CAUSES OF SPATIAL COMPOSITIONAL VARIATIONS IN MARIANA ARC LAVAS - TRACE-ELEMENT EVIDENCE

New inductively coupled plasma mass spectrometry (ICP-MS) trace elemen t data are presented on a suite of are lavas from the northern Mariana and southern Bonin island arcs. The samples were dredged fi om seamou nts in the Central Island Province (CIP), the Northern Seamount Provin ce (NSP) and the Volcano Arc (VA), and they range in composition from low-K tholeiites to shoshonites. Previous studies on these samples con cluded that the primary compositional control was two-component mixing between a fluid-metasomatized mid-ocean ridge basalt (MORB) source an d an enriched, ocean island basalt (OIB)-like, mantle component, with subducted sediment material playing a secondary role. However, the new trace element data suggest that the compositional variations along th e Mariana are can be better explained by the addition of spatially var ying subduction components to a spatially varying mantle source. The d ata suggest that the subduction component in the CIP and VA is dominat ed by aqueous fluids derived from altered oceanic crust and a pelagic sediment component, while the subduction component in the NSP is domin ated by more silicic fluids derived from volcanogenic sediments as wel l as from pelagic sediment and altered oceanic crust. The mantle wedge in the CIP and VA is depleted relative to a normal mid-ocean ridge ba salt source by loss of a small melt fraction, while the mantle wedge i n the NSP is enriched either by possible gain of a small melt fraction or addition of a sediment-derived melt. Because the subduction of sea mounts controls the are and back-are geometries, so the concomitant va riation between subducted material and mantle composition may be no co incidence. The high field strength element (HFSE) data indicate a high degree of melting (similar to 25-30%) throughout the are, similar to 10% of which may be attributed to decompression and similar to 20% to fluid addition.