Extreme redox variations in a superdeep diamond from a subducted slab

Fabrizio Nestola (), Margo E. Regier, Robert W. Luth, D. Graham Pearson, Thomas Stachel, Catherine McCammon, Michelle D. Wenz, Steven D. Jacobsen, Chiara Anzolini, Luca Bindi and Jeffrey W. Harris
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Fabrizio Nestola: University of Padova
Margo E. Regier: University of Alberta
Robert W. Luth: University of Alberta
D. Graham Pearson: University of Alberta
Thomas Stachel: University of Alberta
Catherine McCammon: University of Bayreuth
Michelle D. Wenz: Northwestern University
Steven D. Jacobsen: Northwestern University
Chiara Anzolini: University of Alberta
Luca Bindi: University of Firenze
Jeffrey W. Harris: University of Glasgow

Nature, 2023, vol. 613, issue 7942, 85-89

Abstract: Abstract The introduction of volatile-rich subducting slabs to the mantle may locally generate large redox gradients, affecting phase stability, element partitioning and volatile speciation1. Here we investigate the redox conditions of the deep mantle recorded in inclusions in a diamond from Kankan, Guinea. Enstatite (former bridgmanite), ferropericlase and a uniquely Mg-rich olivine (Mg# 99.9) inclusion indicate formation in highly variable redox conditions near the 660 km seismic discontinuity. We propose a model involving dehydration, rehydration and dehydration in the underside of a warming slab at the transition zone–lower mantle boundary. Fluid liberated by dehydration in a crumpled slab, driven by heating from the lower mantle, ascends into the cooler interior of the slab, where the H2O is sequestered in new hydrous minerals. Consequent fractionation of the remaining fluid produces extremely reducing conditions, forming Mg-end-member ringwoodite. This fractionating fluid also precipitates the host diamond. With continued heating, ringwoodite in the slab surrounding the diamond forms bridgmanite and ferropericlase, which is trapped as the diamond grows in hydrous fluids produced by dehydration of the warming slab.

Date: 2023
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DOI: 10.1038/s41586-022-05392-8

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