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 similar to 5.7 GPa (similar to 180 km) at 1500 K. However, the observed coexistence of CaSiO3-walstromite, larnite (beta-Ca2SiO4) and CaSi2O5-titanite in one multiphase inclusion within the same diamond indicates that the sample investigated is sub-lithospheric with entrapment pressure between similar to 9.5 and similar to 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. (C) 2016 Elsevier B.V. All rights reserved.