Mantle fluids in pyroxenite nodules from Salt Lake Crater (Oahu, Hawaii)

Abstract

Pyroxenite xenoliths in the Salt Lake Crater nephelinites of the Koolay Shield (Oahu, Hawaii) consist of clinopyroxene + spinel ± olivine ± garnet ± orthopyroxene ± amphibole ± phlogopite. Two successive generations of fluid inclusions are recognized: #75(1) Inclusions containing superdense CO 2 (±H 2 O) (d=1.21 g/cm 3 ) 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 o C and final melting in the range −57.9/−50.8 o C. #75(2) Secondary high-density (1.18-0.60 g/cm 3 ) monophase CO 2 -rich inclusions occur in all minerals. Melting temperatures are recorded between −60.0 and −56.6 o C, while homogenization temperatures (L+V→L) vary between −58.4 and +30.0 o C. The wide interval of melting temperatures in both generations suggests the presence of other species than CO 2 . However, Raman analyses did not detect any species other than CO 2 (i.e. CH 4 , N 2 , H 2 , CO, H 2 S). 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 40 Ar/ 36 Ar 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 ≃55 km (17 kb). We suggest that solute-rich CO 2 ±H 2 O fluids coexisted with a compositionally evolved, volatile-rich residual silicate melt. These fluids migrated into the Iherzolites 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