HIMU EM - THE FRENCH-POLYNESIAN CONNECTION

HIMU, EM I and Em II are three of the main geochemical mantle components that give rise to oceanic island basalts [1]. They represent the end members that produce the extreme isotopic compositions measured on intraplate volcanics. In French Polynesia, all three mantle components are represented in volcanic rocks. The characteriStiC HIMU signature is found in Tubuai, Mangaia and Rurutu, EM I is present in the source of Rarotonga and Pitcairn volcanics and EM II dominates the composition of most Society Islands. Intermediate values between the three end members are found on most islands. We suggest that the three components are not independent but are physically related in the mantle. The HIMU component is thought to be recycled oceanic crust that lost part of its Pb through hydrothermal processes prior to and during subduction. EM I and EM II are believed to acquire their isotopic and trace element characteristics through entrainment of sediments that were subducted together with the oceanic crust. The trace element pattern and the isotopic composition of HIMU lavas can be quantitatively modelled using a mixture of approximately 25% old recycled MORB Crust and 75% mantle peridotite. The extreme Pb composition is modelled assuming that Pb was lost from oceanic crust when hydrothermal alteration at the ridge leached Pb from the basalt to redeposit it as sulphides on top of and throughout the crust, followed by preferential dissolution of sulphides during dehydration in the subduction zone. These processes led to a drastic increase of the U/Pb ratio of the subducted material which evolved over 2 Ga to very radiogenic Pb isotopic compositions. Pb isotopic compositions similar to those Of Em I and Em II are modelled assuming that sediments with average crustal Pb isotopic compositions were subducted and recycled into the mantle together with the underlying MORB oceanic crust. Pelagic sediments (mu approximately 5 and kappa approximately 6) account for the Pb isotopic composition Of EM I whereas terrigenous sediments (mu approximately 10 and kappa approximately 4.5) evolve towards the EM II end member. A few percent of sediment in the recycled crust-sediment mixture will destroy the characteristic Pb isotopic signature of the Himu component. This, together with the low probability of isolating oceanic crust in the mantle for greater-than-or-equal-to 2 Ga, explains why the extreme HIMU composition, as seen on Tubuai and St Helena, is sampled so rarely by oceanic volcanism.