Contributions of slab fluid and sediment melt components to magmatism in the Mariana Arc-Trough system: Evidence from geochemical compositions and Sr, Nd, and noble gas isotope systematics

摘要

This study presents new major and trace element, mineral, and Sr, Nd, and noble gas isotope geochemical analyses of basalts, gabbro, and clinopyroxenite from the Mariana Arc (Central Islands and Southern Seamount provinces) including the forearc, and the Mariana Trough (Central Graben and Spreading Ridge). Mantle source compositions beneath the Mariana Arc and the Mariana Trough indicate a mantle source that is depleted in high field strength elements relative to MORB (mid-oceanic ridge basalt). Samples from the Mariana Arc, characterized by high ratios of Ba/Th, U/Th, Kr-84/He-4 and Xe-132/He-4, are explained by addition of fluid from the subducted slab to the mantle wedge. Correlations of noble gas data, as well as large ion lithophile elements, indicate that heavy noble gases (Ar, Kr, and Xe) provide evidence for fluid fluxing into the mantle wedge. On the other hand, major elements and Sr, Nd, He, and Ne isotopic data of basalts from the Mariana Trough are geochemically indistinguishable from MORB. Correlations of He-3/He-4 and Ar-40/Ar-36 in the Mariana Trough samples are explained by mixing between MORB and atmosphere. One sample from the Central Graben indicates extreme enrichment in Ne-20/Ne-22 and Ne-21/Ne-22, suggesting incorporation of solar-type Ne in the magma source. Excess Xe-129 is also observed in this sample suggesting primordial noble gases in the mantle source. The Mariana Trough basalts indicate that both fluid and sediment components contributed to the basalts, with slab-derived fluids dominating beneath the Spreading Ridge, and that sediment melts, characterized by high La/Sm and relatively low U/Th and Zr/Nb, dominate in the source region of basalts from the Central Graben.