Helium isotopic systematics of oceanic islands and mantle heterogeneity

The results of helium isotopic analyses in basaltic phenocrysts from the islands of Gough and Tristan da Cunha are reported which seem to indicate that the mantle beneath these islands are not primitive or undepleted relative to mid-ocean ridge basalt (MORB). New analyses from Loihi Seamount are also reported which do indicate a more primitive source. When these data are combined with values for MORBs previously obtained and plotted with respect to 87Sr/86Sr, the observed trends offer insight into the different source regions for oceanic island basalts and the nature of mantle heterogeneity. (U.K.)