New Samoan lavas from Ofu Island reveal a hemispherically heterogeneous high 3He/4He mantle
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w New measurements of high He-3/He-4 ratios in Samoan lavas from Ofu Island (19.5-33.8 times atmospheric) extend the known range for He-3/He-4 in the southern hemisphere mantle. The Ofu data suggest that the high He-3/He-4 mantle component thought to be common to all oceanic hotspots, called FOZO (Focus Zone), is not homogeneous. Sr, Nd and Pb isotopes in Ofu lavas indicate that the Samoan high He-3/He-4 component is isotopically distinct from the high He-3/He-4 lavas from Hawaii, Iceland and Galapagos. Along with Samoa, the highest He-3/He-4 sample from each southern hemisphere high He-3/He-4 hotspot exhibits lower Nd-143/Nd-144 ratios than their counterparts in the northern hemisphere (excluding lavas erupted in continental, back-arc, and submarine ridge environments). The observation of a large-scale isotopic enrichment (generally higher Sr-87/Sr-86 and lower Nd-143/Nd-144) in the FOZO-A (austral) high He-3/He-4 mantle compared to the FOZO-B (boreal) high He-3/He-4 mantle is similar to the DUPAL anomaly, a globe-encircling feature of isotopic enrichment observed primarily in southern hemisphere ocean island basalts. Additionally, the recent discovery that terrestrial samples have Nd-142/Nd-144 ratios higher than chrondrites has potentially important implications for the origin of the FOZO reservoirs, and suggest that the high He-3/He-4 mantle has been re-enriched. (c) 2007 Elsevier B.V. All rights reserved.