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Published January 1998 | public
Journal Article

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 ^(187)Os/^(186)Os from a selection of MORBs and Loihi lavas. The high Re and Os concentrations measured in sulfides confirm the chalcophile character of these elements. The partition coefficients of Re and Os between sulfide globules and silicate melt are estimated to be D_(Re)=43 and D_(Os)=4.8×10^4. The ^(187)Os/^(186)Os of the samples range from 1.054 to 1.387 for MORBs and from 1.091 to 1.146 for the Loihi samples. The low Re/Os ratios of sulfides make correction for in situ ^(187)Re decay negligible. Several sulfide-rich (Os=500–1000 ppb) and sulfide-containing fractions from a Loihi sample gave essentially the same ^(187)Os/^(186)Os=1.095, demonstrating the possibility of obtaining very reliable data with our procedure. For some MORBs, shifts in ^(187)Os/^(186)Os for concentrates with low [Os] can be explained by the presence of Fe-oxyhydroxides enriched in Os by co-precipitation from seawater. The radiogenic ^(187)Os/^(186)Os ratios found in several sulfides with high [Os] cannot be explained by post-eruption contamination by Fe-oxyhydroxides or isotopic exchange between sulfides and seawater. We infer that some of the radiogenic ^(187)Os/^(186)Os ratios measured in MORB sulfides reflect the isotopic composition of the lava prior to eruption. Correlations were found between Os and B isotopic compositions and suggest that the radiogenic ^(187)Os/^(186)Os measured in MORBs are mainly due to assimilation of seawater-derived Os by the lavas during their ascent through the altered oceanic crust. MORBs with δ^(11)B≤−10‰ seem to be suitable to study the ^(187)Os/^(186)Os ratio of the mantle. Considering only such `uncontaminated' samples, we still find a significant range of ^(187)Os/^(186)Os ratios (1.054–1.093) between two MORBs from the FAMOUS area. This result suggests that the mantle underlying the FAMOUS area is heterogeneous with regard to the Re–Os system and that it may have 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 basalts have similar isotopic ratios that apparently provide a good estimate of the ^(187)Os/^(186)Os ratio of the Loihi source (^(187)Os/^(186)Os=1.095±0.005).

Additional Information

© 1998 Elsevier Science B.V. Received 3 March 1997; revised 16 September 1997; accepted 2 October 1997. W. Smith, J. Nigrini, G. Czamanske, J. Moore, B. Hanan, P. Michael, D. Clague, G.R. Binns, R. Batiza, B. Murton and C. Langmuir have generously provided large amounts of basaltic glasses. We thank B. Hanan for providing unpublished data on South Atlantic samples. We thank E. Hauri, an anonymous reviewer, and A. Zindler for their constructive reviews. We appreciated discussions with G. Czamanske, M. Tredoux and G. Ravizza on various aspects of PGE geochemistry. We have benefited from the help of J. Armstrong, P. Carpenter, G. Huss and S. Russell for the elemental analysis of sulfides. We thank J. Shen for sharing his Re ionization method with us. We thank H. Ngo for discussions on Os analysis and for handling Sr and Nd analyses. We are grateful to L. Hedges for her generous assistance during the set up of the mineral separation and for processing some of the samples presented in this work. This work was supported by DOE grant DOE DE-FG03-88ER13851. Division Contribution No. 5660(907) and CRPG-CNRS Contribution No. 1293. [RO]

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