Laboratory experiments and seismology data have created a clear theoretical picture of the most abundant minerals that comprise the deeper parts of the Earth’s mantle. Discoveries of some of these minerals in ‘super-deep’ diamonds—formed between two hundred and about one thousand kilometres into the lower mantle—have confirmed part of this picture1,2,3,4,5. A notable exception is the high-pressure perovskite-structured polymorph of calcium silicate (CaSiO3). This mineral—expected to be the fourth most abundant in the Earth—has not previously been found in nature. Being the dominant host for calcium and, owing to its accommodating crystal structure, the major sink for heat-producing elements (potassium, uranium and thorium) in the transition zone and lower mantle, it is critical to establish its presence. Here we report the discovery of the perovskite-structured polymorph of CaSiO3 in a diamond from South African Cullinan kimberlite. The mineral is intergrown with about six per cent calcium titanate (CaTiO3). The titanium-rich composition of this inclusion indicates a bulk composition consistent with derivation from basaltic oceanic crust subducted to pressures equivalent to those present at the depths of the uppermost lower mantle. The relatively ‘heavy’ carbon isotopic composition of the surrounding diamond, together with the pristine high-pressure CaSiO3 structure, provides evidence for the recycling of oceanic crust and surficial carbon to lower-mantle depths.
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We thank M. Regier for proofreading the paper. F.N. is supported by the European Research Council (ERC) Starting Grant number 307322. M.K.’s work and sample collection was possible thanks to an NSERC Discovery grant. N.K. acknowledges funding from the Dr. Eduard Gübelin Association through a 2015 research scholarship. D.G.P. was funded by an NSERC CERC award. M.A. was supported by the ERC under the European Union’s Horizon 2020 research and innovation programme (grant 714936) ‘TRUE DEPTHS’ and by the SIR-MIUR grant (RBSI140351) ‘MILE DEEp’. We thank L. Litti and M. Meneghetti of the Laboratory of Nanostructures and Optics of the Department of Chemical Sciences, University of Padova for their help in acquiring and interpreting the Raman data. F.N. and D.G.P. were supported by the Deep Carbon Observatory. M.G.P. was supported by NERC grant NE/M015181/1.
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Mineralogy and Petrology (2018)