Large gem diamonds from metallic liquid in Earth's deep mantle

Abstract

The redox state of Earth's convecting mantle, masked by the lithospheric plates and basaltic magmatism of plate tectonics, is a key unknown in the evolutionary history of our planet. Here we report that large, exceptional gem diamonds like the Cullinan,Constellation, and Koh-i- Noor carry direct evidence of crystallization from a redox-sensitive metallic liquid phase in the deep mantle.These sublithospheric diamonds contain inclusions of solidified iron-nickel-carbon-sulfur melt, accompanied by a thin fluid layer of methane ± hydrogen, and sometimesmajoritic garnet or former calcium silicate perovskite.Themetal-dominatedmineral assemblages and reduced volatiles in large gem diamonds indicate formation under metalsaturated conditions.We verify previous predictions that Earth has highly reducing deep mantle regions capable of precipitating a metallic iron phase that contains dissolved carbon and hydrogen.