Oxygen Isotope Constraints on the Sources of Ocean Island Basalts

Abstract

Oxygen isotope ratios in phenocrysts from ocean island basalts (OIB) can place constraints on the origins of their mantle sources. Values of δ^(18)O in olivines from alkali basalts from Pitcairn Island (which have the extreme "EM1" signature based on radiogenic isotopes) are equal to those of olivines from average mantle peridotite (based on studies of mantle xenoliths and the MORB source region) . This result suggests that the amount of recycled continental sediment in Pitcairn Island EM1 sources is less than ~0.75%. Based on studies of olivine phenocrysts, the sources of both the plume component of Hawaiian lavas and of all measured Juan Fernandez ("PHEM") samples also have oxygen isotope compositions indistinguishable from average upper mantle peridotite. If these OIB sources come from the lower mantle, it is implied to have an oxygen isotope ratio similar to the upper mantle. Low ^(18)O/^(16)O olivines from some Hawaiian lavas are associated with a more MORB-like He and radiogenic isotope component, possibly recycled but also consistent with contamination from the base of the modern Pacific plate.

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