Helium is a unique tracer of the chemical evolution of the Earth’s interior. The presence of high 3He/4He ratios in the Earth’s mantle provides strong evidence for the existence of mantle reservoirs that have largely escaped melt extractiontios in the Earth’s mantle provides strong evidence for the existence of mantle reservoirs that have largely escaped melt extraction. On the other hand, low 3He/4He ratios (< the MORB range of 7-9 RA) found in HIMU ocean islands (high , where  is the U/Pb ratio) are attributed to recycled oceanic crust and sediments. However, recycled crustal material should be essentially devoid of 3He but have high concentrations of 4He produced from the decay of U and Th. It is therefore surprising that the average helium isotopic ratios at HIMU ocean islands are as high as 6-7 RA. A number of hypotheses have been put forward to explain the helium isotopic ratios in HIMU basalts. However, none of these provide a definite explanation on the origin of low helium isotopic ratios in HIMU lavas and there exist key gaps in our understanding of this important mantle component. Extensive studies that combine He with other lithophile isotopic tracers (Sr, Nd, Pb) have been carried out at high 3He/4He ocean islands, such as Iceland, Hawaii, and Galapagos. However, there exist relatively fewer studies on HIMU ocean islands and the mixing systematics in HIMU localities have not been fully ascertained, particularly with respect to helium.

We are measuring 3He/4He ratios in a suite of samples from Raivavae and Rapa from the Cook-Austral Islands to better understand the origin of the low 3He/4He ratios in HIMU basalts. The sample suite from the Cook-Austral islands were selected because: 1) the samples are well characterized with respect to major elements, trace elements, and radiogenic isotopes (Sr, Nd, Hf, Pb, and Os); 2) Raivavae lavas have a strong HIMU signature and also have the most diverse isotopic composition from the Cook-Austral Islands while lavas from the island of Rapa , approximately 500 km south of Raivavae, have an enriched composition. There are no helium isotopic measurements from Raivavae. A couple of helium measurements exists from Rapa (Craig unpublished data; cited in Farley and Neroda, 1998) although the lithophile properties of these samples are not known. The frequent occurrence of HIMU and EM-type volcanism in close geographical proximity has been previously noted. Our helium isotopic composition, when combined with the existing geochemical data on the samples, will give us new information on the meaning of such an association. In addition, the proposed helium isotopic measurements will enable us to evaluate mixing relations between the different mantle components present under Polynesia and will, therefore, provide new insights into the origin of the low 3He/4He ratios of HIMU basalts. Only through careful studies involving helium and the isotopes of Sr, Nd, Pb, and Os measured on the same samples can the origin of low 3He/4He ratios in HIMU lavas be ascertained
ntrations obtained from basalt cobbles in excavations. We hypothesize that erosion of these flat outwash surfaces is much less than on the adjacent moraine slopes and that exposure ages will more closely relate to formation of the deposit.