Ferric Iron in Eclogitic Garnet and Clinopyroxene from the V. Grib Kimberlite Pipe (NW Russia): Evidence of a Highly Oxidized Subducted Slab.

This article focuses on determining the redox conditions of diamond-free Archean eclogites from the Vladimir Pavlovich Grib (V. Grib) kimberlite pipe in the Arkhangelsk Diamondiferous Province (NW Russia) through Fe³⁺/ΣFe measurements in garnet and clinopyroxene using Mössbauer milliprobe spectroscopy. The study finds that these eclogites equilibrated at pressures of 3.0–5.2 GPa and temperatures of 720–1050 °C, with oxygen fugacity (fO₂) ranging from −2.9 to 0.9 log units relative to the fayalite–magnetite–quartz (FMQ) buffer, indicating more oxidized conditions than typical Archean mantle eclogites worldwide. Despite evidence of metasomatism, the elevated Fe³⁺/ΣFe ratios and fO₂ values likely reflect the oxidized nature of the magmatic protoliths and their mantle source rather than late-stage alteration. These findings suggest an unusually oxidized Archean mantle environment beneath the V. Grib area, which may explain the absence of eclogitic diamonds and the predominance of carbonate-stable carbon species in these rocks.