Abstract
Pyroxenite xenoliths in the Salt Lake Crater nephelinites of the Koolau Shield (Oahu, Hawaii) consist of clinopyroxene + spinel + or - olivine + or - garnet + or - orthopyroxene + or - amphibole + or - phlogopite. Two successive generations of fluid inclusions are recognized: - (1) Inclusions containing superdense CO2 (+ or - H2O) (d = 1.21 g/cm3) are sometimes apatite-bearing, and occur only in clinopyroxene. Initial melting occurs precisely at the temperature of partial homogenization to liquid from -58.2 to -56.5 degrees C and final melting in the range -57.9/-50.8 degrees C. - (2) Secondary high-density (1.18-0.60 g/cm3) monophase CO2-rich inclusions occur in all minerals. Melting temperatures are recorded between -60.0 and -56.6 degrees C, while homogenization temperatures (L+V<<L) vary between -58.4 and +30.0 degrees C. The wide interval of melting temperatures in both generations suggests the presence of other species than CO2. However, Raman analyses did not detect any species other than CO2 (i.e. CH4, N2, H2, CO, H2S). The melting point depression is not due to Ar or admixtures with noble gases, as only a few ppm Ar were detected in the fluid. The 40Ar/36Ar in the inclusions is 359, similar to vesiculated basalts from Loihi Seamount, Hawaii. Early inclusions were formed during pyroxenite equilibration processes that occurred during isobaric cooling in the mantle; they are most likely related to the crystallization of pyroxenite veins at a depth of approximately 55 km (17 kb). We suggest that solute-rich CO2 + or - H2O fluids coexisted with a compositionally evolved, volatile-rich residual silicate melt. These fluids migrated into the lherzolites and may have been responsible for the metasomatic enrichment in LREE and Na that is observed in the mantle below Oahu. Late carbonic inclusions were formed during the ascent of nodules from a depth of = 50 km. They resulted from the trapping of the host magma with an immiscible carbonic fluid phase.
