Geochemistry and origin of sulphide minerals in mantle xenoliths: Qilin, southeastern China

Primary sulphides occur both as mico-inclusions in major silicate and oxide phases and as individual grains in spinel lherzolite and pyroxenite xenoli ths from Qilin, southeast China. Most of the lherzolite-hosted sulphide inc lusions, typically 20 50 mu m across, occur as isolated spheres or spheroid s; host grains are olivine, clinopyroxene and orthopyroxene, but not spinel . In contrast, sulphide inclusions in pyroxenite are mostly 20-80 mu m acro ss and are almost exclusively hosted by clinopyroxene and spinel. These sul phide inclusions are typically multifaceted polygons, with their shapes epi taxially controlled by the host minerals. Isolated sulphide grains occur on ly in pyroxenite; they are up to 500 mu m across, show evidence of deformat ion and are spatially associated with spinel. Lherzolite-hosted sulphide gr ains are polyphase assemblages that consist of pentlandite +/- chalcopyrite +/- Ni-poor monosulphide solid solution (mssl) +/- Ni-rich monosulphide so lid solution (mss2) +/- cubanite +/- heazlewoodite +/- millerite +/- bornit e. Pyroxenite-hosted sulphide grains are pyrrhotite with minor chalcopyrite . All assemblages are likely to be the low-T (less than or equal to 300 deg rees C) re-equilibrated products of high-T monosulphide solid solutions (MS S). The bulk compositions of these sulphide grains, estimated using proton microprobe analysis, show no consistent differences between the inclusion s uites and intergranular sulphide grains, withe in spinel lherzolite or pyro xenite samples. Average values of 111 proton microprobe analyses reveal tha t the lherzolite-hosted sulphides are rich in Ni (21%), Cu (<9%), Se (110 p pm) and platinum group elements (PGE) (less than or equal to 30 ppm) but po or in Fe (37%) compared with the pyroxenite-hosted sulphides (Ni 1.4%, Cu < 4%, Se 35 ppm, PGE absent, Fe 61.5%). Other trace elements (co, Zn, As, Mo, Ag, Sn, Sb, Te and Pb) show no significant difference between the two suit es. Lherzolite-hosted sulphides are inferred to be derived from immiscible sulphide melts trapped in residual mantle during partial melting. The sulph ide melts had themselves undergone. MSS fractionation before their incorpor ation into the depleted mantle rocks. In contrast, pyroxenite-hosted sulphi des were produced by sulphur saturation during the crystallization of mafic magmas intruded into lherzolitic mantle.