HR: 1340h
AN: V13A-1458    [Abstracts]
TI: Hf-Nd Isotope Characterization of the Tonga-Kermadec Sub-Arc Mantle
AU: * Worthington, T J
EM: tw@gpi.uni-kiel.de
AF: Institut fuer Geowissenschaften, Universitaet Kiel, Olshausenstr. 40, Kiel, D-24098 Germany
AU: Muenker, C
EM: muenker@nwz.uni-muenster.de
AF: Institut fuer Mineralogie, Universitaet Mnster, Correnstr. 24, Muenster, D-48149 Germany
AU: Stoffers, P
EM: pst@gpi.uni-kiel.de
AF: Institut fuer Geowissenschaften, Universitaet Kiel, Olshausenstr. 40, Kiel, D-24098 Germany
AU: Gamble, J A
EM: j.gamble@ucc.ie
AF: Dept. of Geology, National University of Ireland, University College, Cork, 000 Ireland
AU: Wright, I C
EM: i.wright@niwa.cri.nz
AF: National Institute of Water and Atmospheric Research, PO Box 14-901, Wellington, 6009 New Zealand
AB: Subduction-related magmatism involves fluxing of the mantle wedge by a slab-derived fluid and/or melt. Uncertainty surrounds the thermal, physical and chemical state of the slab and slab-wedge interface beneath the volcanic front, creating doubt over the application of experimental data on slab dehydration and melting to models of magma genesis and mass balance. Constraining the composition of the wedge should provide an important control on such models. However, simple models of wedge convection from the backarc to sub-arc environment are clearly incomplete (e.g. do not explain the lack of correlation between depletion of the sub-arc mantle and either backarc width or extension rate). To investigate the dynamics of wedge convection, we undertook Hf-Nd isotope determinations for lavas spanning most of the Tonga-Kermadec arc. Advantages of these elements include their fluid-immobile character (Hf strongly, Nd moderately) relative to the highly mobile, and thus predominantly slab-derived, Sr and Pb. Previous Sr-Nd-Pb isotope studies of backarc Lau Basin lavas revealed that Indian MORB mantle (IMM) replaced Pacific MORB mantle (PMM) during basin opening, and that IMM underlies the central Tonga arc. In contrast, the backarc mantle south of Valu Fa Ridge and under the Havre Trough remains PMM. Thus, a transition from IMM to PMM was anticipated beneath the south Tonga arc. Hf isotope compositions of the Tonga-Kermadec arc lavas range from +12 to +16 $\epsilon$-units and are 1-2 $\epsilon$Hf units below those of the Lau Basin spreading centres. Tonga lavas are generally in the upper part of this range, whereas Kermadec lavas are more varied. All Tonga-Kermadec lavas plot within the IMM field in $\epsilon$Hf vs $\epsilon$Nd-space, indicating IMM-like mantle is under the entire arc from $15-35\deg$S. This result is further enhanced by subtracting the $\sim$20-40 % Nd added from the subducting slab (PMM-composition). $\epsilon$Hf exhibits a mild negative correlation with Hf/Yb and increasing latitude, consistent with a minor Hf contribution from subducting volcanigenic sediment in the Kermadec arc sector. Our data provide no evidence for a slab melt, even in the southern Kermadecs where the slab is warmer due to slow subduction. The occurrence of IMM beneath the Tonga-Kermadec arc, but PMM below the southern Lau Basin-Havre Trough, is inconsistent with simple models invoking progressive southeastward migration of IMM into the Tonga-Kermadec system accompanying the opening of the Lau-Havre backarc basin. Instead, considerably complexity is required in the geometry of the IMM-PMM interface, the sub-arc wedge flow, or the origin of the IMM-like signature.
DE: 3640 Igneous petrology
DE: 3670 Minor and trace element composition
DE: 1025 Composition of the mantle
DE: 1040 Isotopic composition/chemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting