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

Diamonds from the Kankan district of Guinea frequently contain majoritic garnet and ferropericlase mineral inclusions similar to those from Sao Luiz in Brazil [1–3]. Besides these ultra-high pressure phases in the Guinea diamonds, we have now identified, initially by in-situ single crystal X-ray diffraction, a new inclusion paragenesis of Ca-silicates. One diamond contained walstromite-structured CaSiO3 and three others the mineral assemblage CaSi2O5 (titanite-structured) with larnite (p-Ca2SiO4). The first two phases represent new minerals not described before from natural occurrences. From the phase diagram for mantle CaSiO3 [4], there are indications that primary 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 transition zone and the lower mantle. The presence of coesite (formerly stishovite) in two of the diamonds containing Ca-silicates indicates that these diamonds belong to an ‘eclogitic’ suite [5,6], whereas the presence of ferropericlase together with CaSiO3 and MgSiO3 in a third diamond may imply a ‘peridotitic’ environment.