Alpe Arami: A peridotite massif from depths of more than 300 kilometers

The abundance of FeTiO3 and chromite precipitates in olivine of the Alpe Arami peridotite massif, Switzerland, requires a much higher solubility for highly charged cations than is found in mantle xenoliths from depths of 250 kilometers. Three previously unknown crystal structures of FeTiO3 were identified that indicate that the originally exsolved phase was the high-pressure perovskite polymorph of ilmenite, implying a minimum depth of origin of 300 kilometers. These observations, coupled with the unique lattice preferred orientation of olivine, further suggest that the original composition of olivine may reflect that of a precursor phase, perhaps wadsleyite, that is stable only at depths greater than 400 kilometers.