OSMIUM ISOTOPIC COMPOSITIONS FROM OCEANIC BASALTS

We report Os isotopic analyses of certain oceanic basalts as well as Re and Os concentrations of oceanic and continental mafic igneous rocks. Os-186/Os-186 ratios of OIB from Loihi (1.10), Mauna Loa (1.13) and Reunion (1.11) are slightly more radiogenic than estimates of the bulk silicate earth (approximately 1.06) and the majority of published values for peridotites. An ankaramite from Iceland has a Os-187/Os-187 of 1.07. These values preclude the possibility that these OIB are derived either totally or largely by the fusion of ancient subducted oceanic crust, as such material would have extremely high Os-187/Os-186 ratios. Mixing calculations show that a basalt with a Os-187/Os-186 of 1.11 could have been generated from a source containing approximately 8% of 3 Ga crust, assuming that the depleted mantle had chondritic Re-Os systematics. However, such an interpretation of the more radiogenic OIB cannot be considered unique. The fact that the Iceland sample has a lower Os-187/Os-186 than samples from Reunion and Hawaii suggests that the age of the lithosphere through which the magmas passed may be a factor. During ascent, OIB magmas could have been contaminated with either the oceanic crust itself, which had evolved highly radiogenic Os-187/Os-186 ratios since its formation at the ridge, or with high-Re/Os components of the lithospheric mantle. However, one cannot exclude the possibility that the difference in the Os isotopic compositions between the more radiogenic OIB and those of continental and oceanic peridotites reflects the more depleted history of the uppermost mantle (implying, in one interpretation, that the Os-187/Os-186 of the bulk silicate earth could be greater than 1.06).