Melt depletion and subsequent metasomatism in the shallow mantle beneath Koolau volcano, Oahu (Hawaii)

[1] Xenoliths from Pali (Oahu, Hawaii) include samples of the mantle lithosphere underlying Koolau shield volcano. Most such xenoliths are spinel peridotites, the remainder being plagioclase-spinel peridotites, and garnet-free pyroxenites. Clinopyroxene separates from Pali peridotite xenoliths have relatively depleted Sr-87/Sr-86 (0.70309-0.70346) and Nd-143/Nd-144 (0.512967-0.513206). The mantle lithosphere beneath the Koolau volcano has a range of present-day epsilon(Nd) = 6.9-11. On a Nd-143/Nd-144 versus Sm-147/Nd-144 diagram, they define a 61 +/- 20 Ma errorchron, within error of the estimated age of similar to 80-85 Ma of the Pacific lithosphere at Oahu, and an intercept of epsilon(Nd) = 6.8. We interpret the Pali spinel peridotites as samples of the Pacific lithosphere residual to melt extraction of the Pacific crust. These rocks were not metasomatized by melts having isotopic composition similar to the shield-building Koolau volcano, but they could have been metasomatized by melts related to the Honolulu Volcanic Series. Plagioclase mineral separates from two peridotites are in Sr-87/Sr-86 isotopic disequilibrium with coexisting clinopyroxenes. The plagioclases are light rare earth element depleted and have "enriched'' Sr-87/Sr-86 ratios (0.70436 and 0.70443). The delta(18)O of olivine (5.09-5.12parts per thousand) and clinopyroxene (5.32-5.33parts per thousand) from spinel peridotites are typical for oceanic upper mantle rocks. In contrast, delta(18)O of olivine, pyroxene, and plagioclase in plagioclase-bearing peridotites are enriched by similar to 0.5parts per thousand relative to the spinel peridotites (e.g., delta(18)O ratios of olivines from plagioclase peridotites are 5.4-5.5parts per thousand). It is here interpreted that the plagioclases represent phases that formed by reaction with or precipitation from a melt that intruded the Pacific lithospheric mantle. The Pacific lithospheric isotopic composition is reflected in the clinopyroxene separates. The high delta(18)O and Sr-87/Sr-86 of the plagioclase resemble Koolau lavas and suggest that the melt passing through the lithosphere was plume-related. The elevated delta(18)O of the plagioclase indicates that the melt from which it fractionated had a component of recycled crustal material not acquired from local crustal contamination. It is also estimated here that the duration of the porous melt flow of the Koolau-like melt though the mantle lithosphere was short-lived, similar to 10(2)-10(4) years.