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

摘要

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 87Sr/86Sr (0.70309–0.70346) and 143Nd/144Nd (0.512967–0.513206). The mantle lithosphere beneath the Koolau volcano has a range of present-day epsilonNd = 6.9–11. On a 143Nd/144Nd versus 147Sm/144Nd diagram, they define a 61 ± 20 Ma errorchron, within error of the estimated age of ~80–85 Ma of the Pacific lithosphere at Oahu, and an intercept of epsilonNd = 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 87Sr/86Sr isotopic disequilibrium with coexisting clinopyroxenes. The plagioclases are light rare earth element depleted and have “enriched” 87Sr/86Sr ratios (0.70436 and 0.70443). The delta18O of olivine (5.09–5.12‰) and clinopyroxene (5.32–5.33‰) from spinel peridotites are typical for oceanic upper mantle rocks. In contrast, delta18O of olivine, pyroxene, and plagioclase in plagioclase-bearing peridotites are enriched by ~0.5‰ relative to the spinel peridotites (e.g., delta18O ratios of olivines from plagioclase peridotites are 5.4–5.5‰). 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 delta18O and 87Sr/86Sr of the plagioclase resemble Koolau lavas and suggest that the melt passing through the lithosphere was plume-related. The elevated delta18O 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, ~10^2–10^4 years.