CARBONATE MELTS AND CARBONATITES

Carbonatites are familiar to students of petrology as rare igneous rocks formed predominantly of carbonate, whose only modern expression is a single active volcano, which erupts strongly alkaline carbonate lavas with no direct match in Earth’s geological record. Based on Sr-Nd-Pb isotopic data, stable isotopic compositions, noble gases, and experimental phase equilibria, they are derived from the mantle, showing almost no sign of contamination by crust. As liquids, carbonate melts have remarkable physical properties, which set them apart from the alkaline silicate melts with which they are often temporally associated. They show very high solubilities of many elements considered rare in silicate magmas, and they have the highest known melt capacities for dissolving water and other volatile species like halogens at crustal pressures. They are highly efficient transport agents of carbon from the mantle to the crust, remaining mobile over extraordinary ranges of temperature, and their very low viscosity should enhance connectivity along grain boundaries in the mantle where they are implicated in geochemical enrichment processes related to metasomatism. Carbonatites mostly have unambiguous origins in the mantle and the limit to their depth is not known, but the likelihood that they may exist in the lower mantle (Stoppa et al. 2009; Kaminsky 2012) needs to be appraised since they may exert a dramatic control on the mobility and long-term storage of deep carbon in the Earth. Ultimately the stability of carbonate melt is an extension of the stability of carbonate minerals,subject critically to the mantle oxidation state (Luth 1993; Frost & McCammon 2008); carbonate-melts have also been predicted in the oceanic low-velocity zone and deep mantle (Presnall and Gudfinnsson 2005) by laboratory petrology experiments (Wyllie 1995). Much remains to be discovered about carbonate melts at very high pressures. Beyond the current solid media experiments (piston cylinder, multianvil press, DAC; eg calcite-dolomite-aragonite (Kraft et al. 1991)), we may look to carbonate inclusions in diamonds (Brenker et al. 2007) and high pressure shock environments for clues, including shocked carbonate from impact craters which show isotopic shifts (δ¹³C vary 5 per mil) (Martinez et al. 1994; Martinez, et al. 1995; Jones, Claeys et al. 2000).