Geochemistry of axial lavas from the mid-and southern Mariana Trough, and implications for back-arc magmatic processes

The Mariana Trough, a relatively simple intra-oceanic back-arc basin, is ideal for investigating magmatic processes and mantle- crust interaction in a subduction setting. We present new major- and trace element compositions for 31 basaltic lava and glass samples from the Mariana Trough back-arc spreading center. The studied lavas include phenocrysts of plagioclase, olivine and pyroxene. Major element compositions show that these lavas range from tholeiitic basalt to basaltic andesite, and belong to a sub- alkali tholeiitic series produced by fluid-influenced fractional crystallization of primary basaltic melts. Trace element composi- tions show that these lavas are transitional between typical normal mid-ocean ridge basalts (MORB) and island arc basalt (IAB), and are enriched in large ion lithophile elements (LILEs) and light rare earth elements (LREEs). Trace element ratios, e.g., Ba/Th, Pb/Ce, Th/Nd, La/Sm, Th/Nb, Ba/Nb and Th/Nb, indicate that the mantle from which these lavas were derived underwent modification resulting from the addition of multiple subduction components. Some typical trace element ratios (e.g., Ba/Nb- total subduction component, Ba/Th- shallow subduction, and Th/Nb-deep subduction component) from our new data and the literature suggest that a latitudinal variation exists in addition to subduction components, and indicates a more complex and heterogeneous distribution of subduction components in the Mariana back-arc region. We suggest that, (1) compared to back-arc locations at 18° N and 15.5° N, lavas from back-arc locations at 17° N indicate higher levels of modification by hydrous fluid released from the subducted slab, and (2) compared to back-arc locations at 17° N and 15.5° N, petrogenesis of lavas from back-arc locations at 18° N indicates a greater influence of sediment melt.