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Volume 560 Issue 7716, 2 August 2018

Deep blue

The extent to which Earth’s surface materials are recycled into the deep mantle is difficult to evaluate. In this week’s issue, Evan Smith and his colleagues shed light on the matter through an examination of blue (type IIb) diamonds. These rare gems get their colour from boron, an element predominantly found in the crust, which suggests boron somehow made its way to the high-pressure environment below the surface where diamonds form. The team’s analysis of mineral inclusions trapped inside boron-bearing diamonds reveals that they crystallized in oceanic lithosphere (oceanic tectonic plates) that had been subducted down to the lower mantle (at least 660 km below Earth’s surface, much deeper than the 200 km limit typical for most diamonds), meaning blue diamonds are among the most deeply sourced diamonds known. The researchers propose that boron was carried down in seawater-altered oceanic lithosphere, illustrating a viable pathway for the deep-mantle recycling of crustal elements.

Cover image: Jian Xin Liao/© 2018 GIA

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