Heavy iron in large gem diamonds traces deep subduction of serpentinized ocean floor.

Evan M Smith,Wuyi Wang,Anat Shahar,Stephen H Richardson,Peng Ni,Steven B Shirey

doi:10.1126/sciadv.abe9773

Evan M Smith, Wuyi Wang + Show 4 more

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https://doi.org/10.1126/sciadv.abe9773

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Abstract

Subducting tectonic plates carry water and other surficial components into Earth's interior. Previous studies suggest that serpentinized peridotite is a key part of deep recycling, but this geochemical pathway has not been directly traced. Here, we report Fe-Ni-rich metallic inclusions in sublithospheric diamonds from a depth of 360 to 750 km with isotopically heavy iron (δ56Fe = 0.79 to 0.90‰) and unradiogenic osmium (187Os/188Os = 0.111). These iron values lie outside the range of known mantle compositions or expected reaction products at depth. This signature represents subducted iron from magnetite and/or Fe-Ni alloys precipitated during serpentinization of oceanic peridotite, a lithology known to carry unradiogenic osmium inherited from prior convection and melt depletion. These diamond-hosted inclusions trace serpentinite subduction into the mantle transition zone. We propose that iron-rich phases from serpentinite contribute a labile heavy iron component to the heterogeneous convecting mantle eventually sampled by oceanic basalts.

Highlights

Global subduction of oceanic lithosphere is a fundamental characteristic of terrestrial plate tectonics
We show that the iron isotopic compositions of these inclusions are exceptionally heavy ( 56Fe = 0.79 to 0.90‰, where 56Fe is the parts per mil deviation of the 56Fe/54Fe ratio relative to the IRMM-014 standard) and are best explained if the iron is derived from magnetite and/or Fe-Ni alloys precipitated during serpentinization of the peridotitic mantle portion of slabs before deep subduction
Three iron isotopic compositions were obtained from three inclusion samples, each weighing less than 4 g, by miniaturizing chemical separation and mass spectrometric procedures

Summary

Introduction

Global subduction of oceanic lithosphere is a fundamental characteristic of terrestrial plate tectonics. An integral feature of this recycling is that oceanic lithosphere interacts with seawater over millions of years before it subducts into the mantle, having profound implications for the geochemical cycle of water and other volatiles. The signature of the serpentinized peridotitic portion of these recycled slabs has only been inferred indirectly, based on the geochemistry of oceanic basalts, erupted at ocean islands and midocean ridges. Such basalts contain water and halogens [4], as well as noble gases [8], that suggest that their mantle sources are marked by extensive long-term subduction of serpentinized peridotite

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