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

• Published in: Science (American Association for the Advancement of Science) Vol. 354; no. 6318; pp. 1403 - 1405
• Main Authors: Smith, Evan M., Shirey, Steven B., Nestola, Fabrizio, Bullock, Emma S., Wang, Jianhua, Richardson, Stephen H., Wang, Wuyi
• Format: Journal Article
• Language: English
• Published: United States American Association for the Advancement of Science 16.12.2016; The American Association for the Advancement of Science
• Subjects: Aluminum; Compressive properties; Crystallization; Diamonds; Earth mantle; Geochemistry; Mantle; Metals; Nanostructure; Platinum; Surface chemistry
• ISSN: 0036-8075; 1095-9203
• DOI: 10.1126/science.aal1303

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 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.