‘Not so Hot’ Spots: A Cold - Young HIMU - Source in Gambier Island (Pitcairn Chain)

Mantle plumes have been increasingly studied in the past years, yet they remain not well understood. The Polynesian plume that displays the entire isotopic range known for OIB, provides new perspectives into the global understanding on mantle plume activity on Earth. Here we present new geochemical (whole rock and olivine) and isotopic analyses on samples from Gambier Island, one of the Pitcairn chain islands in Polynesia. Most samples come from a temporal sequence and display small but significant geochemical and isotopic variations. Sr, Nd, Hf and Pb isotopic data do not vary much (e.g., ²⁰⁶Pb/²⁰⁴Pb ≈ 19.00 and ⁸⁷Sr/⁸⁶Sr ≈ 0.703290) and are very different from the EMI-type values reported for Pitcairn Island and seamounts.

Using in situ olivine analyses and REE modeling, we constrain the composition of the mantle source (10% recycled oceanic crust and 90% surrounding peridotite), how both components melt (25-30% for the pyroxenite produced from the oceanic crust and 2-7% for the surrounding peridotite) and by combining these data with published experimental studies, we find that the temperature of Gambier source is rather low temperature compared to other mantle plumes such as Hawaii. We also constrain the age and the nature of the recycled oceanic crust using the isotopic data. The data require the presence of about 3% sedimentary material associated to recycled oceanic basalt and its age is constrained at about 1.5 Ga. A marked Nb-Ta positive anomaly together with elevated Ce/Pb ratios are other indices of the presence of recycled oceanic basalt in the mantle source. Given that these features resemble typical HIMU lavas but that the 1.5 Gy age of the recycled crust is too young to create the extreme Pb isotopic compositions that characterize the HIMU basalts, Gambier Island can be considered as a “young HIMU” island.