Alpe Arami: A Peridotite Massif from Depths of More Than 300 Kilometers

The abundance of FeTiO 3 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 FeTiO 3 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.