Oxygen fugacity and valence state of chromium in ferropericlase: Can Cr²⁺ be a redox indicator for the deep mantle?

Abstract

The valence state of chromium in ferropericlase inclusions within diamonds was expected to serve as a redox indicator applicable to the deep earth environment with an extremely reduced condition. In this study, Cr K-edge X-ray absorption near-edge structure (XANES) spectra and ⁵⁷Fe Mössbauer spectra were obtained for ferropericlase samples synthesized under controlled oxygen fugacity conditions of -7, -9, and -11 in log f_O2. All the ferropericlase samples were shown to have almost identical Fe³⁺/Fe²⁺ ratios; this observation is consistent with a phase diagram of ferropericlase reported previously. All the ferropericlase samples contained Cr²⁺, and the Cr²⁺/ΣCr ratio was almost constant among the samples, within the experimental precision of XANES spectroscopic measurements. Divalent chromium ions are stabilized in ferropericlase by the crystallographic constraint, even under an oxidized condition. The present study proposes that there is room for reconsidering the presence of Cr²⁺ in ferropericlase inclusions in diamonds as evidence of the extremely reduced condition of the lower mantle.