Paper Number: 1606
Melting sediment and subducting bacteria: exploring the connection between the Great Oxidation Event and the rise of δ18O in zircon.
Spencer, C.J.1, Raub, T.D.2, Kirkland, C.L.3, Kinny, P.D.1
1 The Institute of Geoscience Research (TIGeR), Department of Applied Geology, Curtin University, Bentley, 6102, Australia, cspencer@curtin.edu.au
2Department of Earth Sciences, University of St Andrews, St Andrews, KY16 9AL, UK
3Centre for Exploration Targeting–Curtin Node, Department of Applied Geology, Curtin University, Bentley, 6102, Australia
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Zircon is a ubiquitous and robust mineral that survives multiple sedimentary cycles with little modification to its isotopic composition. This feature makes zircon the perfect archive for recording changes in crustal processes over deep time. In particular, oxygen isotopes in zircon inform on the degree to which a magmatic system has incorporated supracrustal material. That is because oxygen isotopes are highly fractionated by near surface processes, resulting in sedimentary rocks becoming strongly enriched in 18O especially during the formation of clays [1, 2]. When these sediments are assimilated into magmatic systems the δ18O composition of the magma also becomes isotopically heavy with the δ18O VSMOW of sedimentary rocks typically from 6-40‰ [3]. In contrast the δ18O signature of mantle is narrowly constrained between 4.7‰ and 6.0‰ [4]. Thus, any significant increase from the mantle value in zircons with concordant U-Pb ages has been interpreted to result from incorporation of supracrustal material [5].
Work over the past decade has shown that the maximum δ18O value of zircons has increased through geologic time, indicating progressive reworking of supracrustal material in Earth’s magmatic systems [5, 6]. During the Archean, δ18O values in zircon remain relatively subdued with maximum values ~8‰ [5, 7] then, between 2.5 and 2.1 Ga, the maximum values rise to ~14‰ and remain relatively constant to the present [6]. The rise has been interpreted as either representing a shift in sediment composition through time [5] or a shift in the magnitude of supracrustal reworking via tectonic processes [6]. Furthermore, it is conspicuous that the rise in atmospheric oxygenation, known as the Great Oxidation Event [8], appears to coincide with the rise of δ18O. We present a suite of zircon δ18O analyses from southwestern Australia that provide better constraint on the timing of δ18O rise in zircon. These data show a step change in δ18O with a ~50 Ma time lag from the disappearance of mass-independent fractionation of sulphur isotopes and enhanced deposition of marine sulphate which are generally associated with the onset of the Great Oxidation Event. We argue that the formation and subsequent subduction of marine sulphate which carries a significant enrichment of 18O is responsible for the strong shift in δ18O values in zircon. This implies a direct link between the composition of the continental crust, atmospheric chemistry, and evolution of Earth’s biosphere.
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