NEW HE, ND, PB, AND SR ISOTOPIC CONSTRAINTS ON THE CONSTITUTION OF THE HAWAIIAN PLUME - RESULTS FROM KOOLAU VOLCANO, OAHU, HAWAII, USA

Most analyzed tholeiitic basalts from Koolau Volcano. Oahu, USA, have strontium (Sr-87/Sr-86 = 0.7040-0.7043), neodymium (Nd-143/Nd-144 0.51270-0.51276), and lead (Pb-206/Pb-204 = 17.8-17.9) isotopic compositions near that of the bulk silicate earth, and He-3/He-4 isotopic ratios of 11-14 times the atmospheric ratio. These helium ratios are higher than MORB, but lower than those of lavas from Loihi seamount. Moreover, the source for the Koolau tholeiites is inferred to have non-bulk earth abundance ratios of highly incompatible elements. Consequently, the source of the Koolau lavas is not primitive, undegassed mantle. The abundance ratios La/Nb, Zr/Nb, and Sr/Nb correlate with Sr-87/Sr-86 and Nd-143/Nd-144 ratios in Hawaiian tholeiites. The enriched (Koolau) source component has relatively high La/Nb, Zr/Nb, and Sr/Nb ratios; in fact, Koolau tholeiites have higher Zr/Nb and La/Nb, and lower Th/Nb than most other OIB. These combined trace element and isotopic signatures of the enriched component are not consistent with derivation from primitive mantle, recycled crustal material, or a carbonatite metasomatized source. A simple explanation is that the enriched component is residual material, formed recently when a small amount of melt was extracted from primitive mantle, perhaps during the incorporation of the Koolau component into the plume.