New Ca-silicate inclusions in diamonds - tracers from the lower mantle

Diamonds from the Kankan district of Guinea frequently contain majoritic ga rnet and ferropericlase mineral inclusions similar to those from Sao Luiz i n Brazil [1-3]. Besides these ultra-high pressure phases in the Guinea diam onds, we have now identified, initially by in-situ single crystal X-ray dif fraction, a new inclusion paragenesis of Ca-silicates. One diamond containe d walstromite-structured CaSiO3 and three others the mineral assemblage CaS i2O5 (titanite-structured) with larnite (beta-Ca2SiO4). The first two phase s represent new minerals not described before from natural occurrences. Fro m the phase diagram for mantle CaSiO3 [4], there are indications that prima ry CaSiO3-perovskite underwent successive retrograde phase transformations. Development of equilibrium textures suggests slow exhumation. Ca-silicates are possibly important carriers of strontium, phosphorus and potassium and thereby contain part of the inventory of radioactive elements in the trans ition zone and the lower mantle. The presence of coesite (formerly stishovi te) in two of the diamonds containing Ca-silicates indicates that these dia monds belong to an 'eclogitic' suite [5,6], whereas the presence of ferrope riclase together with CaSiO3 and MgSiO3 in a third diamond may imply a 'per idotitic' environment. (C) 1999 Elsevier Science B.V. All rights reserved.