Mantle Heterogeneity beneath Oceanic Islands: Some Inferences from Isotopes

Radiogenic isotopes in oceanic basalts are extremely useful as tracers of long-lived heterogeneities in the Earth's mantle. Helium isotopes provide unique information in that high ³He/⁴He ratios are indicative of relatively undegassed mantle reservoirs (i.e. mantle with high time-integrated ³He/(Th+U) ratios). An alternative hypothesis is that high ³He/⁴He ratios may have been produced by ancient melting events, if the solid/melt partition coefficient (K_d) for He is greater than that for Th and U (i.e. yielding relatively high He/(Th+U) in the residue of melting). However, the distribution of helium within basaltic phenocrysts, and olivine/glass helium partitioning within mid-ocean ridge basalts, suggest that helium behaves as an incompatible element during melting (K_d (olivine/glass) < 0.0055), which strongly supports the hypothesis that high ³He/⁴He ratios are derived from undegassed mantle reservoirs. Isotopic measurements of He, Sr, and Pb in Hawaiian volcanoes lavas demonstrate that the mantle sources have changed on extremely short timescales, between 100 and 10000 years before present. The preferred explanation for these variations is that they represent heterogeneities within the Hawaiian mantle plume, combined with late stage melting in the lithosphere for post shield alkali basalts. Helium isotopic data from Kilauea, Hualalai and Mauna Loa suggest that the plume is presently located beneath Kilauea (and Loihi seamount), and constrain the melting zone of the Hawaiian plume to be less than 40 km in radius.