The geochemical characteristics of Re and Os, such
as the extreme chemical fractionation between Re and
Os during magmatic genesis and their siderophilic and
chalcophilic nature, clearly distinguish the 187Re-187Os decay system from the other long-lived geochronometers. This behaviour, which is the basis of the use of the Os isotopic system as a geodynamic tracer (Allègre and Luck, 1980), make this a powerful system to study the geodynamic evolution of the earth. The estimated 187Os/186Os ratios of the present-day oceanic upper mantle, documented from analyses of unaltered abyssal peridotites (e.g. Snow and Reisberg, 1995; Roy-Barman and Allègre, 1994) are restricted to values close to the carbonaceous chondrite average (×1.06). However, preliminary results obtained for mid-ocean ridge basalts (MORB) show much larger variations and have significantly more radiogenic values than the upper mantle estimates (Roy-Barman and Allègre, 1994).
We report here the 187Os/186Os ratios and the Os and Re concentrations obtained for MORB glasses from the Atlantic, Pacific and Indian oceans. The chemistry developed in the laboratory provides reproducible Os blanks of 0.153±0.004 pg/g with 187Os/186Os = 2.71±0.07 and Re blanks of 8.72±0.09 pg/g. The measurement precision for the isotopic ratio is about 5 for a 2pg Os sample. In order to remove any possible secondary seawater contamination due to Fe-Mn oxyhydroxides or MnO2 precipitates, the samples were leached with successively H2O (80°C), 1N HCl (80°C), 0.1N oxalic acid (80°C), 9N HBr (80°C) and 14N HF (20°C) for a total of 9 hours. The 187Os/186Os ratios of the last HBr leachates were similar to those of the residuas.
The Os and Re concentrations for 24 pristine MORB glasses with uniform mg# values, collected from the Atlantic (10 samples), Pacific (7 samples) and Indian (7 samples) mid-ocean ridges. All glasses show similar a range of Re concentrations (215 to 1384 ppt), but Os concentrations in Indian MORB glasses are higher (up to 27.05 ppt), than those in Pacific and Atlantic samples (0.20 to 13.95 ppt).
The comparison of Re and Os with other chemical elements indicate that : a) Re and Os have similar behaviour in the major oceans and b) during MORB genesis, the variations of Os in MORB are controlled by its compatible character, whereas Re concentrations relate to both the chalcophilic character and the moderate incompatibility of Re.
The preliminary results show that Os isotopic compositions for Pacific and Atlantic MORB vary between 1.098 and 1.315. By contrast, Indian MORB are significantly more radiogenic with 187Os/186Os ratios ranging between 1.106 to 1.7041.
Allègre, C.J. & Luck, J.M., Earth Planet. Sci. Lett. 48, 148-154 (1980).
Roy-Barman, M. & Allègre, C.J., Geochim. Cosmochim. Acta 58, 5043-5054 (1994).
Snow, J.E. & Reisberg, L., Earth Planet. Sci. Lett. 133, 411-421 (1995).