Geochemistry and origin of sulphide minerals in Mantle Xenoliths

Primary sulphides occur both as micro-inclusions in major silicate and oxide phases and as individual grains in spinel lherzolite and pyroxenite xenoliths from Qilin, southeast China. Most of the lherzolite-hosted sulphide inclusions, typically 20-50 μm across, occur as isolated apheres of spheroids; host grains are olivine, clinopyroxene and orthopyroxene, but not spinel. In contrast, sulphide inclusions in pyroxenite are mostly 20-80 μm across and are almost exclusively hosted by clinopyroxene and spinel. These sulphide inclusions are typically multifaceted polygons, with their shapes epitaxially controlled by the host minerals. Isolated sulphide grains occur only in pyroxenite; they are up to 500 μm across, show evidence of deformation and are spatially associated with spinel. Lherzolite-hosted sulphide grains are polyphase assemblages that consist of pentlandite ± chalcopyrite ± Ni-poor monosulphide solid solution (mss1) ± Ni-rich monosulphide solid solution (mss2) ± cubanite ± heazlewoodite ± millerite ± bornite. Pyroxenite-hosted sulphide grains are pyrrhotite with minor chacopyrite. All assemblages are likely to be the low-T (≤300°C) re-equilibrated products of high-T monosulphide solid solutions (MSS). The bulk compositions of these sulphide grains, estimated using proton microprobe analysis, show no consistent differences between the inclusion suites and intergranular sulphide grains, either in spinel lherzolite or pyroxenite samples. Average values of 111 proton microprobe analyses reveal that the lherzolite-hosted sulphides are rich in Ni (21%), Cu (<9%), Se (110 ppm) and platinum group elements (PGE) (≤30 ppm) but poor in Fe (37%) compared with the pyroxenite-hosted sulphides (Ni 1.4%, Cu <4%, Se 35 ppm, PGE absent, Fe 61̇5%). Other trace elements (Co, Zn, As, Mo, Ag, Sn, Sb, Te and Pb) show no significant difference between the two suites. Lherzolite-hosted sulphides are inferred to be derived from immiscible sulphide melts trapped in residual mantle during partial melting. The sulphide melts had themselves undergone MSS fractionation before their incorporation into the depleted mantle rocks. In contrast, pyroxenite-hosted sulphides were produced by sulphur saturation during the crystallization of mafic magmas intruded into lherzolitic mantle.