HELIUM ISOTOPE RATIOS IN EASTER MICROPLATE BASALTS

He-3/He-4 ratios in Easter Microplate basalt glasses show clear eviden ce of the effects of a mantle plume. The East Rift of the microplate b etween 26 and 28-degrees-S, identified by La/ Sm, Sr and Pb isotopes a nd ridge crest elevation as the region of maximum plume influence, has He-3/He-4 ratios spanning the entire range from 7.5 Lo 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 ha s elevated He-3/He-4 ratios. Although most of the West Rift basalts co ntain MORB helium (8.0-8.7 R(A)), the basalt closest to the East Rift has an elevated He-3/He-4 ratio (11.3 R(A)), consistent with a signifi cant plume component. The diversity in isotopic signatures also indica tes that homogenization of isotopic anomalies does not occur, even in this region of 'super-fast' spreading. The overall He-3/He-4-Pb-206/Pb -204 and He-3/He-4-Sr-87/Sr-86 trends have positive correlations, alth ough the high He-3/He-4, Sr-87/Sr-86 and Pb-206/Pb-204 component is no t a single value but rather a broad band. The apparent 'decoupling' be tween the He and Sr isotope distribution is modeled in the context of a plume source-migrating ridge sink. During channeling of the plume to ward the ridge, helium is preferentially lost from the center of the c hanneled plume, resulting in lower He/Pb and He/Sr concentration ratio s in the high He-3/He-4 component. Mixing trajectories in He-Sr isotop ic space between a LILE depleted asthenosphere and a variably degassed plume component provide a reasonably good fit to the data and may exp lain the isotope systematics of plume-ridge interactions in the contex t of modern theories of plume dynamics.