HR: 1340h
AN: V43D-1632    [Abstracts]
TI: New constraints from Tonga-Kermadec on the origin of O-Hf-Os isotope signatures in oceanic arc lavas
AU: * Turner, S
EM: sturner@els.mq.edu.au
AF: GEMOC, Department of Earth and Planetary Sciences, Macquarie University, Sydney, NSW 2109, Australia
AU: Handler, M
EM: monica.handler@vuw.ac.nz
AF: School of Geography, Environment and Earth Sciences, Victoria University of Wellington, Wellington, PO Box 600, New Zealand
AU: Bindeman, I
EM: bindeman@uoregon.edu
AF: Department of Geological Sciences, University of Oregon, Eugene, OR 97403, United States
AB: O, Hf and Os isotope ratios have been measured on selected lavas from the Tonga-Kermadec arc that have previously been analysed for Be isotopes which provides an unambiguous tracer of subducted sediment. O isotope values overlap with those of MORB and indicate minimal interaction with the arc crust. O isotopes do not increase northwards with increasing subduction rate as would be expected if slab derived fluids had high O isotope ratios. The northward decrease in HFSE concentrations reflects prior depletion not increasing extents of melting. Hf isotopes are strongly negatively correlated with Be isotopes. Simple binary mixing of subducted pelagic sediment into the mantle wedge can replicate the Hf-Be isotope correlation, without recourse to fluid mobility of Hf. However, the sediment component must have been stored in the mantle wedge for several Myr, as suggested previously. The same mixing model can replicate those samples with the lowest Os isotopes and one sample has subchondritic Os indicating that some melt depletion of the mantle wedge pre-dates the opening of the Lau Basin. Thus, some slab-derived Os isotope signatures are preserved which requires rapid magma ascent to minimise melt – wall rock interaction. Higher Os isotope ratios in some samples may reflect interaction with the arc crust. There is no overlap between the Hf isotope composition of the north Tonga arc lavas and either the Samoan plume or the subducting Louisville volcaniclastic sediments. Thus, the unusual Pb and Nd isotope signatures in these lavas must derive from the Louisville sediments but be added with minimal mobilisation of Hf indicating conservative behaviour for this element.
DE: 1009 Geochemical modeling (3610, 8410)
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting