First measurements of the Fe oxidation state of spinel inclusions inolivine single crystals from Vulture (Italy) with the in situ synchrotronmicro-Mossbauer technique

Abstract

The redox state of the Earth's upper mantle (i.e., oxygen fugacity, f(O2)) is a key variable that influences numerous processes occurring at depth like the mobility of volatile species, partial melting, and metasomatism. It is linked to the oxidation state of peridotite rocks, which is normally determined through the available oxythermobarometers after measuring the chemical composition of equilibrated rock-forming minerals and the Fe3+ in redox-sensitive minerals like spinel or garnet. To date, accurate measurements of Fe3+ / Sigma Fe in peridotites have been limited to those peridotites (e.g., harzburgites and lherzolites) for which an oxythermobarometer exists and where spinel (or garnet) crystals can be easily separated and measured by conventional Fe-57 Mossbauer spectroscopy. Wehrlitic rocks have been generally formed by the interaction of a lherzolite with carbonatitic melts and, therefore, have recorded the passage of (metasomatic) fluids at mantle conditions. However, no oxythermobarometer exists to determine their equilibrium f(O2).The aim of this study was to retrieve the f(O2) of the mantle beneath Mt. Vulture volcano (Italy) through the study of a wehrlitic lapillus emitted during the last eruption (similar to 140 kyr ago) that contain olivines with multiple tiny spinel inclusions with sizes < 40 mu m. To our knowledge, the Fe oxidation state of these inclusions has been never determined with the Mossbauer technique due to their small sizes.Here, we present measurements of the Fe3+ / Sigma Fe using in situ synchrotron Mossbauer spectroscopy coupled with chemical and spectroscopic analysis of both host olivine and spinel inclusions.The results show Fe3+ / Sigma Fe ratios of 0.03-0.05 for olivine and 0.40-0.45 for the included spinels, the latter of which appear higher than those reported in literature for mantle spinel harzburgites and lherzolites. Given the evidence of the mantle origin of the trapped spinels, we propose that the high f(O2) (between 0.81 and 1.00 log above the fayalite-magnetite-quartz buffer; FMQ) likely results from the interaction between the pristine spinel lherzolite and a CO2-rich metasomatic agent prior to the spinel entrapment in olivines at mantle depths.