Cin-Ty Lee á Roberta L. Rudnick William F. McDonough á Ingo Horn Petrologic and geochemical investigation of carbonates in peridotite xenoliths from northeastern Tanzania Received: 30 July 1999 /Accepted: 5 February 2000 Abstract Primary carbonates in peridotite xenoliths from the East African Rift in northeastern Tanzania occur as intergranular patches with accessory minerals olivine and spinel), as patches with accessory magmatic minerals nepheline), and as round monomineralic inclusions in primary olivine grains. All are character- ized by calcitic compositions Ca/Ca + Mg + Fe from 0.83 to 0.99), extremely low SiO 2 + Al 2 O 3 + Na 2 O+K 2 O, low trace element abundance [total rare- earth element REE) abundance <25 ppm], uniform extinction, and lack of reaction textures with the host xenolith. Calculated Fe±Mg exchange coecients between carbonate and primary olivine indicate dis- equilibrium in most samples. Combined with the lack of signi®cant reaction textures, this suggests that the carbonates were introduced shortly before or during eruption of the host magma. A global compilation of electron microprobe analyses of mantle-derived car- bonates in xenoliths, xenocrysts, and megacrysts) reveals compositional clusters near end member calcite, end member magnesite, and stoichiometric dolomite. Eutectic liquid compositions are less common, suggest- ing that many carbonate inclusions reported worldwide may be crystalline precipitates. Likewise, the calcites in this study are not interpreted to represent quenched carbonatitic melts, but are interpreted instead to be crystalline cumulates from such melts. These inferences are consistent with recent experiments, which show that carbonatitic melts cannot become more calcitic than CaCO 3 80 wt%. Low trace element abundance may be a diagnostic feature of cumulate carbonate, and in combination with petrography and major element composition, serve to distinguish it from quenched carbonated liquid. Introduction Although primary, mantle-derived carbonates are rare in mantle-derived rocks Canil 1990), there are increasing reports of occurrences from around the world and from diverse tectonic environments McGetchin and Besancon 1973; Hunter and Smith 1981; Berg 1986; Amundsen 1987; Bulanova and Pavlova 1987; Smith 1987; Ionov et al. 1993; Yang et al. 1993; Frezzotti et al. 1994; McInnes and Cameron 1994; Pyle and Haggerty 1994; Schiano et al. 1994; Zhang and Liou 1994; Kogarko et al. 1995; Ionov et al. 1996; Ionov 1998; Norman and Pearson 1998; Saal et al. 1998). These carbonates po- tentially provide insights into the origin of carbonatite melts and mantle metasomatism and to the problem of determining where and in what form carbon is stored in the mantle Barker 1996). For instance, if these carbon- ates are quenched liquids, then their compositions can be used to determine whether carbonatite lavas are 1) pri- mary mantle melts, 2) primary melts modi®ed by wall- rock interaction, or 3) products of crystal fractionation or carbonate±silicate liquid immiscibility e.g., Wallace and Green 1988; Dalton and Wood 1993a; W.-J. Lee and Wyllie 1998; Wyllie and W.-J. Lee 1998). Additionally, experiments suggest that carbonatitic melts have low viscosities and low wetting angles Minarik and Watson 1995; Dobson et al. 1996). Thus, by analogy with car- bonatitic lavas, which are highly enriched in incompati- ble elements, carbonate melts are prime candidates for metasomatic agents in the lithospheric mantle Meen 1987; Green and Wallace 1988; Yaxley et al. 1991; Dautria et al. 1992; Dalton and Wood 1993a; Hauri et al. 1993; Rudnick et al. 1993; Yaxley and Green 1996; Wiechert et al. 1997; Yaxley et al. 1998; Coltorti et al. Contrib Mineral Petrol 2000) 139: 470±484 Ó Springer-Verlag 2000 C.-T. Lee &) á R. L. Rudnick á W. F. McDonough á I. Horn Department of Earth and Planetary Sciences, Harvard University, 20 Oxford St., Cambridge, MA 02138, USA e-mail: ctlee@eps.harvard.edu Tel.: +1-617-484-2397; Fax: +1-617-496-0434 Present Address: Lab for Inorganic Chemistry, ETH ZuÈrich, University St. 6, CH-8092, ZuÈrich, Switzerland Editorial responsibility: T. L. Grove