Mantle metasomatism drives oxidation of arc peridotite revealed by significant Fe3+/FeT variations in spinels from Kamchatka Peninsula

Redox conditions strongly influence mantle phase equilibria and arc magmatism, yet the processes driving elevated oxygen fugacity ( f O 2 ) in subarc mantle remain unresolved. We investigate six harzburgitic peridotite xenoliths from Shiveluch volcano, Kamchatka, to estimate f O 2 shifts induced by metasomatic reactions. Subduction–derived silicate melts and hydrous fluids generated fibrous pyroxenes at the expense of olivine, produced amphibole, phlogopite, anhydrite, and reaction textures at spinel rims. Spinel displays systematic core–to–rim increase in Fe 3+ /Fe T (from 0.17–0.39 to 0.48–0.57) and inverse correlation between Mg# and Cr#. Fe 3+ /Fe T was reset at the spinel rim, followed by progressive oxidation through diffusion–driven A– and B–site substitutions involving Mg 2+ –Fe 2+ and Fe 3+ –Al 3+ –Cr 3+ exchange. f O 2 (ΔFMQ) estimates, using the Ballhaus and Wood oxybarometers (hereafter BO and WO), range from 0.9–2.5 in spinel cores to 2.8–4.1 in rims by BO and from 1.0–2.4 to 2.8–3.2 by WO. Individual xenoliths record redox increases of up to 2.9 log units (BO) or 1.9 log units (WO). The two oxybarometers agree within ±0.5 log units at moderate f O 2 but diverge at higher oxidation states (Fe 3+ /ΣR 3+ spinel >0.30). Spinel core f O 2 values overlap those of previously studied local harzburgites, whereas rim values overlap those of clinopyroxenites from Shiveluch volcano, revealing significant redox heterogeneity (BO: 0.9–4.1; WO: 1.0–3.2) in the Kamchatka arc mantle spanning a large segment of the global arc mantle f O 2 spectrum. The metasomatically modified rims in all samples record f O 2 values of approximately FMQ +3 (WO) suggesting the strong oxidising character of the melt/fluid–deriving source beneath the Kamchatka arc, highlighting this region as among the most oxidised mantle domains yet documented.