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

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

doi:10.1126/sciadv.abe9773

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

Sci Adv. 2021 Mar 31;7(14):eabe9773. doi: 10.1126/sciadv.abe9773. Print 2021 Mar.

Authors

Evan M Smith¹, Peng Ni², Steven B Shirey³, Stephen H Richardson⁴, Wuyi Wang⁵, Anat Shahar³

Affiliations

¹ Gemological Institute of America, New York, NY 10036, USA. evan.smith@gia.edu pni@carnegiescience.edu.
² Earth and Planets Laboratory, Carnegie Institution for Science, Washington, DC 20015, USA. evan.smith@gia.edu pni@carnegiescience.edu.
³ Earth and Planets Laboratory, Carnegie Institution for Science, Washington, DC 20015, USA.
⁴ Department of Geological Sciences, University of Cape Town, Rondebosch 7701, South Africa.
⁵ Gemological Institute of America, New York, NY 10036, USA.

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.

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