The Incompatible Element Characteristics of an Ancient Subducted Sedimentary Component in Ocean Island Basalts from French Polynesia

Abstract

The concentrations of incompatible elements in basaltic rocks from the Cook–Austral and Pitcairn–Gambier island chains of French Polynesia were determined to refine geochemical models for the origin of EM I- type ocean island basalts. It has been proposed previously that the addition of pelagic sediment to a HIMU source could yield mantle with EMI-like characteristics. The very high abundances of many large ion lithophile elements (e.g. Pb) in pelagic sediments would control incompatible element ratios in the proposed EMI source. Calculations indicate that partial melts of this HIMU–pelagic sediment mix would have incompatible element ratios that do not overlap with those of EMI basalts. An investigation of the effect of subduction on a pelagic sediment composition shows that this is a critical process: the sediment undergoes significant loss of large ion lithophile elements during sub-arc devolatilization and metamorphism. Incompatible element ratios in partial melts of a model EMI source which includes this residual metasediment component are shown to be consistent with the range of EMI-type lavas from French Polynesia. In addition, the EMI source may include a smaller ratio of altered oceanic crust to depleted mantle peridotite than is present in a HIMU source.