CONTENT AND ISOTOPIC COMPOSITION OF SULFUR IN ULTRAMAFIC XENOLITHS FROM CENTRAL-ASIA

Sulphur contents and isotopic compositions have been determined in 90 fresh mantle-derived garnet and spinel peridotite and pyroxenite xenoliths from six regions of Cenozoic alkali basaltic volcanism in southern Siberia and Mongolia. Sulphur contents in most of the peridotite nodules fall in the range 6-25 ppm, with largely positive delta-S-34 values clustering between -1 and +7 parts per thousand (average +2 parts per thousand). By contrast, the pyroxenites are richer in sulphur (25-140 ppm), and their delta-S-34 values are close to 0 parts per thousand (-1.5 to +1.4 parts per thousand). In the peridotite nodules, sulphur content correlates negatively with MgO, while their delta-S-34 values correlate positively with MgO. Most of the fertile lherzolites (MgO = 37-39%, CaO = 3-4%) have delta-S-34 values close to 0 parts per thousand (-1 to + 2 parts per thousand), similar to meteorite values, while in the harzburgites these values reach +3 to +5 parts per thousand. These features apparently reflect sulphur depletion during partial melting of peridotite mantle accompanied by sulphur isotopic fractionation between residual peridotites and generated basaltic melts. Clinopyroxene-poor xenoliths from southern Mongolia yielded highest delta-S-34 values of +5 to +7 parts per thousand accompanied by high Ba and K contents and high Sr-87/Sr-86 ratios; these may be a result of interaction with fluids derived from subducted oceanic crust which followed the partial melting and magmatic fractionation episodes. Low average sulphur concentrations (< 50 ppm) and largely positive delta-S-34 values may predominate in the continental lithospheric mantle worldwide. Sulphur isotope compositions typical of tholeiitic basalts and MORB (0 to +1 parts per thousand [1,2]) may be produced by melting of moderately depleted lherzolites. Primary melts with positive delta-S-34 values may be generated from mantle peridotites with larger degrees of depletion and/or from rocks metasomatised by subduction-related fluids.