SULPHIDE MINERALOGY AND CHALCOPHILE/SIDEROPHILE ELEMENT GEOCHEMISTRY OF ABYSSAL PERIDOTITES FROM THE MARK AREA (20-24°N MID-ATLANTIC RIDGE, SOUTH OF THE KANE TRANSFORM)

Abstract

A coupled study of sulphide mineralogy and chalcophile/ siderophile elements geochemistry (i.e., S, Se, Cu, As, V, Cd, Co, Zn, platinum-group elements and Au) has been undertaken in abyssal peridotites from the MARK area. The 28 samples analysed were mostly collected by submarine along the southern wall of the Kane transform fault. Compared to the peridotites drilled in the same area, our samples show a wide range of modal compositions (lherzolites, cpx-poor lherzolites and harzburgites) and of bulk-rock chemistry (1.51.85; SmN/YbN up to 1.4, N= chondrite normalized). Lherzolites and cpx-poor lherzolites show coarsergrained isolated cpx crystals in addition to the smaller cpx forming cpx+opx±sp clusters. These coarse-grained cpx are ascribed to in-situ precipitation of clinopyroxene from partial melts (Seyler et al., in prep.). Sulphides of indisputable magmatic origin have been observed in all of the peridotites. In spite of sea-water alteration and serpentinization, the number of magmatic sulphide grains positively correlates with fertility in the residual samples, decreasing from 1.2-3.7 grains/cm2 in the lherzolites to less than 0.5 grain/cm2 in the most refractory harzburgites. These latter still contain residual sulphides which provides further hypothesis of immiscible sulphide persisting in the mantle source of MORB-melts. Magmatic sulphides are closely associated with the small clusters-forming cpx crystals, being either enclosed or forming discrete grains adjacent to this silicate. In the weathered samples, magmatic sulphides are totally replaced by hydrous iron hydroxides (limonites) whereas the samples free from weathered products preserved sulfide assemblages typical of the reducing conditions generated by low T-serpentinization (i.e. Fe-rich pentlandite + magnetite ± mackinawite ± digenite ± troilite ± awaruite). Nevertheless, the peridotites refertilized by exotic MORB-type magmas display systematic sulphide enrichments (up to 5 grain/cm2) with respect to the sulphide vs. fertility correlation documented in the residual samples. Sulphides preferentially occur along or within the grain boundaries of the coarse-grained cpx. Similar sulphide enrichments have been observed in a few residual cpx-poor lherzolites devoid of melt/rock reaction evidence; however sulphides are disseminated within the serpentinized matrix at distance from the cpx. Clearly, several sulphide precipitation mechanisms operated in the oceanic mantle sampled by the Kane transform fault. The contents of moderately chalcophile elements (Cu, Zn, V, Co, Cd) and chalcogenides (S, Se, As) have been variably upgraded by sea-floor alterations. Weathered samples are systematically enriched compared to typical mantle compositions (up to 265 ppb Se; 200-400 ppm S with 2 occasional peaks at 2,000 ppm; up to 60 ppm As; up to 120 ppm Cu). None of the moderately chalcophiles positively correlates with the number of sulphide grains per polished thin sections and correlations with fertility indices are poor. By contrast, positive correlations such as Cu vs. Se, As vs. Se; Cu vs. As; Cd vs. Se and Zn vs. Cu suggest that these elements were contaminated by fluids from hydrothermal vents. However, apart from the weathered samples, S, Se as well as numerous moderately chalcophiles still show concentration ranges plotting on mantle melting trends. Some lherzolites display sub-chondritic S/Se ratios (~2,800) and Se and S contents that fit the range of oceanic mantle compositions recomputed from primitive MORBs (~200 ppm S; ~80 ppb Se). Gold contents (0.8-5 ppb) are almost systematically higher than mantle abundances, probably because of contaminations from hydrothermal vents. This element poorly correlates with indicators of melt depletion or magmatic sulphide precipitation. On the contrary, platinum-group elements were mostly insensitive to hydrothermal alterations, whether serpentinization or sea-floor weathering. Weathered and unweathered peridotites display PGE concentration ranges typical of mantle rocks (2-4 ppb Os, 2-4 ppb Ir; 4-8 ppb Ru; 0.7-1.4 ppb Rh; 3-9 ppb Pt; 0.2-8 ppb Pd). These concentration range are not affected by the alteration of magmatic sulphides into iron hydroxides. Strongly serpentinized harzburgites are the poorest in PGEs, perhaps because of the well-known diluting effect of serpentinization on trace elements. CI-chondrites normalized PGE patterns are similar to those reported for on-land unserpentinized mantle rocks, ranging from nearly flat to slightly fractionated. Compatible PGEs (Os, Ir, Ru and Rh) were not affected by partial melting or melt/rock reactions as their concentration ranges are uncorrelated with petrogenetic indicators of these processes. Os/Ir ratio is within the range of chondrites (0.9-1.2) whereas Ru/Ir and Rh/Ir are slightly higher than chondritic (1.7-2.3 and 0.3-0.4, respectively). Our sample provide further evidence that compatible PGEs are not extracted from the mantle as long as residual sulphides survive in partial melting residues. Pt Pd systematics are more variable. The most refractory harzburgites preserve negatively-trending chondrite normalized PGE patterns (0.7