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Non-chondritic sulphur isotope composition of the terrestrial mantle

Abstract : Core-mantle differentiation is the largest event experienced by a growing planet during its early history. Terrestrial core segregation imprinted the residual mantle composition by scavenging siderophile (iron-loving) elements such as tungsten, cobalt and sulphur. Cosmochemical constraints suggest that about 97% of Earth's sulphur should at present reside in the core, which implies that the residual silicate mantle should exhibit fractionated 34S/32S ratios according to the relevant metal-silicate partition coefficients, together with fractionated siderophile element abundances. However, Earth's mantle has long been thought to be both homogeneous and chondritic for 34S/32S, similar to Canyon Diablo troilite, as it is for most siderophile elements. This belief was consistent with a mantle sulphur budget dominated by late-accreted chondritic components. Here we show that the mantle, as sampled by mid-ocean ridge basalts from the south Atlantic ridge, displays heterogeneous 34S/32S ratios, directly correlated to the strontium and neodymium isotope ratios 87Sr/86Sr and 143Nd/144Nd. These isotope trends are compatible with binary mixing between a low-34S/32S ambient mantle and a high-34S/32S recycled component that we infer to be subducted sediments. The depleted end-member is characterized by a significantly negative δ34S of -1.28 +/- 0.33‰ that cannot reach a chondritic value even when surface sulphur (from continents, altered oceanic crust, sediments and oceans) is added. Such a non-chondritic 34S/32S ratio for the silicate Earth could be accounted for by a core-mantle differentiation record in which the core has a 34S/32S ratio slightly higher than that of chondrites (δ34S = +0.07‰). Despite evidence for late-veneer addition of siderophile elements (and therefore sulphur) after core formation, our results imply that the mantle sulphur budget retains fingerprints of core-mantle differentiation.
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Submitted on : Sunday, February 20, 2022 - 3:26:41 PM
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J. Labidi, P. Cartigny, M. Moreira. Non-chondritic sulphur isotope composition of the terrestrial mantle. Nature, Nature Publishing Group, 2013, 501, pp.208-211. ⟨10.1038/nature12490⟩. ⟨insu-03581766⟩

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