研究業績

Olivine-free metasomatic mantle-derived xenoliths, frequently recovered from kimberlite and lamprophyre intrusions on the continents, are seldom described from oceanic settings. We report the mineralogy, geochemistry and Sr-Nd-Hf-Pb-Os isotopic compositions of a unique K-rich, hydrous mantle nodule sourced from a 34?Ma ultramafic lamprophyre (aln?ite) pipe on the island of Malaita, Ontong Java Plateau, SW Pacific. The Ontong Java Plateau, the most voluminous oceanic large igeous province, has a >100?km thick lithospheric root reminiscent of thick lithospheres that characterize cratonic settings. The phlogopite, amphibole, clinopyroxene and ilmenite-rich nature of the Malaita metasomatic nodule bears striking similarity to cratonic mica-amphibole-rutile-ilmenite-diopside (MARID) suite xenoliths, and it provides a tangible example of heavily overprinted oceanic mantle lithosphere. The nodule phlogopite 40Ar/39Ar age of 44.7?±?1.8?Ma (95% confidence level) predates the 34?Ma aln?ites, but is contemporaneous with 44?Ma alkali basalts on Malaita. Geodynamic reconstructions of the Ontong Java Plateau position within the Pacific realm demonstrate that aln?ite magma and K-rich metasomatic nodule formation occurred within a strictly oceanic environment during the Eocene, away from subduction zones. The elevated incompatible trace element concentrations coupled with low highly siderophile element contents suggest that the K-rich metasomatic nodule formed by olivine-absent crystallisation from low-volume mantle-derived melt comparable to aln?ite, but not the Malaita alkali basalts. A genetic link between the Malaita metasomatic nodule and aln?ite is further suggested by overlapping Sr-Nd-Hf-Pb isotopic compositions (87Sr/86Sr45Ma?=?0.70419?0.70423; εNd45Ma?=?+3.5; εHf45Ma?=?+5.3; 206Pb/204Pb45Ma?=?18.66?18.71; 207Pb/204Pb45Ma?=?15.61). These isotopic compositions are generally more enriched than those of mantle-derived peridotites and 122?Ma plateau-building basalts at Ontong Java, but share similarities with pyroxenite xenoliths from Malaita previously interpreted to represent ancient recycled crustal material. Mixing models between melts derived from fertile mantle and the more enriched pyroxenite, as well as recycled sedimentary material, can account for the composition of the K-rich metasomatic nodule. Extremely low contents of highly siderophile elements and high 187Os/188Os45Ma (0.1824?0.1997) can also be reconciled with the involvement of recycled crustal components in the complex origin of the K-rich hydrous nodule. K-rich hydrous metasomatized mantle lithosphere could represent an enriched source component that is required for the formation of many oceanic intraplate basaltic provinces. As such, strongly metasomatized oceanic lithosphere provides an alternative to recycled crust components within the convecting mantle to explain enriched mantle signatures observed in oceanic basalts worldwide. However, constraining the volumetric abundance of strongly metasomatized oceanic lithosphere, possibly revealed by seismic mid-lithospheric discontinuities, is required before its importance can be established. The existence of K-rich metasomatic components within oceanic plateau lithosphere may also have implications for Archean continental crust formation models. The K-rich mantle component required to explain sanukitoid magma formation associated with continental crust building TTG magmatism may not be restricted to a subduction channel environment, but it can equally reside within overthickened oceanic lithosphere. Thus, oceanic plateaus away from subduction zones could have served as nucleation points for growth of continental crust during the Archean.