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  • Title: Large gem diamonds from metallic liquid in Earth's deep mantle.
    Author: Smith EM, Shirey SB, Nestola F, Bullock ES, Wang J, Richardson SH, Wang W.
    Journal: Science; 2016 Dec 16; 354(6318):1403-1405. PubMed ID: 27980206.
    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 sometimes majoritic garnet or former calcium silicate perovskite. The metal-dominated mineral assemblages and reduced volatiles in large gem diamonds indicate formation under metal-saturated 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.
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