HIMU ocean islands (high 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 5-7 RA. This proposal requests funding to measure 3He/4He ratios in a suite of samples from Raivavae and Rapa from the Cook-Austral Islands to better understand the origin of the 3He/4He ratios in HIMU basalts. This suite of HIMU basalts 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. The frequent occurrence of HIMU and EM-type volcanism in close geographical proximity has been previously noted. The proposed helium isotopic measurements, 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 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.
Helium isotopic composition provides the strongest constraint on the existence of a primitive mantle reservoir. Understanding the transport behavior and the variations in helium isotopic composition in mantle-derived rocks is key to providing strong constraints on the origin and chemical evolution of the Earth as well as the nature and dynamics of mantle convection. In addition, the proposed research incorporates undergraduate student education and involves them in research at the forefront of mantle geochemistry. Early involvement in exciting research is important for recruiting and retaining talented individuals to the Earth Sciences
