The decomposition of diopside to CaSiO3 cubic and MgSiO3 orthorhombic perovskites was demonstrated by in situ X-ray diffraction measurements under simultaneous high pressure and high temperature conditions, using a combination of synchrotron radiation and a multianvil apparatus at SPring-8. Quench experiments also confirmed the decomposition of diopside in a wide temperature range of 1000--1900 ¿C and at pressures 23--27 GPa. These results strongly support the idea that some mineral inclusions with pyroxene stoichiometry discovered in natural diamonds may have been originated from the lower mantle. Moreover, observed temperature dependence of MgSiO3 content dissolved in CaSiO3 perovskite suggests that some of the inclusions were formed at relatively low temperatures (<1200 ¿C) in the deep mantle, presumably related to slab subductions. ¿ 2000 American Geophysical Union