Osmium isotope systematics of historical lavas from Piton de la Fournaise (Réunion Island, Indian Ocean)
Résumé
Re-Os isotope and elemental data have been obtained for 20 historical picrites and basalts (1931-2006) from the Piton de la Fournaise volcano on Réunion Island and two old (>0.78 Ma) cumulates from a drill hole in the eastern part of the volcano. The 187Os/188Os ratios of the historical lava samples, selected to cover the MgO concentration and Pb isotopic ranges of Piton de la Fournaise lavas, range from 0.1311 to 0.1374. This result, together with previous results on 66-Ma-old lavas from the Deccan Traps (Allègre et al. in. Earth Planet Sci Lett, 170:197-204, 1999), supports the idea that the Os isotopic signature of the Réunion plume is relatively uniform and is at the less radiogenic end of the ocean island basalt spectrum. In detail, lavas erupted before 1992 seem to have higher 187Os/188Os than the lavas erupted after the 1992-1998 period of quiescence. Comparison of 187Os/188Os ratios with Pb, Sr and Nd isotopic data on the same set of samples shows no correlation between Os and Sr-Nd isotopes, whereas a broad positive relationship with Pb isotopes is observed, which is interpreted to reflect coupled fractionation of Re/Os and U-Th/Pb in the mantle due to the partitioning of Pb and Os into sulphides. Lavas inferred to be recording the Os isotopic signature of the Réunion plume source have higher 187Os/188Os ratios than the primitive mantle values. While this might be ascribed to melting of a lithologically heterogeneous source comprising recycled oceanic crust and/or continental sediment, the expected coupled Os-Sr-Nd-Pb isotopic variations are not observed. It is thus proposed that the mantle source for Piton de la Fournaise has inherently slightly radiogenic 187Os/188Os values that could reflect a mantle domain almost isolated from recycling processes.