CARBONATE MELTS AND DIAMOND FORMATION IN SUBDUCTED CONTINENTAL CRUST

Despite the fact that carbonate melts have been clearly identified in mantle xenoliths the possibility of formation of such melts in deeply subducted crustal rocks has never been discussed. Carbonates are present in subducted sediments as well as in subducted continental crust and hence there is plenty of source material to form carbonatite melts. However, it is very difficult to preserve evidence of carbonate melts because they are extremely reactive. Additionally, exhumation of deeply subducted crust is generally related to extensive recrystallisation of the rocks. We investigated diamond-bearing meta-carbonates from the Kokchetav Massif (Kazakhstan), which contain garnet, K-rich clinopyroxene and either matrix dolomite or Mg-calcite. In both rock types there are polyphase Mg-calcite inclusions in garnet and occasionally in clinopyroxene, which display straight grain boundaries indicative of a negative crystal shape of the host mineral. Micro-diamonds and minor silicate phases occur occasionally in these inclusions. In-situ trace element determinations of these inclusions showed that they are enriched in Ba and LREE and depleted in HREE with respect to the matrix carbonates. The textural and chemical evidence thus strongly suggests that these inclusions represent trapped carbonate melts. These melts must have formed close to peak metamorphic conditions of 45-60kbar, 950-1000°C within the stability field of diamond and the host minerals. The presence of such carbonate melts is important for the understanding of metamorphic diamond formation in a subduction zone environment. Textures and trace element composition of major minerals indicate that diamonds and carbonate melts formed by interaction of the meta-carbonates with a hydrous granitic melt derived from the country rock metapelites. The finding of carbonate melts in deeply subducted continental crust might be important for subduction zone processes. Carbonatite melts are likely to transport different types of trace elements than aqueous fluids or hydrous silicate melts and therefore they may produce a different type of sub-arc mantle metasomatism in subduction zones. This study was supported by the Russian Foundation for Basic Research (N 01-05-65093).