New Constraints on the HIMU Source from Helium and Neon Isotopic Compositions of Basalts from the Cook-Austral Islands

Low 3He/4He ratios found in HIMU ocean islands (high μ, where μ is the U/Pb ratio) are attributed to the presence of recycled oceanic crust. However, recycled crustal material should be essentially devoid of 3He and have high concentrations of 4He produced from the decay of U and Th. Thus, it is surprising that the average helium isotopic ratios at HIMU ocean islands are as high as 6-7 RA. To better constrain the origin of the 3He/4He ratios in HIMU basalts, we have measured He and Ne isotopic compositions in a suite of geochemically well-characterized basalts from the Cook-Austral Islands. Helium and neon isotopic measurements were made by crushing olivine phenocrysts in vacuo. Samples from Raivavae, Mangaia, and Tubuai have 3He/4He ratios that range from 5.1 to 7.2 RA. In contrast, samples from Rapa and young Rurutu have 3He/4He ratios that range from 8.4 to 12.7 RA, consistent with the more enriched Sr-Nd-Pb-Os isotopic signatures found in these islands. Neon isotopic compositions distinct from air have so far been detected at Raivavae, Mangaia, and Tubuai. The external precision on the isotopic ratios of 20Ne/^{22}Ne and 21Ne/^{22}Ne are small, 2‰ and 4‰ respectively. Thus, small isotopic anomalies can be detected precisely. 20Ne/^{22}Ne ratios range up to 10.95 and the neon isotopic compositions of the samples are either less radiogenic than MORB or fall on the MORB line. The combined He and Ne isotopic measurements demonstrate that the noble gas signature of HIMU basalts cannot arise either from simple diffusive equilibration of a subducted slab with a MORB mantle source, or from mixing of melts that are derived from subducted slabs and the MORB mantle source. We observe a good correlation between helium and osmium isotopic ratios (r=0.76) that further rules out a significant involvement of the MORB mantle. Rather, the correlation demonstrates that the entire spectrum of helium isotopic composition seen at the Cook-Austral Islands results from a mixture of a radiogenic component, probably recycled crust, with a less radiogenic component, probably FOZO or PHEM. Such a scenario also appears to be consistent with the neon isotopic signature in the Cook-Austral basalts.