Extreme HIMU in an oceanic setting: the geochemistry of Mangaia Island (Polynesia), and temporal evolution of the Cook—Austral hotspot

Abstract

New isotope and trace-element data are presented for volcanic rocks from the island of Mangaia in Polynesia, which forms the most extreme example of the HIMU mantle endmember known at the present time.Observations from these data, coupled with published results, provide strong support for an origin of the HIMU signature related to recycling of oceanic crust. Evidence exists for carbonate metasomatism within the mantle beneath Polynesia; however, it is likely that this is also related to the recycling process, rather than being responsible itself for generating HIMU. The elevated U/Pb ratios observed in the HIMU source appear to be largely a result of Pb loss rather than U uptake. The apparent rarity of “pure” HIMU and its relation to other OIB types is presented within a simple recycling framework; to a first approximation, the spectrum of OIB compositions between HIMU and EM may reflect simple mixing between subducted oceanic crust and sediment sources.Temporal/genetic relationships with the other islands in the Cook-Austral archipelago are investigated. Within the Macdonald-Mangaia segment relatively well-defined age progressions require a common heat source. In contrast, isotopic compositions do not follow a simple trend—there is a decrease of the HIMU influence with time, but it is not clear whether this change was abrupt or gradual. The southern Cook islands appear to be largely unrelated to volcanism within the Macdonald-Mangaia segment.