Ca-Fe-rich exsolution lamellae from olivine in a wehrlitic xenolith, Monti Vulsini Volcanic District, Central Italy

A wehrlitic xenolith containing an assemblage of Cr-spinel, forsterite, Cr-diopside, phlogopite and leucite was sampled in the pyroclastic products of the Monti Vulsini Volcanic District of the Roman Comagmatic Province (Central Italy). The peculiarity of this ultramafic xenolith is the presence of a large number of exsolution lamellae in the olivine crystals. These exsolutions have a peculiar dendritic shape and a Ca-Fe-rich composition due to the microscopic intergrowths of clinopyoxene and magnetite. Melting experiments and mass balance show that the host olivine crystallised from a primitive K-basaltic melt with an uncommon calcium content, before exsolution. Geothermobarometric calculations suggest that the exsolution formed at subsolidus temperature and 6-9 kbar of pressure.