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Abstract
Plate tectonics and associated subduction are unique to the Earth. Studies of Archean rocks show significant changes in composition and structural style around 3.0 to 2.5 Ga that are related to changing tectonic regime, possibly associated with the onset of subduction. Whole rock Hf isotope systematics of black shales from the Australian Pilbara craton, selected to exclude detrital zircon components, are employed to evaluate the evolution of the Archean crust. This approach avoids limitations of Hf-in-zircon analyses, which only provide input from rocks of sufficient Zr-concentration, and therefore usually represent domains that already underwent a degree of differentiation. In this study, we demonstrate the applicability of this method through analysis of shales that range in age from 3.5 to 2.8 Ga, and serve as representatives of their crustal sources through time. Their Hf isotopic compositions show a trend from strongly positive εHfinitial values for the oldest samples, to strongly negative values for the younger samples, indicating a shift from juvenile to differentiated material. These results confirm a significant change in the character of the source region of the black shales by 3 Ga, consistent with models invoking a change in global dynamics from crustal growth towards crustal reworking around this time.
Highlights
The onset of plate tectonics, associated crustal evolution, and related changes in crustal chemistry are strongly debated e.g
Their robustness against weathering and re-melting together with single grain dating, host-melt records of O isotope signatures, and their possibility to ‘freeze’ the Hf isotopic composition of their source rock, due to the extreme low Lu and high Hf concentrations, make them ideal reservoirs to study past crustal evolution e.g
We investigated a series of Archean black shales with known ages between 3.46 and 2.74 Ga, overlapping the inferred time of changes in global geodynamics
Summary
The onset of plate tectonics, associated crustal evolution, and related changes in crustal chemistry are strongly debated e.g.1–3. Recent geochemical studies of Archean crustal domains have shown that their bulk composition records a change from a predominantly mafic tholeiitic to a more evolved calc-alkaline character between 3 Ga and 2.5 Ga e.g.7,8. This shift in crustal chemistry has been linked to changes in global geodynamics, most likely associated with the onset of subduction[8,9]. Hf isotopes from mineral archives, such as zircons, are a popular method to provide a window into Earth’s early crustal evolution Their robustness against weathering and re-melting together with single grain dating, host-melt records of O isotope signatures, and their possibility to ‘freeze’ the Hf isotopic composition of their source rock, due to the extreme low Lu and high Hf concentrations, make them ideal reservoirs to study past crustal evolution e.g.13–15
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