Evidence for carbonate in the mantle

Abstract

The presence of primary carbonate in kimberlite magmas, which erupt tp the Earth's surface from depths of ≳150 km, carrying with them the rare elemental form of carbon, diamond, is evidence for a reservoir of carbon in the Earth's mantle. Hypotheses about the form in which this carbon is stored were largely speculative until laboratory experiments^1,2 established that at depths of ≳50 km carbon can be incorporated in the solid carbonates, dolomite (CaMg(CO₃)₂) and magnesite (MgCO₃). However, only one instance of minute inclusions of dolomite in a sample of rock from the mantle has ever been recorded³, despite 25 years of intensive study of these rocks. Here I report the presence of hitherto undescribed brucite/calcite intergrowths (Mg(OH)₂/CaCO₃) in mantle-derived xenoliths which occur in the diamond-bearing volcanic kimberlite pipes of Kimberley, South Africa. These inter-growths could reflect the presence of relatively abundant dolomite in the mantle. They seem to have been overlooked to date because of their fine textures and susceptibility to replacement by secondary serpentine.