ELEMENT TRANSPORT FROM SLAB TO VOLCANIC FRONT AT THE MARIANA ARC

We present a comprehensive geochemical data set for the most recent vo lcanics from the Mariana Islands, which provides new constraints on th e timing and nature of fluxes from the subducting slab, The lavas disp lay many features typical of island are volcanics, with all samples sh owing large negative niobium anomalies and enrichments in alkaline ear th elements and lead (e.g., high Ba/La and Pb/Ce). Importantly, many o f these key ratios correlate with a large range in U-238 excesses, (U- 238/Th-230) = 0.97-1.56. Geochemical features show island to island va riations; lavas from Guguan have the largest U-238-excesses, Pb/Ce and Ba/La ratios, while Agrigan lavas have small U-238 excesses, the leas t radiogenic Nd-143/Nd-144, and the largest negative cerium and niobiu m anomalies. These highly systematic variations enable two discrete sl ab additions to the subarc mantle to be identified. The geochemical fe atures of the Agrigan lavas are most consistent with a dominant subduc ted sediment contribution. The added sedimentary component is not iden tical to bulk subducted sediment and notably shows a marked enrichment of Th relative to Nb. This is most readily explained by melt fraction ation of the sediment with residual rutile and transfer of sedimentary material as a melt phase. For most of the highly incompatible element s, the sedimentary contribution dominates the total elemental budgets of the lavas. The characteristics best exemplified by the Guguan lavas are attributed to a slab-derived aqueous fluid phase, and Pb and Sr i sotope compositions point toward the subducted, altered oceanic crust as a source of this fluid. Variable addition of the sedimentary compon ent, but near-constant aqueous fluid flux along are strike, can create the compositional trends observed in the Mariana lavas. High field st rength element ratios (Ta/Nb and Zr/Nb) of the sediment poor Guguan la vas are higher than those of most mid-oceanic ridge basalts and sugges t a highly depleted subarc mantle prior to any slab additions. The U-2 38-(230)Tn systematics indicate >350 kyr between sediment and mantle m elting but <30 kyr between slab dehydration and eruption of the lavas. This necessitates rapid magma migration rates and suggests that the a queous fluid itself may trigger major mantle melting.