187Os/186Os in oceanic island basalts: tracing oceanic crust recycling in the mantle

We measured 187Os/186Os, Re and Os concentrations in 14 lavas from different oceanic islands using negative thermal ionization mass spectrometry. The counting precision (2σ) on 187Os/186Os is 0.1-0.6%. Uncertainties introduced by blank and age corrections can increase the total error up to 3%. 187Os/186Os corrected for blank and187Re in situ decay range from 1.082 ± 0.001 (Reunion) to 1.485 ± 0.045 (Mangaia). Os concentrations can be explained by the chalcophile behaviour of these elements during magmatic processes. A weak correlation between 187Os/186Os and 206Pb/204Pb was found. Principal component analysis in OsSrNdPb isotopic ratio space shows that the scattering of the data is related to a partial decoupling between 187Os/186Os and 206Pb/204Pb. 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 187Os/186Os (Mangaia, Tubuai, Comores). Small quantities of sediments (< 1%) subducted with the oceanic crust can explain the lack of good correlation between 187Os/186Os and 206Pb/204Pb. Since 187Os/186Os is much less sensitive to sediment subduction than 206Pb/204Pb, 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 187Os/186Os of some Os-poor lavas and, therefore, it contributes to the OsPb decoupling.