title: 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 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.
authors.affiliation: Gemological Institute of America, New York, NY 10036, USA. evan.smith@gia.edu.
authors.forename: Evan M
authors.initials: EM
authors.lastname: Smith
authors.affiliation: Department of Terrestrial Magnetism, Carnegie Institution for Science, Washington, DC 20015, USA.
authors.forename: Steven B
authors.initials: SB
authors.lastname: Shirey
authors.affiliation: Department of Geosciences, University of Padova, Padova 35131, Italy.
authors.forename: Fabrizio
authors.initials: F
authors.lastname: Nestola
authors.affiliation: Geophysical Laboratory, Carnegie Institution for Science, Washington, DC 20015, USA.
authors.forename: Emma S
authors.initials: ES
authors.lastname: Bullock
authors.affiliation: Department of Terrestrial Magnetism, Carnegie Institution for Science, Washington, DC 20015, USA.
authors.forename: Jianhua
authors.initials: J
authors.lastname: Wang
authors.affiliation: Department of Geological Sciences, University of Cape Town, Rondebosch 7701, South Africa.
authors.forename: Stephen H
authors.initials: SH
authors.lastname: Richardson
authors.affiliation: Gemological Institute of America, New York, NY 10036, USA.
authors.forename: Wuyi
authors.initials: W
authors.lastname: Wang