Geobarometric evidence for a LM/TZ origin of CaSiO3 in a sublithospheric diamond

Breyite is the second most abundant mineral inclusion in super-deep diamonds after ferropericlase. Though breyite stability extends to 300 km along typical mantle geotherm, this phase is often assumed to be the product of retrograde transformation of CaSiO3-perovskite, and thus has the potential to retain informa-tion from as deep as 800-1000 km. In this study, we determined the depth of formation of a breyite inclusion still enclosed in its host diamond from Juina, Brazil, by X-ray diffraction. The measured >5 % smaller unit cell for breyite indicates a stored residual pressure showing that the breyite was entrapped between about 9(1) and 10(1) GPa. These are the highest estimates of formation pressure ever determined for a breyite inclusion. For ambient mantle temperatures higher than 1400-1500 degrees C, these pressures would exceed the maximum P of the breyite stability field. Breyite in this diamond cannot be primary but is rather a back -transformation product from CaSiO3-perovskite formed in the transition zone or the lower mantle. The co-existence magnesite in diamond JU55 and the slab -association of sublithospheric diamonds is evidence of carbon transport to lower mantle depths.