Ocean Island basalts analyzed to date include extreme examples of all known mantle isotopic associations. Os isotope ratios, particularly at HIMU localities, range to very radiogenic values and are quite variable. There are several possible explanations. 1) Heterogeneous mantle source materials are tapped selectively by differing magma generation mechanisms in such a way as to preclude radiogenic Os compositions in Os-rich magmas. The full range of Os isotopic compositions must be attainable, however in Os-poor magmas. 2) Accumulation and possible resorption of lithospheric cumulate olivine, resulting in Os-rich magmas, modulates the Os isotopic compositions of these magmas. 3) Assimilation of manganese-encrusted altered basaltic material, in the ocean crust or oceanic- island volcanic pile, is sufficient to elevate the 187Os/186Os ratios of Os-poor magmas, but not Os-rich magmas. Investigation of the relative importance of non- mantle mechanisms for the modification of Os isotope ratios in magmas is imperative. To this end, we propose a series of experiments to be performed on the island of La Palma in the Canary Islands.
