On the formation of cumulates, characteristics of oceanic lithosphere, and the process of carbonatite metasomatism: A study of ultramafic xenoliths from South Pacific islands.

摘要

ltramafic xenoliths, which are abundant in lavas on South Pacific islands in the Samoan, Society and Marquesas chains, include samples of both residual mantle harzburgite and cumulates. Based on petrography, samples were selected for analysis of major and trace elements--for bulk-rock by XRF and ICP-MS, and for individual mineral grains by electron and ion microprobe. Residual and cumulate xenoliths can be distinguished in thin section and by distinct differences in phase chemistry (e.g., residual olivines have Fo 90-92 and clinopyroxenes have TiO;The cumulate xenoliths are differentiates of hotspot lavas, mostly from alkalic melts, but a few crystallized from tholeiites. This supports the hypothesis that South Pacific hotspot volcanoes are formed largely of tholeiitic shields, which must be predominately submerged since tholeiitic lavas are rare on these volcanoes.;Percentages of trapped intercumulus melt were calculated from rare earth element data for the Ua Huka cumulate dunites, wehrlites and olivine clinopyroxenites. These rocks are near-perfect adcumulates (i.e., have almost no trapped melt), thus demonstrating the extraordinary efficacy of melt expulsion by compaction and recrystallization during cooling beneath an oceanic island.;The residual xenoliths are pieces of abyssal peridotite--very shallow mantle which underwent extensive partial melting at a mid-ocean ridge. Mineral compositions of the Tahitian residual xenoliths are indistinguishable from those in Atlantic and Indian Ocean abyssal peridotites, but modally the Tahitian xenoliths are as refractory as abyssal harzburgites obtained near hotspots (i.e.,