Earth’s oxygen-rich atmosphere creates extremely oxidizing conditions at the surface. Anyone who has seen rusty metal has observed oxidation in action: the oxygen steals electrons from the metal, creating crusty, red, iron oxide. Within Earth’s core, however, low levels of oxygen create much more reduced conditions. Intuitively, many researchers have supposed that Earth’s layers become less oxidized with depth, but few samples have emerged from the deep Earth to support or disprove this idea.

In a new paper in Nature Geoscience, DCO researchers report the surprising news that iron in the mantle may become more oxidized at deeper levels. DCO members Kate Kiseeva (University of Oxford, UK), Thomas Stachel (University of Alberta, Canada), Aleksandr Chumakov, Valerio Cerantola (both at European Synchrotron Radiation Facility, France), Jeff Harris (University of Glasgow, UK), Catherine McCammon, and Leonid Dubrovinsky (both at Bayreuth University, Germany), analyzed the oxidation state of iron from garnets. The garnets formed in the mantle between 240 and at least 500 kilometers deep and traveled to the surface as inclusions within diamonds. The researchers suspect that the increasing oxidation of iron in garnets with depth is due to oxidized carbonate compounds that react with iron and form diamonds.

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