High-pressure phase transformation in CaMgSi2O6 and implications for origin of ultra-deep diamond inclusions

The decomposition of diopside to CaSiO3 cubic and MgSiO3 orthorhombic perov skites was demonstrated by in situ X-ray diffraction measurements under sim ultaneous high pressure and high temperature conditions, using a combinatio n of synchrotron radiation and a multianvil apparatus at SPring-8. Quench e xperiments also confirmed the decomposition of diopside in a wide temperatu re range of 1000-1900 degreesC and at pressures 23-27GPa. These results str ongly support the idea that some mineral inclusions with pyroxene stoichiom etry discovered in natural diamonds may have been originated from the lower mantle. Moreover, observed temperature dependence of MgSiO3 content dissol ved in CaSiO3 perovskite suggests that some of the inclusions were formed a t relatively low temperatures (<1200<degrees>C) in the deep mantle, presuma bly related to slab subductions.