OS ISOTOPE SYSTEMATICS OF LA PALMA, CANARY-ISLANDS - EVIDENCE FOR RECYCLED CRUST IN THE MANTLE SOURCE OF HIMU OCEAN ISLANDS

Sub-aerial lavas from the single ocean island of La Palma, Canary Islands show as large a variation in Os-187/Os-186 isotope ratios (1.13-1.59) as found across all of French Polynesia [1]. The La Palma lavas, however, display a restricted range of chemical composition and have all been erupted within the last 3.5 Ma. The highest Os isotopic compositions are observed in lavas with low Os concentrations. An uplifted sequence of lavas, that represent the early phase of submarine growth of the island, show extremely heterogeneous Os-187/Os-186 isotope ratios, from 1.21 to 3.53, with the most radiogenic values found in pillow rinds. Assimilation of these pillow rinds by ascending magma can readily account for highly radiogenic ratios (Os-187/Os-186 > 1.3) found in lavas with Os concentrations below 30 ppt. Samples with Os concentrations too high to be significantly affected by assimilation still display a range in Os isotope ratios from 1.13 to 1.25. We argue that these radiogenic values reflect a HIMU mantle source that contains ancient recycled oceanic crust. Characteristic incompatible trace element ratios suggest further similarities between the mantle beneath La Palma and other HIMU islands. When potentially contaminated low-Os OIBs are screened from literature data, HIMU islands are found to display the highest Os isotope ratios (up to 1.25). Pb-Os systematics for uncontaminated OIBs do not define a simple two-component mixing relationship between ambient mantle and recycled oceanic crust of a single composition. We suggest that this is due to variable alteration and subduction-induced perturbation of the U/Pb ratio in the recycled material that forms a component of the HIMU source.