Tag: trace elements

Impact melting and early Earth’s crust, a new take from detrital zircons

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In early July 2026 I commented on a quite plausible hypothesis that energy from repeated impacts played the primary role in forming the first Hadean crust.  That would also help explain why rocks from those early times are vanishingly rare: they may have been remelted and recrystallised numerous times. But there are Hadean zircon grains from the Archaean Jack Hills sandstone of Western Australia that have underpinned a cottage industry for some geochemists for three decades. Such grains must have crystallised from magma, and inherited trace elements from the igneous processes that formed them. An experiment to test the early impact-melt hypothesis appears in a new publication soon to appear in Nature (Szumila, I. and 11 others 2026. A missing continental impact melt sheet signal in the early Earth detrital zircon record. Nature Communications, In press. DOI: 10.1038/s41467-026-76529-w).  Led by Ian Szumila of the Carnegie Institute, Washington DC a group of geochemists from the US, UK and Canada compared a range of elements in zircons from the well-established 1.85 Ga impact melt sheet at Sudbury – the largest of its kind preserved on Earth – with those from more typical, internal igneous processes. They used the data from these known settings to ‘train’ a machine-learning model that can be applied to ancient, detrital zircon grains: the ‘unknowns’ with ages between 3.2 to 4.3 Ga.

The Sudbury complex is made up of a basal sheet of mafic igneous rocks (norite and quartz gabbro) and an upper granitic (granophyre) sheet. Essentially, trace-element data from Sudbury zircons result from ‘exogenous’ melting triggered by a massive impact, whereas those formed by internal magmatic processes are of ‘endogenous’ origin. Finding statistically different geochemical features is, however, a tall order as earlier investigations demonstrate. One study comparing the titanium concentration in Sudbury and Hadean zircons found that crystallisation temperatures of those in the accredited impact melt sheet differed from those of Hadean age. This suggested impact melting was not a primary source for the Hadean crystals. Another based on mineral inclusions in the two populations of zircons did support Hadean impact melting, as did a repeat of the Ti approach to crystallisation temperatures. Szumila et al.’s approach using a more sophisticated comparison of a wider range of the zircons’ trace elements seeks to resolve the issue: their U/Yb, Th/U, Yb, and Gd/Yb ratios.

Predicted identity of detrital zircons dated to between 3200 and 4300 Ma: endogenous – pink; exogenous – blue. Credit Szumila et al. Fig 4.

Of the 250 detrital zircon grains dated between 3200 to 4300 Ma-old only a single 4150 Ma-old grain was assigned to an exogenous/impact origin.  Apparently, the repeated impact-melt hypothesis for the formation of Earth’s original, Hadean crust is not supported by this experiment. Maybe it did happen, indeed the lunar cratering proxy record for Hadean bombardment of the Earth is unchallengeable. Possibly repeated mega-impacts obliterated and churned up their predecessors’ legacy thereby thermally modifying the tiny zircon grains’ geochemistry before erosion freed them to early and repeatedly reworked sediments. Such a conclusion begs the question of whether or not sustained bombardment repeatedly heated surface water or even boiled it away episodically. On that hangs the vexed question of when self-reproducing organic chemistry was able to result in the earliest life forms: I doubt we will ever know when that fait accompli happened …

See also: Early Earth’s detrital zircons lack evidence of a continental impact melt sheet. Scienmag, 11 August 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.