic rocks was probably recycled continually and literally into the Hadean ‘melting pot’. Convection would have dominated Hadean tectonics, but rather than taking the modern form of isolated plumes it would have been chaotic.

Simulated convective patterns for a Hadean upper mantle subject only to radiogenic heating (A) compared with its dynamic behaviour when heated by continuous heavy bombardment The grey areas represent dense residues left by very high degrees of partial melting at more shallow depths (B). Credit: Johnson et al., Fig 3 A and B.

Suddenly, beginning about 3.9 Ga a rich record of albeit disputed tectonics emerges during the Palaeoarchaean and then evolves onwards to modern planetary behaviour. The heavy bombardment had stopped.

See also: Asteroid assault made ancient Earth too hot and chaotic for continents to form. EurekAlert; 25 June 2026. Why Earth Could Not Hold On to Its First Continents Until the Asteroids Stopped Falling. Science Blog; 25 June 2026.

Vanished continents of the Hadean Eon: the zircon key

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Over the last few decades improved analytical techniques have made it possible to analyse tiny mineral grains for a variety of trace elements and several isotopes. Zircons obtained directly from crushed granitic igneous rocks vary in chemistry according to the magmatic processes that generated them and their tectonic context. Elevated ratios between uranium and niobium (U/Nb) and scandium and ytterbium (Sc/Yb) are characteristic of zircons in intermediate granites. These contain 52 to 63 % SiO2 – between mafic and felsic magmas – which formed by melting of hydrated mafic crust in settings akin to modern continental arcs; i.e. in subduction zones. But such partial melting can also take place where the base of continental crust delaminates and ‘drips’ into the mantle. That process is part of what is known as stagnant lid tectonics, believed by many to have been important in the Palaeoarchaean and Hadean. Such a process would have involved nearly anhydrous conditions and thus different geochemical partitioning of elements in the magmas and minerals that crystallised from them. Exposures of crystalline continental crust become increasingly rare further back in geological time, and there are none older than 4.0 Ga – i.e. of Hadean age – with a granitic component. Consequently studying the generation of continental crust in the Hadean and the early Archaean is almost entirely dependent on ancient zircons that found their way into much younger sedimentary rocks. The most famous of these occur as detrital grains in the 3.6 Ga Jack Hills conglomerate of Western Australia. Others have been extracted from similar ~3.3 Ga sedimentary rocks in the Barberton Greenstone Belt of South Africa and Eswatini.

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)

John Valley of the University of Wisconsin-Madison, USA, and co-workers from the US, Germany, Australia and France have worked on a large number of zircons newly extracted from Jack Hills. They have radiometrically dated them, and analysed Nb, Sc, U and Yb trace elements and hafnium (Hf) and oxygen isotopes Together with data from earlier studies, including Barberton zircons, they have teased out some remarkable insights into  ‘continent-forming’ magmatism as far back in time as 4.4 billion years ago (Valley, J.W. and 11 others 2026. Contemporaneous mobile- and stagnant-lid tectonics on the Hadean Earth. Nature, Open access; DOI: 10.1038/s41586-025-10066-2). More than 70% of the >4.0 Ga Jack Hills zircons have elevated U/Nb and Sc/Yb ratios, which suggest that they formed in a setting akin to continental-arc subduction (CAS) zones, to produce now-vanished Hadean continental crust. The remainder seem to represent processes at mid-ocean ridge (MOR) and oceanic island (OI) settings. In contrast, the bulk of Barberton zircons of Hadean age show OI affinities, with only around 22% showing Nb–Sc–U–Yb signatures of probable CAS origins. From about 4.4 to 3.8 Ga two distinct forms of continental crust generation seem to have operated on Earth. In the erosional source region for the Barberton zircons their host granites seem to have formed during the Hadean and Eoarchaean by remelting of foundered lower crust, i.e. probably in a stagnant-lid-like tectonic setting. But at around 3.6 Ga they ‘flip’ to a subduction-like setting. The zircons yielded by Jack Hills conglomerates suggest substantially different conditions: alternating CAS and OI settings during the Hadean and a fall-off in crust generation during the Eoarchaean (4.0 to 3.8 Ga).

Plots of Sc/Yb and U/Nb against ages of zircons (vertical scale logarithmic). Black points are from Jack Hills, red from Barberton. The yellow field represents zircons formed in subduction zones; green suggests stagnant lid tectonics; grey the overlap between the two settings. Credit: Valley et al. Fig 3 a and b.

The mixed Hadean zircon signatures from Jack Hills possibly indicate that they were derived by erosion and transport from several distinct terranes that had been generated by two different processes: some kind of upper crustal recycling and stagnant lid tectonics. Meanwhile, that part of the Hadean Earth represented by the Barberton zircons may have been a long-lived regime of stagnant lid tectonics, replaced by dominant subduction at the end of the Eoarchaean.  Yet the data suggest that into the Palaeoarchaean (3.6 to 3.2 Ga) and perhaps later, lid tectonics continued to operate somewhere, but at no time after 4.4 Ga was the Earth entirely subject to lid tectonics. Likewise, the authors insist that subduction was not of the plate-tectonic style, referring to some form of recycling of hydrated upper crustal mafic and ultramafic rocks into the mantle to undergo partial melting. Plate tectonics as we know it probably developed later in the Archaean. The early Earth had much higher heat flow than in later times, and thus the lithosphere was more ductile rather than brittle. The essence of modern tectonics is a series of rigid plates that extend down to the asthenosphere. When they deform it is largely through brittle failure of the entire lithosphere.

How the earliest continental crust may have formed

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Detrital zircon grains extracted from sandstones deposited ~3 billion year (Ga) ago in Western Australia yield the ages at which these grains crystallised. The oldest formed at about 4.4 Ga; only 150 Ma after the origin of the Earth (4.55 Ga). Various lines of evidence suggest that they originally crystallized from magmas with roughly andesitic compositions, which some geochemists suggest to have formed the first continental crust (see: Zircons and early continents no longer to be sneezed at; February 2006). So far, no actual rocks of that age and composition have come to light. The oldest of these zircon grains also contain anomalously high levels of 18O, a sign that water played a role in the formation of these silicic magmas. Modern andesitic magmas – ultimately the source of most continental crust – typically form above steeply-dipping subduction zones where fluids expelled from descending oceanic crust encourage partial melting of the overriding lithospheric mantle. Higher radiogenic heat production in the Hadean and the early Archaean would probably have ensured that the increased density of later oceanic lithosphere needed for steep subduction could not have been achieved. If subduction occurred at all, it would have been at a shallow angle and unable to exert the slab-pull force that perpetuated plate tectonics in later times (see: Formation of continents without subduction, March, 2017).

Landsat image mosaic of the Palaeoarchaean granite-greenstone terrain of the Pilbara Craton, Western Australia. Granite bodies show as pale blobs, the volcanic and sedimentary greenstone belts in shades of grey.

Geoscientists have been trying to resolve this paradox for quite a while. Now a group from Australia, Germany and Austria have made what seems to be an important advance (Hartnady, M. I. H and 8 others 2025. Incipient continent formation by shallow melting of an altered mafic protocrust. Nature Communications, v. 16, article 4557; DOI: 10.1038/s41467-025-59075-9). It emerged from their geochemical studies of rocks in the Pilbara Craton of Western Australia that are about a billion years younger than the aforementioned ancient zircon grains. These are high-grade Palaeoarchaean metamorphic rocks known as migmatites that lie beneath lower-grade ‘granite-greenstone’ terrains that dominate the Craton, which Proterozoic deformation has forced to the surface. Their bulk composition is that of basalt which has been converted to amphibolite by high temperature, low pressure metamorphism (680 to 730°C at a depth of about 30 km). These metabasic rocks are laced with irregular streaks and patches of pale coloured rock made up mainly of sodium-rich feldspar and quartz, some of which cut across the foliation of the amphibolites. The authors interpret these as products of partial melting during metamorphism, and they show signs of having crystallised from a water-rich magma; i.e. their parental basaltic crust had been hydrothermally altered, probably by seawater soon after it formed. The composition of the melt rocks is that of trondhjemite, one of the most common types of granite found in Archaean continental crust. Interestingly, small amounts of trondhjemite are found in modern oceanic crust and ophiolites.

A typical migmatite from Antarctica showing dark amphibolites laced with quartzofeldspathic products of partial melting. Credit: Lunar and Planetary Laboratory, University of Arizona

The authors radiometrically dated zircon and titanite (CaTiSiO₅) – otherwise known as sphene – in the trondhjemites, to give an age of 3565 Ma. The metamorphism and partial melting took place around 30 Ma before the overlying granite-greenstone assemblages formed. They regard the amphibolites as the Palaeoarchaean equivalent of basaltic oceanic crust. Under the higher heat production of the time such primary crust would probably have approached the thickness of that at modern oceanic plateaux, such as Iceland and Ontong-Java, that formed above large mantle plumes. Michael Hartnady and colleagues surmise that this intracrustal partial melting formed a nucleus on which the Pilbara granite-greenstone terrain formed as the oldest substantial component of the Australian continent. The same nucleation may have occurred during the formation of similar early Archaean terrains that form the cores of most cratons that occur in all modern continents.

The origin of life on Earth: new developments

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Debates around the origin of Earth’s life and what the first organism was like resemble the mythical search for the Holy Grail. Chivalric romanticists of the late 12th and early 13th centuries were pretty clear about the Grail – some kind of receptacle connected either with the Last Supper or Christ’s crucifixion – but never found it. Two big quests that engage modern science centre on how the chemical building blocks of the earliest cells arose and the last universal common ancestor (LUCA) of all living things. Like the Grail’s location, neither is likely to be fully resolved because they can only be sought in a very roundabout way: both verge on the imaginary. The fossil record is limited to organisms that left skeletal remains, traces of their former presence, and a few degraded organic molecules. The further back in geological time the more sedimentary rock has either been removed by erosion or fundamentally changed at high temperatures and pressures. Both great conundrums can only be addressed by trying to reconstruct processes and organisms that occurred or existed more than 4 billion years ago.

Artistic impression of the early Earth dominated by oceans (Credit: Sci-news.com)

In the 1950s Harold Urey of the University of Chicago and his student Stanley Miller mixed water, methane, ammonia and hydrogen sulfide in lab glassware, heated it up and passed electrical discharges through it. They believed the simple set-up crudely mimicked Hadean conditions at the Earth surface. They were successful in generating more complex organic chemicals than their starting materials, though the early atmosphere and oceans are now considered to have been chemically quite different. Such a ‘Frankenstein’ approach has been repeated since with more success (see Earth-logs April 2024), creating 10 of the 20 amino acids plus the peptide bonds that link them up to make all known proteins, and even amphiphiles, the likely founders of cell walls. The latest attempt has been made by Spanish scientists at the Andalusian Earth Sciences Institute, the Universities of Valladolid and Cadiz, and the International Physics Centre in San Sebastian (Jenewein, C. et al 2024. Concomitant formation of protocells and prebiotic compounds under a plausible early Earth atmosphere. Proceedings of the National Academy of Sciences, v. 122, article 413816122; DOI: 10.1073/pnas.241381612).

Biomorphs formed by polymerisation of HCN (Credit: Jenewein, C. et al 2024, Figure 2)

Jenewein and colleagues claim to have created cell-like structures, or ‘biomorphs’ at nanometre- and micrometre scale – spheres and polyp-like bodies – from a more plausible atmosphere of CO2 , H2O, and N2. These ‘protocells’ seem to have formed from minutely thin (150 to 3000 nanometres) polymer films built from hydrogen cyanide that grew  on the surface of the reaction chamber as electric discharges and UV light generated HCN and more complex ‘prebiotic’ chemicals. Apparently, these films were catalysed by SiO2 (silica) molecules from the glass reactor. Note:  In the Hadean breakdown of olivine to serpentinite as sea water reacted with ultramafic lavas would have released abundant silica. Serpentinisation also generates hydrogen. Intimate release of gas formed bubbles to create the spherical and polyp-like ‘protocells’. The authors imagine the Hadean global ocean permanently teeming with such microscopic receptacles. Such a veritable ‘primordial soup’ would be able to isolate other small molecules, such as amino acids, oligopeptides, nucleobases, and fatty acids, to generate more complex organic molecules in micro-reactors en route  to the kind of complex, self-sustaining systems we know as life.

So, is it possible to make a reasonable stab at what that first kind of life may have been? It was without doubt single celled. To reproduce it must have carried a genetic code enshrined in DNA, which is unique not only to all species, but to individuals. The key to tracking down LUCA is that it represents the point at which the evolutionary trees of the fundamental domains of modern life life – eukarya (including animals, plants and fungi), bacteria, and archaea – converge to a single evolutionary stem. There is little point in using fossils to resolve this issue because only multicelled life leaves tangible traces, and the first of those was found in 2,100 Ma old sediments in Gabon (see: The earliest multicelled life; July 2010). The key is using AI to compare the genetic sequences of the hugely diverse modern biosphere. Modern molecular phylogenetics and computing power can discern from their similarities and differences the relative order in which various species and broader groups split from others. It can also trace the origins of specific genes that provides clues about earlier genetic associations. Given a rate of mutation the modern differences provide estimates of when each branching occurred. The most recent genetic delving has been achieved by a consortium based at various institutions in Britain, the Netherlands, Hungary and Japan  (Moody, E.R.R. and 18 others 2024. The nature of the last universal common ancestor and its impact on the early Earth system. Nature Ecology & Evolution, v.8, pages 1654–1666; DOI: 10.1038/s41559-024-02461-1).

Moody et al have pushed back the estimated age of LUCA to halfway through the Hadean, between 4.09 to 4.33 billion years (Ga), well beyond the geologically known age of the earliest traces of life (3.5 Ga). That age for LUCA in itself is quite astonishing: it could have been only a couple of hundred million years after the Moon-forming interplanetary collision. Moreover, they have estimated that Darwin’s Ur-organism had a genome of around 2 million base pairs that encoded about 2600 proteins: roughly comparable to living species of bacteria and archaea, and thus probably quite advanced in evolutionary terms. The gene types probably carried by LUCA suggest that it may have been an anaerobic acetogen; i.e. an organism whose metabolism generated acetate (CH3COO−) ions. Acetogens may produce their own food as autotrophs, or metabolise other organisms (heterotrophs). If LUCA was a heterotroph, then it must have subsisted in an ecosystem together with autotrophs which it consumed, possibly by fermentation. To function it also required hydrogen that can be supplied by the breakdown of ultramafic rocks to serpentinites, which tallies with the likely ocean-world with ultramafic igneous crust of the Hadean (see the earlier paragraphs about protocells). If an autotroph, LUCA would have had an abundance of CO2 and H2 to sustain it, and may have provided food for heterotrophs in the early ecosystem. The most remarkable possibility discerned by Moody et al is that LUCA may have had a kind of immune system to stave off viral infection.

The carbon cycle on the Hadean Earth (Credit: Moody et al. 2024; Figure 3e)

The Hadean environment was vastly different to that of modern times: a waterworld seething with volcanism; no continents; a target for errant asteroids and comets; more rapidly spinning with a 12 hour day; a much closer Moon and thus far bigger tides. The genetic template for the biosphere of the following four billion years was laid down then. LUCA and its companions may well have been unique to the Earth, as are their descendants. It is hard to believe that other worlds with the potential for life, even those in the solar system, could have followed a similar biogeochemical course. They may have life, but probably not as we know it  . . .

See also: Ball, P. 2025. Luca is the progenitor of all life on Earth. But its genesis has implications far beyond our planet. The Observer, 19 January 2025.

A fully revised edition of Steve Drury’s book Stepping Stones: The Making of Our Home World can now be downloaded as a free eBook

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.

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

Evidence for oldest microbes from Arctic Canada

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Among the oldest known rocks are metamorphosed pillow basalts on Nuvvuagittuk Island in Quebec on the east side of Hudson Bay, Canada. They contain red and orange, iron-rich sediments probably formed by hydrothermal activity associated with sea water passing through hot basalts. The ironstones are made of silica in the form of jasper (SiO2) and carbonates that are coloured by hematite (Fe2O3). This rock sequence is cut by silica-rich intrusive igneous rocks dated between 3750 and 3775 Ma: a minimum, Eoarchaean age for the sequence. This is roughly the same as the age of the famous Isua supracrustal rocks of West Greenland, but dating of the basalts using the samarium–neodymium method suggested that they formed in the Hadean about 4300 Ma ago, which would make them by far the oldest known rocks. However, that date clashes with a zircon U-Pb age of 3780 Ma for associated metasedimentary mica schists: a still ‘live’ controversy. The ironstones have been suggested to contain signs of life, in the form of minute tubes and filaments similar to those formed in modern hydrothermal vents by iron-oxidising bacteria (see: Earliest hydrothermal vent and evidence for life, March 2017). If that can be proven this would push back the age of the earliest known life by at least 300 Ma and maybe far more if the Hadean Sm-Nd age is confirmed

The Nuvvuagittuk material has recently been re-examined by its original discoverers using a variety of advanced microscope techniques (Papineau, D. et al 2022. Metabolically diverse primordial microbial communities in Earth’s oldest seafloor-hydrothermal jasper. Science Advances, v. 8, article 2296; DOI: 10.1126/sciadv.abm2296.). The most revealing of these involve two very-high resolution imaging systems: X-ray micro-tomography and electron microscopy armed with a focused ion beam that repeatedly shaves away 200 nm of rock from a sample. Both build up highly detailed 3-D images of any minute structures within a sample. The techniques revealed details of twisted filaments, tubes, knob-like and branching structures up to a centimetre long. While the first three could possibly have some inorganic origin, a ‘comb-like’ branch, likened to a moth’s antenna, has never been known to have formed by chemical reactions alone.

An image of hematite tubes from microfossils discovered in hydrothermal vent precipitates in the Nuvvuagittuk ironstones, reconstructed from X-ray and ion-beam micro-tomography (credit: Matthew Dodd, UCL)

All the structures are formed from hematite within a silica or carbonate (mainly calcite CaCO3 and ankerite Ca(Fe,Mg,Mn)(CO3)2) matrix. Some of the hematite (dominated by Fe3+) contains significant amounts of reduced Fe2+. The structures also contain tiny grains of graphite (C), phosphate (apatite Ca5(PO4)3(F,Cl,OH)) and various metal (Mn, Co, Cu, Zn, Ni, Cd) sulfides. The presence of graphite obviously suggests – but does not prove – a biological origin. However, all Phanerozoic jaspers formed from hydrothermal fluids contain undisputed organic material and appear little different from these ancient examples. Filaments, tubes and comb-like structures are displayed by various iron-oxidising bacteria found living in modern sea-floor hydrothermal vent systems. The sulfur isotopes in metal sulfides suggest their formation in an environment with vanishingly low oxygen content. Carbon isotopes in graphite are more enriched in light 12C relative to 13C than those in associated carbonates, a feature produced by living organic processes today. Patterns in plots of rare-earth elements (REE) from the Nuvvuagittuk jaspers are similar to those from modern examples and suggest high-temperature interactions between sea water and basaltic igneous rocks.

It is clear from the paper just how comprehensively the team of authors have considered and tested various biotic and abiotic options for the origin of the features found in the Nuvvuagittuk jasper samples. They conclude that they probably do represent an ancient microbial ecosystem associated with sea-floor hydrothermal vents; a now widely supported scenario for the origin of life on Earth. But what metabolic processes did the Nuvvuagittuk microbes use? Their intimate association with Fe3+ oxides that contain some reduced Fe2+ suggests that they exploited chemical ‘energy’ from oxidation reactions that acted on Fe2+ dissolved in hydrothermal fluids. This would have been impossible by inorganic means because of the very low oxygen content of seawater shown by the sulfur isotopes in associated sulfide minerals. Iron oxidation and precipitation of iron oxide by organic processes must have involved dissociation of water to yield the necessary oxygen and loss of electrons from available Fe2+, a process used by modern deep-water bacteria that depends on the presence of nitrates. That can power the metabolism of inorganic carbon dissolved in water as, for instance, bicarbonate ions and water to yield cell-building carbohydrates: a form of autotrophy. There may have been other metabolic routes, such as reducing dissolved sulfate ions to sulfur, as suggested by the association of metal sulfides. If the sea floor was shallow enough to be lit CO2 and water may have been converted to carbohydrates by a form of photosynthesis that does not release oxygen, analogous to modern purple bacteria.

There may have been considerable biodiversity in the Nuvvuagittuk ecosystem. So despite its vast age – it may have been active only 300 Ma after the Earth formed, if the oldest date is verified – it has to be remembered that a great many earlier evolutionary steps, both inorganic and organic, must have been accomplished to have allowed these organisms to exist. The materials do not signify the origin of life, but life that was chemically extremely sophisticated: far more so than anything attempted so far in laboratories to figure out the tricks performed by natural inorganic systems. DNA and RNA alone are quite a challenge!

See also: Video by authors of the paper (YouTube) Diverse life forms may have evolved earlier than previously thought. ScienceDaily, 13

Earliest plate tectonics tied down?

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Papers that ponder the question of when plate tectonics first powered the engine of internal geological processes are sure to get read: tectonics lies at the heart of Earth science. Opinion has swung back and forth from ‘sometime in the Proterozoic’ to ‘since the very birth of the Earth’, which is no surprise. There are simply no rocks that formed during the Hadean Eon of any greater extent than 20 km2. Those occur in the 4.2 billion year (Ga) old Nuvvuagittuq greenstone belt on Hudson Bay, which have been grossly mangled by later events. But there are grains of the sturdy mineral zircon ZrSiO4)  that occur in much younger sedimentary rocks, famously from the Jack Hills of Western Australia, whose ages range back to 4.4 Ga, based on uranium-lead radiometric dating. You can buy zircons from Jack Hills on eBay as a result of a cottage industry that sprang up following news of their great antiquity: that is, if you do a lot of mineral separation from the dust and rock chips that are on offer, and they are very small. Given a laser-fuelled SHRIMP mass spectrometer and a lot of other preparation kit, you could date them. Having gone to that expense, you might as well analyse them chemically using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to check out their trace-element contents. Geochemist Simon Turner of Macquarie University in Sydney, Australia, and colleagues from Curtin University in Western Australia and Geowissenschaftliches Zentrum Göttingen in Germany, have done all this for 32 newly extracted Jack Hills zircons, whose ages range from 4.3 to 3.3 Ga (Turner, S. et al. 2020. An andesitic source for Jack Hills zircon supports onset of plate tectonics in the Hadean. Nature Communications, v. 11, article 1241; DOI: 10.1038/s41467-020-14857-1). Then they applied sophisticated geochemical modelling to tease out what kinds of Hadean rock once hosted these grains that were eventually eroded out and transported to come to rest in a much younger sedimentary rock.

Artist’s impression of the old-style hellish Hadean (Credit : Dan Durday, Southwest Research Institute)

Zircons only form duuring the crystallisation of igneous magmas, at around 700°C, the original magma having formed under somewhat hotter conditions – up to 1200°C for mafic compositions. In the course of their crystallising, minerals take in not only the elements of which they are mainly composed, zirconium, silicon and oxygen in the case of zircon , but many other elements that the magma contains in low concentrations. The relative proportions of these trace elements that are partitioned from the magma into the growing mineral grains are more or less constant and unique to that mineral, depending on the particular composition of the magma itself. Using the proportions of these trace elements in the mineral gives a clue to the original bulk composition of the parent magma. The Jack Hills zircons  mainly  reflect an origin in magmas of andesitic composition, intermediate in composition between high-silica granites and basalts that have lower silica contents. Andesitic magmas only form today by partial melting of more mafic rocks under the influence of water-rich fluid driven upwards from subducting oceanic lithosphere. The proportions of trace elements in the zircons could only have formed in this way, according to the authors.

Interestingly, the 4.2 Ga Nuvvuagittuq greenstone belt contains metamorphosed mafic andesites, though any zircons in them have yet to be analysed in the manner used by Turner et al., although they were used to date those late-Hadean rocks. The deep post-Archaean continental crust, broadly speaking, has an andesitic composition, strongly suggesting its generation above subduction zones. Yet that portion of Archaean age is not andesitic on average, but a mixture of three geochemically different rocks. It is referred to as TTG crust from those three rock types (trondhjemite, tonalite and granodiorite). That TTG nature of the most ancient continental crust has encouraged most geochemists to reject the idea of magmatic activity controlled by plate tectonics during the Archaean and, by extension, during the preceding Hadean. What is truly remarkable is that if mafic andesites – such as those implied by the Jack Hills zircons and found in the Nuvvuagittuq greenstone belt – partially melted under high pressures that formed garnet in them, they would have yielded magmas of TTG composition. This, it seems, puts plate tectonics in the frame for the whole of Earth’s evolution since it stabilised several million years after the catastrophic collision that flung off the Moon and completely melted the outer layers of our planet. Up to now, controversy about what kind of planet-wide processes operated then have swung this way and that, often into quite strange scenarios. Turner and colleagues may have opened a new, hopefully more unified, episode of geochemical studies that revisit the early Earth . It could complement the work described in An Early Archaean Waterworld published on Earth-logs earlier in March 2020.

A unifying idea for the origin of life

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The nickel in stainless steel, the platinum in catalytic converters and the gold in jewellery, electronic circuits and Fort Knox should all be much harder to find in the Earth’s crust. Had the early Earth formed only by accretion and then the massive chemical resetting mechanism of the collision that produced the Moon all three would lie far beyond reach. Both formation events would have led to an extremely hot young Earth; indeed the second is believed to have left the outer Earth and Moon completely molten. All three are siderophile metals and have such a strong affinity for metallic iron that they would mostly have been dragged down to each body’s core as it formed in the early few hundred million years of the Earth-Moon system, leaving very much less in the mantle than rock analyses show. This emerged as a central theme at the Origin of Life Conference held in Atlanta GA, USA in October 2018. The idea stemmed from two papers published in 2015 that reported excessive amounts in basaltic material from both Earth and Moon of a tungsten isotope (182W) that forms when a radioactive isotope of hafnium (182Hf), another strongly siderophile metal, decays. Hafnium too must have been strongly depleted in the outer parts of both bodies when their cores formed. The excesses are explained by substantial accretion of material rich in metallic iron to their outer layers shortly after Moon-formation, some being in large metallic asteroids able to penetrate to hundreds of kilometres. Hot iron is capable of removing oxygen from water vapour and other gases containing oxygen, thereby being oxidised. The counterpart would have been the release of massive amounts of hydrogen, carbon and other elements that form gases when combined with oxygen. The Earth’s atmosphere would have become highly reducing.

Had the atmosphere started out as an oxidising environment, as thought for many decades, it would have posed considerable difficulties for the generation at the surface of hydrocarbon compounds that are the sine qua non for the origin of life. That is why theories about abiogenesis (life formed from inorganic matter) hitherto have focussed on highly reducing environments such as deep-sea hydrothermal vents where hydrogen is produced by alteration of mantle minerals. The new idea revitalises Darwin’s original idea of life having originated in ‘a warm little pond’. How it has changed the game as regards the first step in life, the so-called ‘RNA World’ can be found in a detailed summary of the seemingly almost frenzied Origin of Life Conference (Service, R.F. 2019. Seeing the dawn. Science, v. 363, p. 116-119; DOI: 10.1126/science.363.6423.116).

Isotope geochemistry has also entered the mix in other regards, particularly that gleaned from tiny grains of the mineral zircon that survived intact from as little as 70 Ma after the Moon-forming and late-accretion events to end up (3 billion years ago) in the now famous Mount Narryer Quartzite of Western Australia. The oldest of these zircons (4.4 Ga) suggest that granitic rocks had formed the earliest vestiges of continental crust far back in the Hadean Eon: Only silica-rich magmas contain enough zirconium for zircon (ZrSiO4) to crystallise. Oxygen isotope studies of them suggest that at that very early date they had come into contact with liquid water, presumably at the Earth’s surface. That suggests that perhaps there were isolated islands of early continental materials; now vanished from the geological record. A 4.1 Ga zircon population revealed something more surprising: graphite flakes with carbon isotopes enriched in 12C that suggests the zircons may have incorporated carbon from living organisms.

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A possible timeline for the origin of life during the Hadean Eon (Credit: Service, R.F. 2019, Science)

Such a suite of evidence has given organic chemists more environmental leeway to suggest a wealth of complex reactions at the Hadean surface that may have generated the early organic compounds needed as building blocks for RNA, such as aldehydes and sugars (specifically ribose that is part of both RNA and DNA), and the amino acids forming the A-C-G-U ‘letters’ of RNA, some catalysed by the now abundant siderophile metal nickel. One author seems gleefully to have resurrected Darwin’s ‘warm little pond’ by suggesting periodic exposure above sea level of abiogenic precursors to volcanic sulfur dioxide that could hasten some key reactions and create large masses of such precursors which rain would have channelled into ‘puddles and lakes’. The upshot is that the RNA World precursor to the self-replication conferred on subsequent life by DNA is speculated to have been around 4.35 Ga, 50 Ma after the Earth had cooled sufficiently to have surface water dotted with specks of continental material.

There are caveats in Robert Services summary, but the Atlanta conferences seems set to form a turning point in experimental palaeobiology studies.

Read more on Palaeobiology and Planetary science

Hadean potentially fertile for life

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The earliest incontrovertible signs of life on Earth are in the 3.48 billion-year-old Dresser Formation in the Pilbara craton of Western Australia, which take the form of carbon-coated, bubble-like structures in fine-grained silica sediments ascribed to a terrestrial hot-spring environment. In the same Formation are stromatolites that are knobbly, finely banded structures made of carbonates. By analogy with similar structures being produced today by bacterial mats in a variety of chemically stressed environments that are inhospitable for multicelled organisms that might know them away, stromatolites are taken to signify thriving, carbonate secreting bacteria. There are also streaks of carbon associated with wave ripples that may have been other types of biofilm. A less certain record of the presence of life are stromatolite-like features in metasediments from the Isua supracrustal belt of West Greenland, dated at around 3.8 Ga, which also contain graphite with carbon-isotopic signs that it formed from biogenic carbon. Purely geochemical evidence that carbonaceous compounds may have formed in living systems are ambiguous since quite complex hydrocarbons can be synthesised abiogenically by Fischer-Tropsch reactions between carbon monoxide and hydrogen.

At present there is little chance of extending life’s record further back in time than four billion years because the Hadean is mainly represented by pre 4 Ga ages of zircon grains found in much younger sedimentary rocks – resistant relics of Hadean crustal erosion. The eastern shore of Hudson Bay does preserve a tiny (20 km2) patch of metamorphosed basaltic igneous rocks, known as the Nuvvuagittuq Greenstone Belt. Dated at 3.77 Ga by one method but 4.28 Ga by another, this could be Hadean. Like the Isua sequence that in Quebec also contains metasediments, including banded ironstones with associated iron-rich hydrothermal deposits. Silica from the vent system shows dramatically lifelike tubules. Yet the ambiguity in dating upsets any claims to genuine Hadean life. There has also been a physical stumbling block to the notion that life may have originated and thrived during the Hadean: the bombardment record.

English: An outcrop of metamorphosed volcanose...
Metamorphosed volcanosedimentary rocks from the Nuvvuagittuq supracrustal belt, Canada. Some of these rocks contain quite convincing examples of fossil cells. (credit: Wikipedia)

While oxygen-isotope data from 4.4 Ga zircons hints strongly at subsurface and perhaps surface water on Earth at that time, continued accretion of large planetesimals would have created the hellish conditions associated with the name of the first Eon in Earth’s history. Liquid water is essential for life to have formed, on top of a supply of the essential biological elements C, H, O, N, P and S. The sheer amount of interstellar dust that accompanied the Hadean impact record would have ensured fertile chemical conditions, but would the surface and near-surface of the early Earth have remained continually wet? Judging by the lunar surface and that of other bodies in the solar system, after the cataclysmic events that formed the Moon, many Hadean impacts on Earth were in the range of 100 to 1000 km across, with a Late Heavy Bombardment (LHB)that not only increased the intensity of projectile delivery but witnessed the most energetic single events such as those that created the lunar maria and probably far larger structures on Earth. The thermal energy, accompanied, by incandescent silicate vapour ejected from craters, may have evaporated oceans and even subsurface water with calamitous consequences for early life or prebiotic chemistry. Until 2017 no researchers had been able to model the energetic of the Hadean convincingly.

After assessing the projectile flux up to and through the LHB, and the consequent impact heating Bob Grimm and Simone Marchi of the Southwest Research Institute in Boulder, Colorado modelled the likely thermal evolution of the outer Earth through the Hadean. This allowed them to calculate the likely thermal gradients in the near-surface, the volumes of rock each event would have affected and the times taken for cooling after impacts (Grimm, R.E. & Marchi, S. 2018. Direct thermal effects of the Hadean bombardment did not limit early subsurface habitability. Earth and Planetary Science Letters, v. 485, p. 1-9; doi:10.1016/j.epsl.2017.12.043). They found that subsurface ‘habitability’ would have grown continuously throughout the Hadean, even during the worst events of the LHB. Sterilizing Earth and thus destroying and interrupting any life processes could only have been achieved by ten times more projectiles arriving ten times more frequently over the 600 Ma history of the Hadean and LHB. Although surface water may have been evaporated by impact-flash heating and vaporized silicate ejecta, the subsurface would have been wet at least somewhere on the early Earth. Provided it either originated in or colonised surface sedimentary cover it would have been feasible for life to have survived the Hadean. However, nobody knows how long it would have taken for the necessary accumulation of prebiotic chemicals and to achieve the complex sequence of processes that lead to nucleic acids encapsulated in cells and thus self-replication and life itself.

Steam-bath Earth

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

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

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

Archaean continents derived from Hadean oceanic crust

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

English: An outcrop of metamorphosed volcanose...
Metamorphosed volcanosedimentary rocks from the Porpoise Cove locality, Nuvvuagittuq supracrustal belt, Canada. Possibly the oldest rocks on Earth. (credit: Wikipedia)

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

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

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Tectonics of the early Earth

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Tectonics on any rocky planet is an expression of the way heat is transferred from its deep interior to the surface to be lost by radiation to outer space. Radiative heat loss is vastly more efficient than either conduction or convection since the power emitted by a body is proportion to the fourth power of its absolute temperature. Unless it is superheated from outside by its star, a planet cannot stay molten at its surface for long because cooling by radiation releases all of the heat that makes its way to the surface.  Any football supporter who has rushed to get a microwaved pie at half time will have learned this quickly: a cool crust can hide a damagingly hot centre.

Thermal power is delivered to a planet’s surface by convection deep down and conduction nearer the surface because rocks, both solid and molten, are almost opaque to radiation. The vigour of the outward flow of heat might seem to be related mainly to the amount of internal heat but it is also governed by limits imposed by temperature on the form of convection. Of the Inner Planets only Earth shows surface signs of deep convection in the form of plate tectonics driven mainly by the pull exerted by steep subduction of cool, dense slabs of old oceanic lithosphere. Only Jupiter’s moon Io shows comparable surface signs of inner dynamics, but in the form of immense volcanoes rather than lateral movements of slabs. Io has about 40 times the surface heat flow of Earth, thanks largely to huge tidal forces imposed by Jupiter. So it seems that a different mode of convection is needed to shift the tidal heat production; similar in many ways to Earth’s relatively puny and isolated hot spots and mantle plumes.

Most of the yellow and orange hues of Io are d...
An analogy for the early Earth, Jupiter’s moon Io is speckled with large active volcanoes; signs of vigorous internal heat transport but not of plate tectonics. Its colour is dominated by various forms of sulfur rather than mafic igneous rocks. (credit: Wikipedia)

Shortly after Earth’s accretion it would have contained far more heat than now: gravitational energy of accretion itself; greater tidal heating from a close Moon and up to five times more from internal radioactive decay. The time at which plate tectonics can be deduced from evidence in ancient rocks has been disputed since the 1970s, but now an approach inspired by Io’s behaviour approaches the issue from the opposite direction: what might have been the mode of Earth’s heat transport shortly after accretion (Moore, W.B. & Webb, A.A.G. 2013. Heat-pipe Earth. Nature, v.  501, p. 501-505). The two American geophysicists modelled Rayleigh-Bénard convection – multicelled convection akin to that of the ‘heat pipes’ inside Io – for a range of possible thermal conditions in the Hadean. The modelled planet, dominated by volcanic centres turned out to have some surprising properties.

The sheer efficiency of heat-pipe dominated heat transfer and radiative heat lost results in development of a thick cold lithosphere between the pipes, that advects surface material downwards. Decreasing the heat sources results in a ‘flip’ to convection very like plate tectonics. In itself, this notion of sudden shift from Rayleigh-Bénard convection to plate tectonics is not new – several Archaean specialists, including me, debated this in the late 1970s – but the convincing modelling is. The authors also assemble a plausible list of evidence for it from the Archaean geological record: the presence in pre- 3.2 Ga greenstone belts of abundant ultramafic lavas marking high fractions of mantle melting; the dome-trough structure of granite-greenstone terrains; granitic magmas formed by melting of wet mafic rocks at around 45 km depth, extending back to second-hand evidence from Hadean zircons preserved in much younger rocks. They dwell on the oldest sizeable terranes in West Greenland (the Itsaq gneiss complex), South Africa and Western Australia (Barberton and the Pilbara) as a plausible and tangible products of ‘heat-pipe’ tectonics. They suggest that the transition to plate-tectonic dominance was around 3.2 Ga, yet ‘heat pipes’ remain to the present in the form of plumes so nicely defined in the preceding item Mantle structures beneath the central Pacific.

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A glimpse of the Hadean

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There is something deeply unsatisfying, even untidy, about a geoscientific history from which the first half billion years is more or less a blank. Every likely stone has been turned and every isotope hurled as a curve-ball through a mass spectrometer in the quest for either direct evidence of Hadean events or an acrid whiff that lingers in later matter. All, that is, except for one…

Formed in a proposed supernova that likely helped trigger formation of the Sun and Solar System, 150Gd quickly decayed to produce 146Sm, which itself had a half-life of about 68 Ma. That is too short for any significant trace of that radioactive rare-earth element to remain in terrestrial rocks, but its daughter isotope 142Nd bears witness to its former existence. Checking the proportion of 142Nd against the heavier 144Nd is a means of assessing isotopic fractionation according to atomic mass between a solid source of a magma, and between residual magma and solids that crystallised from it.

A popular and well-supported view of the Hadean is that shortly after accretion of the Earth a stupendous impact left a deep ‘ocean’ of magma and flung off mass that produced the Moon. Solidification of that ocean, which would have involved denser minerals sinking and lighter ones rising to higher levels, has been suggested to have resulted in differentiation of the mantle into two portions, one enriched, the other depleted; an event on which the entire later geochemical history of our planet has depended. Should either part of the mantle melt again, the igneous rocks that would result should carry a neodymium isotope signature of one or the other. Little sign of either emerges from studies of igneous rocks younger than 2.5 Ga, but older rocks from Greenland that go back to 3.8 Ga demonstrate that almost all of them melted from the Hadean depleted mantle. Without rocks carrying 142Nd/144Nd ratios signifying the other side of the more ancient mantle division, an enriched source, the grand idea was flawed. But this one-sidedness appears now to have been balanced by other Archaean igneous rocks (Rizo, H. et al. 2012. The elusive Hadean enriched reservoir revealed by 142Nd deficits in Isua Archaean rocks. Nature, v. 491, p. 96-100).

3.8 billion year-old Amitsoq gneisses, West Greenland (Image credit: Stephen Moorbath, via Royal Society)

The analysed rocks are interesting for another reason, for they are 3.4 Ga old vertical sheets of basalt or dykes that cut through the more ancient west Greenland crust. They are the first evidence of a brittle crust that cracked under tension to be followed by mantle-derived magma. Some members of the Ameralik dyke swarm show just the isotopic signature predicted for the enriched member of the postulated fundamental mantle division. However, for some yet to be recognised reason, few post-Archaean rocks show any sign of widespread mantle heterogeneity. Such matters could be addressed with any confidence only after mass spectrometry allowed precise discrimination between isotopes of a whole variety of both common and rare elements. That was not so long ago, so a rich trove of future revelations can be anticipated.

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Charting the growth of continental crust

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Česky: Budynáž nedaleko obce Kangerlussuaq, zá...
Archaean gneisses from West Greenland (Photo credit: Wikipedia)

When continents first appeared; the pace at which they grew; the tectonic and magmatic processes responsible for continental crust, and whether or not crustal material is consumed by the mantle to any great extent have been tough issues for geologists and geochemists to ponder on for the last four decades. Clearly, continental material was rare if not absent in the earliest days of the solid Earth, otherwise Hadean crust should have been found by now. Despite the hints at some differentiated, high silica rocks that may have hosted >4 billion-year old zircon crystals from much younger sediments, the oldest tangible crust – the Acasta Gneiss of northern Canada – just breaks the 4 Ga barrier: half a billion years short of the known age of the Earth (http://earth-pages.co.uk/2008/11/01/at-last-4-0-ga-barrier-broken/). Radiometric ages for crustal rocks steadily accumulated following what was in the early 1970s the astonishing discovery by Stephen Moorbath and colleagues at Oxford University and the Geological Survey of Greenland of a 3.8 billion year age for gneisses from West Greenland.  For a while it seemed as if there had been great pulses that formed new crust, such as one between 2.8 and 2.5 Ga (the Neoarchaean) separated by quieter episodes. Yet dividing genuinely new material coming from the mantle from older crust that later thermal and tectonic events had reworked and remelted required – and still does – lengthy and expensive radiometric analysis of rock samples with different original complements of radioactive isotopes.

One approach to dating has been to separate tiny grains of zircon from igneous and metamorphic rocks and date them using the U-Pb method as a route to the age at which the rock formed, but that too was slow and costly. Yet zircons, being among the most intransigent of Earth materials, end up in younger sedimentary rocks after their parents have been weathered and eroded. It was an investigation of what earlier history a sediment’s zircons might yield that lead to the discovery of grains almost as old as the Earth itself (http://earth-pages.co.uk/2011/12/21/mistaken-conclusions-from-earths-oldest-materials/ http://earth-pages.co.uk/2005/05/01/zircon-and-the-quest-for-life%E2%80%99s-origin/). That approach is beginning to pay dividends as regards resolving crustal history as a whole. Almost 7000 detrital zircon grains separated from sediments have been precisely dated using lead and hafnium isotopes. Using the age distribution alone suggests that the bulk of continental crust formed in the Precambrian, between 3 and 1 Ga ago, at a faster rate than it formed during the Phanerozoic. However, that assumes that a zircon’s radiometric age signifies the time of separation from the mantle of the magmas from which the grain crystallised. Yet other dating methods have shown that zircon-bearing magmas also form when old crust is remelted, and so it is important to find a means of distinguishing zircons from entirely new blocks of crust and those which result from crustal reworking. It turns out that zircons from mantle-derived crust have different oxygen isotope compositions from those which crystallised from remelted crust.

U-Pb ages of detrital zircons from sediments o...
An example of ages of detrital zircons from sediments, in this case from five Russian rivers (credit: Wikipedia)

Bruno Dhuime and colleagues from St.Andrew’s and Bristol universities in the UK measures hafnium model ages and δ18O  values in a sample of almost 1400 detrital zircons collected across the world from sediments of different ages (Dhuime, B. et al. 2012. A change in the geodynamics of continental growth 3 billion years ago. Science, v. 335, p. 1334-1336). Plotting δ18O  against Hf model age reveals two things: there are more zircons from reworked crust than from mantle-derived materials; plotting the proportion of new crust ages to those of reworked crust form 100 Ma intervals through geological time reveals dramatic changes in the relative amounts of ‘mantle-new’ crust being produced. Before 3 Ga about three quarters of all continental crust emerged directly from the mantle. Instead of the period from 3 to 1 Ga being one of massive growth in the volume of the crust, apparently the production rate of new crust fell to about a fifth of all crust in each 100 Ma time span by around 2 Ga and then rose to reach almost 100% in the Mesozoic and Cenozoic. This suggests that the late Archaean and most of the Proterozoic were characterised by repeated reworking of earlier crust, perhaps associated with the repeated formation and break-up of supercontinents by collision orogeny and then tectonic break up and continental drift.

Dhuine and colleagues then use the record of varying new crust proportions to ‘correct’ the much larger database of detrital zircon ages. What emerges is a well-defined pattern in the rate of crustal growth through time. In the Hadean and early Archaean the net growth of the continents was 3.0 km3 yr-1, whereas throughout later time this suddenly fell to and remained at 0.8 km3 yr-1. Their explanation is that the Earth only came to be dominated by plate tectonic processes mainly driven by slab-pull at subduction zones after 3 Ga. Subduction not only produces mantle-derived magmas but inevitably allows continents to drift and collide, thereby leading to massive deformation and thermal reworking of older crust in orogenic belts and an apparent peak in zircon ages. The greater rate of new crust generation before 3 Ga may therefore have been due to other tectonic processes than the familiar dominance of subduction. Yet, since there is convincing evidence for subduction in a few ancient crustal blocks, such as west Greenland and around Hudson’s Bay in NE Canada, plate tectonics must have existed but was overwhelmed perhaps by processes more directly linked to mantle plumes.

More on continental growth can be found here

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Mistaken conclusions from Earth’s oldest materials

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Microscope projection close-upThe oldest materials on the planet are tiny zircon grains that were washed into conglomerate in Western  Australia about 2650 to 3050 Ma ago. It wasn’t the fact that the grains are zircons, which are among the most durable materials around, but the range of ages that they revealed when routinely analysed. U-Pb dating of detrital zircons is a well tested means of finding the provenance of sedimentary materials as an indicator of orogenic and igneous events that formed the crust from which they were eroded. In the original study of the Jack Hills zircons some showed ages that might reasonably have been expected from late sediments in an Archaean craton: around 3.5 billion years is about the maximum age for orogenic events there. What astonished all geoscientists was that a proportion of the grains gave ages of more than 4 billion years, some as old as 4.4 Ga: here was a window on the missing first half billion years of Earth history, the Hadean.

Subsequent work on yet more zircons confirmed the original age span but other kinds of analysis led to a variety of claims: that continental crust was around in abundance within 100 Ma of Earth having formed; geothermal heat =flow was not especially high;  liquid water was available for geological processes, including the origin of life; plate tectonics may have started early…. The topic has cropped up several times in EPN since the issue of 1 January 2001. Quite a lot of the claims emerged from studies of other minerals enclosed by the ancient zircons, such as quartz and micas, and now they have been checked again by geochemists from Western Australia (Rasmussen, B. et al. 2011. Metamorphic replacement of mineral inclusions in detrital zircons from Jack Hills, Australia: Implications for the Hadean Earth. Geology, v. 39, p. 1143-1146). It turns out that the inclusions formed at temperatures well below those of magmas, between 350 to 490°C: more like those of metamorphism. Indeed, uranium-bearing rare-earth phosphate minerals, xenotime and monazite, also locked in the zircons not only turn out to be metamorphic in origin too (both are also formed magmatically) but date to between 2700 and 800 Ma.

While the  Hadean zircon dates remain robust, a closer look at their inclusions shows that they did not remain geochemically closed systems thereafter. It was on the assumption of zircons being geological ‘time capsules’ that much of the excitement rested. Even using the presence of zircons from 4.4 Ga – they are most common in granites but do occur in mafic and intermediate igneous rocks – to suggest early ‘sialic’ continental crust is suspect. Despite having some tiny bits from Earth’s early days, it seems we are none the wiser.

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