OSMIUM ISOTOPIC COMPOSITIONS AND RE-OS CONCENTRATIONS IN SULFIDE GLOBULES FROM BASALTIC GLASSES

Sulfide globules present in basaltic glass were analyzed for Re and Os concentrations ([Re], [Os]) and Os-187/Os-186 from a selection of MOR Bs and Loihi lavas. The high Re and Os concentrations measured in sulf ides confirm the chalcophile character of these elements. The partitio n coefficients of Re and Os between sulfide globules and silicate melt are estimated to be D-Re = 43 and D-Os = 4.8 x 10(4). The Os-187/Os-1 86 of the samples range from 1.054 to 1.387 far MORBs and from 1.091 t o 1.146 for the Loihi samples. The low Re/Os ratios of sulfides make c orrection for in situ Re-187 decay negligible. Several sulfide-rich (O s = 500-1000 ppb) and sulfide-containing fractions from a Loihi sample gave essentially the same Os-187/Os-186 = 1.095, demonstrating the po ssibility of obtaining very reliable data with our procedure, For some MORBs, shifts in Os-187/Os-186 for concentrates with low [Os] can be explained by the presence of Fe-oxyhydroxides enriched in Os by co-pre cipitation from seawater. The radiogenic Os-187/Os-186 ratios found in several sulfides with high [Os] cannot be explained by post-eruption contamination by Fe-oxyhydroxides or isotopic exchange between sulfide s and seawater. We infer that some of the radiogenic Os-187/Os-186 rat ios measured in MORE sulfides reflect the isotopic composition of the lava prior to eruption, Correlations were found between Os and B isoto pic compositions and suggest that the radiogenic Os-187/Os-186 measure d in MORBs are mainly due to assimilation of seawater-derived Os by th e lavas during their ascent through the altered oceanic crust. MORBs w ith delta(11)B less than or equal to -10 parts per thousand seem to be suitable to study the Os-187/Os-186 ratio of the mantle. Considering only such 'uncontaminated' samples, we still find a significant range of Os-187/Os-186 ratios (1.054-1.093) between two MORBs from the FAMOU S area. This result suggests that the mantle underlying the FAMOUS are a is heterogeneous with regard to the Re-Os system and that it may hav e an Os signature distinct from that of abyssal peridotites collected on the Mid-Atlantic Ridge. Osmium isotopic ratios in Loihi lavas seem less sensitive to contamination than in MORBs. Two Os-rich Loihi basal ts have similar isotopic ratios that apparently provide a good estimat e of the(187)Os/Os-186 ratio of the Loihi source (Os-187/Os-186 = 1.09 5 +/- 0.005). (C) 1998 Elsevier Science B.V.