Credit:  Robert Simmon, NASA GSFC.

Diamonds form at high pressures deep within the Earth and hold important chemical clues to the interior composition of our planet and its dynamic mantle. A study co-led by Evan Smith of the Gemological Institute of America and Peng Ni of the Carnegie Institution for Science uses the geochemistry of diamonds to trace one enigmatic aspect of plate tectonics and subduction.

Oceanic crust and the underlying upper layer of mantle interact with seawater for millions of years prior to being subducted into the deeper mantle. During this time, seawater can penetrate these layers to form an altered version of mantle rock called serpentinite. These serpentinized layers have long been thought to carry surface materials and water into the deepest regions of Earth’s mantle during subduction. However, the geochemical fingerprint of this process has not been directly detected until now.

Smith and Ni and colleagues use iron- and nickel-rich metal inclusions in large, naturally occurring gem diamonds (similar to the well-known Cullinan diamond) to elucidate this process. These diamonds formed at 360 to 750 kilometers depth, consistent with the transition zone between Earth’s upper and lower mantle, and then erupted to the surface. The researchers analyzed the metal inclusions for their iron isotopic composition and showed that their isotopic signature could not be explained by known mantle compositions or reactions taking place at great depths. However, iron-rich oxide minerals such as magnetite, which are produced during serpentinization near the surface, are an isotopic match. The authors conclude that the isotopic composition of the diamond-hosted metal inclusions is inherited from subducted serpentinite so that these inclusions can be used to trace the process of subduction directly.

These results offer new insights into the geochemical cycling of water between the surface and mantle by serpentinite that is subducted into the deeper portions of Earth. Previously, some researchers believed that water trapped in serpentinite was eliminated during reactions at much shallower depths. The results of this work may also help to explain other long-standing puzzles in geology, such as the iron isotopic heterogeneity observed between mantle rocks and erupted oceanic island basalts. READ MORE