Osmium isotopes in Baffin Island and West Greenland picrites: Implications for the Os-187/Os-188 composition of the convecting mantle and the nature of high He-3/He-4 mantle

Identifying the Os isotope composition of the prevalent, largely peridotitic, convecting mantle places important constraints on the Earth's accretion, differentiation and evolution and also has implications for the interpretation of Re-depletion ages in mantle peridotites. As partial melting preferentially samples mantle components with the lowest melting temperatures, large degree melts such as picrites should most closely reflect the peridotitic components within the source. Thus, Re-Os analyses of thirty picrites from Baffin Island and West Greenland are thought to provide a good estimate of the bulk Os-187/Os-188 composition of their convecting mantle source, which is indistinguishable from DMM in terms of lithophile isotopes and trace elements. In addition, the high He-3/He-4 of these rocks allows us to comment on the possible origins of high He-3/He-4 mantle. Ingrowth-corrected Os-187/Os-188 of the picrites ranges from 0.1267 to 0.1322. The higher Os-187/Os-188 samples have correspondingly lower Nd-143/Nd-144 which can be explained by contribution (similar to 5%) from old recycled oceanic crust, including sediment. However, Baffin Island and the earliest West Greenland picrites are remarkably uniform in composition with Os-187/Os-188 between 0.1267 and 0.1280, and a mean and mode of 0.1272 +/- 0.0007. Such Os isotope compositions are less radiogenic than estimates of primitive upper mantle but are similar to the least radiogenic mid-ocean ridge basalts (MORB) and the most common composition of ophiolite-derived platinum-group alloys and chromites. These compositions appear to represent a source dominated by peridotite.

The picrites studied record the highest known He-3/He-4 in the silicate Earth (up to 50 R.). For this signature to reflect isolated domains of ancient melt depletion would require significantly less radiogenic Os isotope compositions than observed (Os-187/Os-188: < 0.115), unless radiogenic Os, but not He, has been subsequently added. Conversely, a bulk outer core contribution would impart a supra-chondritic Os-187/Os-188 signature to the picrites, and thus Os isotopes preclude the core as a source of high He-3/He-4, unless core-mantle transfer of Os and He is decoupled. It is possible to broadly account for the Os-He and Os-Nd isotope variations by mixing of depleted MORB mantle, recycled oceanic crust and high He-3/He-4 primitive mantle, but it is difficult to explain each individual sample composition in this way. Alternatively, as the high He-3/He-4 signature is found in samples with variable Os and Nd isotope compositions, it seems likely that He is decoupled from other isotopic tracers and is dominated by minor addition of a He-rich, high He-3/He-4 component probably of primordial nature, although the ultimate source is unclear from our data. (c) 2008 Elsevier B.V. All rights reserved.