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churikova

HR: 09:15h
AN: T31F-04
TI: The HFSE budget of arc magmas: new models from Hf isotopes and isotope dilution measurements of Nb/Ta, Zr/Hf and Lu/Hf in Kamchatka arc rocks
AU: * M\"{u}nker, C
EM: muenker@nwz.uni-muenster.de
AF: Institut f\"{u}r Mineralogie, Corrensstr. 24 , M\"{u}nster, 48149 Germany
AU: W\"{o}rner, G
AF: Geowissenschaftliches Zentrum der Universit„t G\"{o}ttingen GZG), Abt. Geochemie, Goldschmidtstr. 1, G\"{o}ttingen, 37077 Germany
AU: Churikova, T
AF: Institute of Volcanology and Geochemistry, Piip Avenue 9, Petropavlowsk, Russian Federation
AU: Mezger, K
AF: Institut f\"{u}r Mineralogie, Corrensstr. 24 , M\"{u}nster, 48149 Germany
AB: There is still no consensus as to whether the HFSE depletion in subduction rocks is caused by the immobile behaviour of these elements during slab dehydration or by the presence of residual accessory phases in the magma sources. To assess this problem, we analysed Nb/Ta, Zr/Hf and Lu/Hf by isotope dilution, together with Hf isotopes, in arc rocks from Kamchatka. Using a mixed $^{180}$Ta-$^{94}$Zr-$^{180}$Hf-$^{176}$Lu tracer and the MC-ICPMS in M�nster, we are able to achieve external precisions and accuracies of $\pm$0.5 to $\pm$1$\%$ for Lu/Hf and Zr/Hf and of $\pm$5$\%$ for Nb/Ta (2$\sigma$ uncertainties). In contrast to older techniques (e.g. quadropole ICPMS), this analytical protocol results in a nearly 10-fold improvement in analytical resolution. The investigated suite of Kamchatka arc rocks comprises a cross-arc transect at $56\deg$ (element budget largely controlled by variable fluid flux into the subarc mantle, Dohrendorf et al. 2000) and a suite from volcanoes in the Northern Central Kamchatka depression (NCKD, largely controlled by slab melts, Yogodzinski et al. 2001). Coupled Hf-Nd isotope variations ($\epsilon$Hf = 12-18, $\epsilon$Nd = 6-10) in all samples from the central Kamchatka depression (CKD) and from the back-arc suggest mixing between an OIB source and a MORB source in the wedge. However, samples from the arc front are slightly displaced from the Hf-Nd array towards less radiogenic Nd, indicating selective addition of minor sediment derived Nd to the mantle wedge by fluids high in Nd/Hf. The Zr/Hf (30-42) in all arc rocks are anti-correlated with Lu/Hf ($^{176}$Lu/$^{177}$Hf= 0.01-0.03), suggesting that the budget of Zr and Hf is controlled by the degree of mantle depletion rather than by the slab component. NCKD samples are slightly offset from the southern array towards lower Lu/Hf-Zr/Hf, suggesting that Zr-Hf is controlled by slab melts. The back-arc samples show superchondritic Zr/Hf ($>$35), consistent with the presence of an OIB source component (typical Zr/Hf$>$35) in their source. Nb/Ta in the Kamchatka rocks range from 11-18 and are decoupled from Zr/Hf and Lu/Hf. The samples from the arc front show an increase in Nb/Ta with Sr/Nd and Ba/Th, indicating that the Nb-Ta budget in these samples is controlled by fluids derived from the subducted slab. Samples from the NCKD (slab melt controlled) overlap in their Nb/Ta and Zr/Hf with samples from the CKD further south (fluid controlled). Such overlapping HFSE patterns, similar in range to MORB, suggest that accessory phases in the slab, if present, can only cause minor Zr/Hf and Nb/Ta fractionation in the subarc fluids or melts. References Dorendorf, F., Wiechert, U. and Worner, G. (2000): Earth and Planetary Science Letters 175: 69-86. Yogodzinski, G.M., Lees, J.M., Churikova, T., Dorenforf, F., Worner, G., and Volynets, O.N., (2000): Nature 409, 500-504.
DE: 1040 Isotopic composition/chemistry
DE: 1065 Trace elements (3670)
DE: 3640 Igneous petrology
SC: T
MN: 2001 AGU Fall Meeting