First measurements of the Fe oxidation state of spinel inclusions in olivine single crystals from Vulture (Italy) with the in situ synchrotron micro-Mössbauer technique

The redox state of the Earth’s upper mantle (i.e., oxygen fugacity, fO2 ) 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 Fe3C in
redox-sensitive minerals like spinel or garnet. To date, accurate measurements of Fe3C =
P
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 57Fe Mössbauer 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 fO2 .
The aim of this study was to retrieve the fO2 of the mantle beneath Mt. Vulture volcano (Italy) through the
study of a wehrlitic lapillus emitted during the last eruption ( 140 kyr ago) that contain olivines with multiple
tiny spinel inclusions with sizes <40 μm. To our knowledge, the Fe oxidation state of these inclusions has been
never determined with the Mössbauer technique due to their small sizes.
Here, we present measurements of the Fe3C =
P
Fe using in situ synchrotron Mössbauer spectroscopy coupled
with chemical and spectroscopic analysis of both host olivine and spinel inclusions.
The results show Fe3C =
P
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 fO2 (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.