Helium isotope ratios in Easter microplate basalts

Abstract

3He/ 4He ratios in Easter Microplate basalt glasses show clear evidence of the effects of a mantle plume. The East Rift of the microplate between 26 and 28°S, identified by La/Sm, Sr and Pb isotopes and ridge crest elevation as the region of maximum plume influence, has 3He/ 4He ratios spanning the entire range from 7.5 to 11.7 R A. The Easter Microplate is the only section of the entire East Pacific Rise that is associated with a known ‘hotspot’ track (mantle plume) and has elevated 3He/ 4He ratios. Although most of the West Rift basalts contain MORB helium (8.0–8.7 R A), the basalt closest to the East Rift has an elevated 3He/ 4He ratio (11.3 R A), consistent with a significant plume component. The diversity in isotopic signatures also indicates that homogenization of isotopic anomalies does not occur, even in this region of ‘super-fast’ spreading. The overall 3He/ 4He/ 206Pb/ 204Pb and 3He/ 4He 87Sr/ 86Sr trends have positive correlations, although the high 3He/ 4He, 87Sr/ 86Sr and 206Pb/ 204Pb component is not a single value but rather a broad band. The apparent ‘decoupling’ between the He and Sr isotope distribution is modeled in the context of a plume source-migrating ridge sink. During channeling of the plume toward the ridge, helium is preferentially lost from the center of the channeled plume, resulting in lower He/Pb and He/Sr concentration ratios in the high 3He/ 4He component. Mixing trajectories in He Sr isotopic space between a LILE depleted asthenosphere and a variably degassed plume component provide a reasonably good fit to the data and may explain the isotope systematics of plume-ridge interactions in the context of modern theories of plume dynamics.