Element Transport Processes Beneath the Mariana Arc

Tim Elliott Faculteit Aardwetenschappen, Vrije Universiteit, de Boelelaan 1085, 1081 HV Amsterdam, Netherlands


Terry Plank Department of Geology, 120 Lindley Hall, University of Kansas, Lawrence, KS 66045-2124, USA

Alan Zindler National High Magnetic Field Laboratory and Department of Geology, Florida State University, Tallahassee, FL 32306-4005, USA

An extensive suite of sub-aerial, recent volcanics (basalts and basaltic andesites) from the Mariana arc have been analysed for major and trace elements by XRF and ICP-MS, and for radiogenic and U-series isotopes. This high quality dataset reveals unusually coherent geochemical systematics, that provide important insights into the processes at work in the Mariana sub-arc.

The volcanics are derived from melting of an originally very depleted mantle source to which separate components from subducted sediment and altered oceanic crust have been added. For most of the highly incompatible elements considered in this study, the sedimentary contribution is the dominant in the elemental budgets of the lavas. Thus, within-arc chemical variations are largely the result of variable additions of sedimentary derived melt to the sub-arc mantle. The added sedimentary component requires significantly fractionated trace element ratios compared to bulk subducted sediment, in particular, marked enrichments of large ion lithophile elements (LILEs) and Th relative to high field strength elements (HSFEs). These features are readily explained if sedimentary material is transported to the mantle wedge as a melt phase derived from a protolith containing residual rutile. Elemental and isotopic systematics rule out melt percolation as a significant cause of high LILE/HSFE ratios in the Mariana lavas.

The aqueous fluid fluxed into the sub-arc mantle has a Sr and Pb isotopic signatures compatible with derivation from altered, basaltic oceanic crust. The aqueous fluid contributes significantly to the Ba and Pb budget of the lavas, and also is somewhat U enriched, resulting in 238U excesses in the lava sources and ultimately the lavas. Volcanics derived from the least sediment enriched sources show the greatest 238U excesses which is consistent with a near constant fluid flux into a variably sediment enriched mantle. The array of historic Mariana lavas define a pseudo-isochron of 30ka. This implies a very short time between aqueous fluid addition and eruption of melts at the surface, which would require melt velocities of some 4mm/a. 238U-230Th systematics also permit the some constraints on the timing of sediment melt addition, which must be at least 350ka prior to major mantle melting.