OS ISOTOPE SYSTEMATICS IN OCEAN ISLAND BASALTS

New Re-Os isotopic results for Os-poor basalts from St. Helena, the Co mores, Samoa, Pitcairn and Kerguelen dramatically expand the known ran ge of initial Os-186/Os-187 ratios in OIBs to values as high as 1.7. I n contrast to the Os isotopic uniformity of Os-rich basalts from the H IMU islands of Tubuai and Mangaia found by Hauri and Hart [1], our val ues for St, Helena span most of the known range of Os isotopic variabi lity in oceanic basalts (initial Os-187/Os-186 ranges from 1.2 to 1.7) . Generation of such radiogenic Os in the mantle requires melting of s ource materials that contain large proportions of recycled oceanic cru st. The very low Os concentrations of most of the basalts analyzed her e, however, leave them susceptible to modification via interaction wit h materials containing radiogenic Os in the near-surface environment. Thus the high Os-186/Os-187 ratios may result from assimilation of rad iogenic Os-rich marine sediments, such as Mn oxides, within the volcan ic piles traversed by these magmas en route to the surface. Furthermor e, the Os isotopic signatures of Os-rich, olivine-laden OIBs may refle ct the accumulation of lithospheric olivine, rather than simply their mantle source characteristics. The extent to which these processes alt er the view of the mantle obtained via study of Re-Os systematics in o ceanic basalts is uncertain. These effects must be quantified before R e-Os systematics in OIBs can be used with confidence to investigate th e nature of mantle heterogeneity and its causes.