In situ Os isotopes in abyssal peridotites bridge the isotopic gap between MORBs and their source mantle

Abstract

Abyssal peridotites are assumed to represent the mantle residue of mid-ocean-ridge basalts (MORBs). However, the osmium isotopic compositions of abyssal peridotites and MORB do not appear to be in equilibrium1,2,3,4,5,6,7,8, raising questions about the cogenetic relationship between those two reservoirs. However, the cause of this isotopic mismatch is mainly due to a drastic filtering of the data based on the possibility of osmium contamination by sea water5. Here we present a detailed study of magmatic sulphides (the main carrier of osmium) in abyssal peridotites and show that the 187Os/188Os ratio of these sulphides is of primary mantle origin and can reach radiogenic values suggesting equilibrium with MORB. Thus, the effect of sea water on the osmium systematics of abyssal peridotites has been overestimated and consequently there is no true osmium isotopic gap between MORBs and abyssal peridotites.

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Figure 1: Os concentration versus isotopic composition of whole-rock AP and MORB glasses.
Figure 2: 187Os/188Os versus 187Re/188Os and (Pd/Ir)N for KN3-4 whole rock and sulphides.

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Acknowledgements

O.A. thanks the Royal Society for financial support. Discussions with M. Godard, J. L. Bodinier, M. Gounelle and L. Reisberg helped to improve the manuscript. This is an ARC National Key Centre for Geochemical Evolution and Metallogeny of Continents (GEMOC) publication.

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Correspondence to Olivier Alard.

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Supplementary Table S1

Highly siderophile element abundances and Re-Os isotopic composition of abyssal (and ophiolites) peridotites, whole rock and in situ analyses of sulphides.

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Alard, O., Luguet, A., Pearson, N. et al. In situ Os isotopes in abyssal peridotites bridge the isotopic gap between MORBs and their source mantle. Nature 436, 1005–1008 (2005). https://doi.org/10.1038/nature03902

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