Petrologic and geochemical investigation of carbonates in peridotite xenoliths from northeastern Tanzania

Primary carbonates in peridotite xenoliths from the East African Rift in no rtheastern Tanzania occur as intergranular patches with accessory minerals (olivine and spinel), as patches with accessory magmatic minerals (nephelin e), and as round monomineralic inclusions in primary olivine grains. All ar e characterized by calcitic compositions (Ca/Ca + Mg + Fe from 0.83 to 0.99 ), extremely low SiO2 + Al2O3 + Na2O + K2O, low trace element abundance [to tal rare-earth element (REE) abundance <25 ppm], uniform extinction, and la ck of reaction textures with the host xenolith. Calculated Fe-Mg exchange c oefficients between carbonate and primary olivine indicate disequilibrium i n most samples. Combined with the lack of significant reaction textures, th is suggests that the carbonates were introduced shortly before or during er uption of the host magma. A global compilation of electron microprobe analy ses of mantle-derived carbonates (in xenoliths, xenocrysts, and megacrysts) reveals compositional clusters near end member calcite, end member magnesi te, and stoichiometric dolomite. Eutectic liquid compositions an less commo n, suggesting that many carbonate inclusions reported worldwide may be crys talline precipitates. Likewise, the calcites in this study are not interpre ted to represent quenched carbonatitic melts, but are interpreted instead t o be crystalline cumulates from such melts. These inferences art: consisten t with recent experiments, which show that carbonatitic melts cannot become more calcitic than CaCO(3)similar to 80 wt%. Low trace element abundance m ay be a diagnostic feature of cumulate carbonate, and in combination with p etrography and major element composition, serve to distinguish it from quen ched carbonated liquid.