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 0.51 270-0.51276), and lead (Pb-206/Pb-204 = 17.8-17.9) isotopic compositio ns 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 ea rth abundance ratios of highly incompatible elements. Consequently, th e 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 an d Nd-143/Nd-144 ratios in Hawaiian tholeiites. The enriched (Koolau) s ource 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 signatu res of the enriched component are not consistent with derivation from primitive mantle, recycled crustal material, or a carbonatite metasoma tized source. A simple explanation is that the enriched component is r esidual material, formed recently when a small amount of melt was extr acted from primitive mantle, perhaps during the incorporation of the K oolau component into the plume.