EarthRef.org Reference Database (ERR) -- Roy-Barman & Allegre 1995

We measured Os-187/Os-186, Re, and Os concentrations in 14 lavas from different oceanic islands using negative thermal ionization mass spectrometry. The counting precision (2 sigma) on Os-187/Os-186 is 0.1-0.6%. Uncertainties introduced by blank and age corrections can increase the total error up to 3%. Os-187/Os-186 corrected for blank and Re-187 in situ decay range from 1.082 +/- 0.001 (Reunion) to 1.485 +/- 0.045 (Mangaia). Re and Os concentrations can be explained by the chalcophile behaviour of these elements during magmatic processes. A weak correlation between Os-187/Os-186 and Pb-206/Pb-204 was found. Principal component analysis in Os-Sr-Nd-Pb isotopic ratio space shows that the scattering of the data is related to a partial decoupling between Os-187/Os-186 and Pb-206/Pb-204. Modeling of oceanic crust, terrigenous and pelagic sediments recycling in the mantle shows that mixing of old oceanic crust in OIB sources can account for the highest Os-187/Os-186 (Mangaia, Tubuai, Comores). Small quantities of sediments (<1%) subducted with the oceanic crust can explain the lack of good correlation between Os-187/Os-186 and Pb-206/Pb-204. Since Os-187/Os-186 is much less sensitive to sediment subduction than Pb-206/Pb-204, it could be a more suitable tool for studying the recycling of old oceanic crust in the mantle. However, even if the assimilation of altered material by the lavas during their ascent through the oceanic crust is not a dominant process, it can significantly affect the Os-187/Os-186 Of some Os-poor lavas and, therefore, it contributes to the Os-Pb decoupling.