Depth of formation of CaSiO₃-walstromite included in super-deep diamonds

“Super-deep” diamonds are thought to crystallize between 300 and 800 km depth because some of the inclusions trapped within them are considered to be the products of retrograde transformation from lower mantle or transition zone precursors. In particular, single inclusion CaSiO₃-walstromite is believed to derive from CaSiO₃-perovskite, although its real depth of origin has never been proven. Our aim is therefore to determine for the first time the pressure of formation of the diamond-CaSiO₃-walstromite pair by “single-inclusion elastic barometry” and to determine whether CaSiO₃-walstromite derives from CaSiO₃-perovskite or not. We investigated several single phases and assemblages of Ca-silicate inclusions still trapped in a diamond coming from Juina (Brazil) by in-situ analyses (single-crystal X-ray diffraction and micro-Raman spectroscopy) and we obtained a minimum entrapment pressure of ~ 5.7 GPa (∼ 180 km) at 1500 K. However, the observed coexistence of CaSiO₃-walstromite, larnite (β-Ca₂SiO₄) and CaSi₂O₅-titanite in one multiphase inclusion within the same diamond indicates that the sample investigated is sub-lithospheric with entrapment pressure between ~ 9.5 and ~ 11.5 GPa at 1500 K, based on experimentally-determined phase equilibria. In addition, thermodynamic calculations suggested that, within a diamond, single inclusions of CaSiO₃-walstromite cannot derive from CaSiO₃-perovskite, unless the diamond around the inclusion expands by ~ 30% in volume.