Host: The Japanese Association of Mineralogists, Petrologists and Economic Geologists
The 34 Ma alnoite from Malaita, Solomon Islands, contains a wide variety of mantle xenoliths regarded as fragments of the lithosphere beneath the Ontong Java Plateau. Most such xenoliths are spinel-bearing peridotites and classified into garnet-spinel lherzolite, spinel lherzolite and spinel harzburgite with low-equilibrated temperature (750-1100°C). This indicates that the xenoliths were composed of the shallower lithospheric mantle. Trace element compositions of clinopyroxene show good agreement for spot analyses of thin sections by SIMS, and bulk mineral analyses by ICP-MS and ID-TIMS, suggesting the chemical homogeneity of clinopyroxene within a single xenolith. In contrast, chondrite-normalized REE patterns of individual xenoliths differ extremely ranging from smooth LREE depleted, enriched LREE with inflection, to convex-upward REE patterns. The chemical diversity can be explained by the integration of following processes; (1) partial melting of an initially fertile and homogeneous peridotite, (2) subsequent metasomatic enrichment caused by a mobile melt/fluid, and (3) continuous inter-mineral redistributions accompanied by metamorphic recrystallization until host eruption.In order to constrain the origin of the source peridotite, we focused on Sr and Nd isotopic systematics of less-metasomatised samples (eight spinel lherzolites and one garnet-spinel lherzolite). Clinopyroxenes of these xenoliths show smooth LREE depleted patterns (except for La) and systematic depletion of Sr, Sm and Nd abundances. They have relatively depleted Sr-Nd isotopic compositions (present-day values: 87Sr/86Sr = 0.70237-0.70311 and 143Nd/144Nd = 0.51311-0.51331). Although no correlation can be found between two isotopic ratios, 143Nd/144Nd ratios are positively correlated with 147Sm/144Nd ratios, suggesting that these samples preserve the information about depletion age and initial 143Nd/144Nd ratio. By using mass-balance calculation, we evaluated bulk compositions of these two ratios for individual xenoliths with assumptions of isotopic equilibrium within single xenoliths at the time of host eruption, and negligible effect of metasomatism. Reconstructed bulk isochron yields a depletion age of 168±38 Ma with an intercept of εNd = 7.1. These values significantly deviate from those of overlying crust originated from the Ontong Java Plateau (121-125 Ma with εNd = 3.7-6.0), indicating that the peridotites are not genetically related with the Ontong Java Plateau basalts. We interpret that the peridotites were originated from residual to melt extraction of the Pacific MORB crust, which possibly exists beneath large volume of the Ontong Java Plateau basalts. The estimated age is consistent with those of neighboring crusts inferred from the magnetic reversal patterns (approximately 120-160 Ma). From these observations, we conclude that the shallow mantle beneath the Ontong Java Plateau was previously constructed at the mid-oceanic ridge, followed by subsequent emplacement of the Ontong Java Plateau at the off-ridge location.
