Combined petrological, geochemical and isotopic modeling of a plume source: Example of Gambier Island, Pitcairn chain

Abstract

Mantle plumes have been increasingly studied in the past years, yet they remain not well understood. While geophysical approaches attempt to locate the depth of sources, petrological approaches characterize the nature of the materials that melt and geochemical approaches trace their history and origin. However, combined studies remain rare while they could provide strong constraints on the processes that give rise to mantle plumes. Here we present a new model that combines petrological and geochemical approaches to constrain the nature of source materials, their temperature of melting, their composition and their age. We focus on Gambier Island, Pitcairn chain in Polynesia because it has the advantage of combining two important features, (a) most lavas are tholeiites, a necessary feature to perform an accurate petrological modeling that can be compared to other plumes and (b) it belongs to the Pitcairn Chain, whose composition varies through time and samples today the extreme EM1-type source.We present geochemical analyses as well as Sr–Nd–Pb–Hf isotopes of lavas and high-precision olivine analyses. Using in situ olivine analyses and REE modeling, we constrain the composition of the mantle source (5% recycled oceanic crust – 95% peridotite), how both components melt (25–30% versus 0.5–1.5%) and we estimate the temperature of Gambier source at about 1400 °C, a rather low temperature compared to Hawaii for example. We also constrain both the age and the nature of the recycled material using the isotopic data. They require the presence of about 3% sediment associated to recycled basalt in the pyroxenitic component and its age is about 1.5 Gyr. We also attribute the marked Nb–Ta positive anomalies and the elevated Ce/Pb ratios to the presence of recycled basalt in the mantle source. These features resemble typical HIMU lavas but the younger age of the recycled material together with the presence of sediment in the plume source explains the lower Pb isotopic ratios. The clear isotopic change between Gambier and Pitcairn Islands suggests that the plume source changed radically within 5 Ma. More generally, our new data and model suggest that the FOZO composition as sampled by Gambier lavas does not necessarily represent a large and rather primitive mantle source.