HR: 0800h
AN: V41B-1384    [Abstracts]
TI: Hf isotope and concentration systematics of the Mariana arc
AU: * Tollstrup, D L
EM: darrent@es.ucsc.edu
AF: University of California, Department of Earth Sciences 1156 High St., Santa Cruz, CA 95064
AU: Gill, J B
EM: jgill@es.ucsc.edu
AF: University of California, Department of Earth Sciences 1156 High St., Santa Cruz, CA 95064
AB: Negative Hf concentration anomalies are common but little-discussed geochemical features of island arcs. Because both light rare earth elements (LREE) and Hf may be mobile even in `fluid-dominated' island arcs, it is important to relate their isotopic and elemental ratios to models of slab-mantle mixing. We report new Hf isotope and trace element data for K-rich submarine basalts from the Kasuga seamounts located 10-20 km behind the volcanic front of the southern Northern Seamount Province (NSP) of the Mariana arc. These data, when combined with published data for other Mariana samples, span the full range from low-K tholeiites to high-K shoshonites. Rear-arc Kasuga seamounts seamounts of the NSP have lower $^{143}$Nd/$^{144}$Nd and $^{176}$Hf/$^{177}$Hf ratios than arc-front volcanoes of the Mariana Central Island Province (CIP). Within the CIP, Hf concentration anomalies correlate positively with $^{176}$Hf/$^{177}$Hf ratios. Radiogenic Hf and little or no concentration anomalies characterize samples from fluid-dominated volcanoes (Guguan and Maug), whereas samples from sediment-melt dominated volcanoes (Anatahan and Sarigan) have less radiogenic Hf and larger concentration anomalies. Samples from the Kasuga and Hiyoshi seamounts have even larger negative concentration anomalies and less radiogenic Hf, although the two are not always correlated. These data are consistent with mixing between a depleted mantle and a partial melt of subducted sediment that is saturated with trace accessory phases including zircon, rutile, and monazite. A more volcaniclastic source is needed for the NSP than the CIP. Implications of these findings are three-fold. Partial melts of subducting sediment affect the HFSE and REE budgets of even fluid-dominated island arcs. Slab temperatures must be high enough for a peraluminous melt to be present, even where old, cold slabs are subducting. Refractory accessory phases have the potential to become exotic "nuggets" in the convecting mantle, potentially controlling the global budget of the affected elements.
DE: 3640 Igneous petrology
DE: 3670 Minor and trace element composition
DE: 1040 Isotopic composition/chemistry
DE: 1065 Trace elements (3670)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting