Abstract
We present new geochemical data (major and trace element, and Sr, Nd and Pb isotopic compositions) for volcanic rocks from Rurutu Island in Polynesia. The rocks are divided into two age groups: 13-10.8 Ma old basalts derived from the plume that is now active beneath the Macdonald seamount, and 1.8-1.1 Ma old volcanics that were produced during a younger volcanic event. Marked chemical differences exist between the two generations of lavas. Typical HIMU isotopic compositions are found in the old lavas but not in the young lavas, and unusual trace element fractionations (Ce/Pb, Zr/Hf and Ti/Eu) are restricted to the young lavas. These results are interpreted in a model in which the composition of the Quaternary lavas results from the interaction between magmas from an ascending young plume and the oceanic lithosphere under the island. The contaminating component in the lithosphere is thought to be a residual carbonatite liquid from the first volcanic episode. This contaminant has isotopic compositions similar to those of the first volcanic event but very high trace-element concentrations characterized by significant negative Ti and Pb anomalies. This model raises questions about the use of isotopic and trace element data from islands located downstream from another plume to understand the general composition and origin of the source of plume volcanism. However, it does not put into question the existence of extreme isotopic compositions (HIMU, EM I and EM II) in the mantle sources of oceanic island basalts.