HR: 0800h
AN: V41D-1494    [Abstracts]
TI: U-series Disequilibria in Ocean Island Basalts: Competing Roles of Mantle Mineralogy and Melting Processes
AU: * Prytulak, J
EM: J.Prytulak@bristol.ac.uk
AF: Department of Earth Sciences, University of Bristol, Wills Memorial Building Queens Road, Bristol, BS8 1RJ United Kingdom
AU: Elliott, T
EM: Tim.Elliott@bristol.ac.uk
AF: Department of Earth Sciences, University of Bristol, Wills Memorial Building Queens Road, Bristol, BS8 1RJ United Kingdom
AU: Heumann, A
EM:
AF: Univerisity of Goettingen, Goldschmidtstr 3, Goettingen, 37077 Germany
AU: Blichert-Toft, J
EM:
AF: Ecole Normale Superiure de Lyon, Cedex, Lyon, 69364 France
AB: Conventional radiogenic isotopes provide clear evidence for the presence of heterogeities in the mantle but not their form or mineralogy. Mineralogy and melting are intimately linked, and investigation of melting process allows us to test the hypothesis that recycled eclogite `plums'[1] ubiquitously populate the mantle, imparting isotopic enrichment to the source of OIB. Eclogitic `plums' are an order of magnitude more productive during decompression melting than their host periodotite [2], and so contribute disproportionally to melts. U-series disequilibria should be particularly sensitive to the differences in melting rates between plums and peridotite. We use this approach, in conjunction with other geochemical indices, to constrain melting processes and mantle mineralogy beneath ocean islands. We present new U-Th-Pa-Ra disequilibria in primitive alkali OIB from Sao Miguel (SM) and Pico islands of the Azores archipelago. SM has been the focus of numerous studies, as it shows remarkable geochemical variation over small length scales [3]. The isotopically depleted end of SM has an equivalent Hf-Nd isotopic signature to depleted Pico samples. Therefore, analyzing samples encompassing the range of isotopic variability within SM and contrasting Pico with isotopically complementary samples from SM are two ideal tests of the effect of changing mantle composition and melting regime beneath the Azores We find that despite the pronounced range in Sr, Nd, Hf and Pb isotopes in SM, the primary U-Th and U-Pa disequilibria show little variation, with excesses of ~15% and ~50%, respectively. The incongruity of disequilibria with other geochemical parameters argues strongly against plums carrying the enriched signature in the SM source. The disequilibria from Pico are more puzzling. Indicators such as δ_(Lu/Hf) and δ_(Sm/Nd) along with trace element modeling, suggest Pico results from a larger degree of melt than SM, consistent with its position on younger, thinner lithosphere. However, the Th excesses measured in Pico (23%) are higher than SM whilst the Pa excesses (30%) are lower. Such variation is neither compatible with net U-Th fractionation at larger degrees of melting nor higher melting rates towards the centre of the Azores melting anomaly as has previously been suggested. A combination of different source mineralogy and melting parameters is apparently required to account for the differences between the isotopically similar SM and Pico lavas. [1] Morgan and Morgan, 1999 [2] Pertermann and Hirshmann, 2003 [3] Hawkesworth et al., 1979
DE: 1009 Geochemical modeling (3610, 8410)
DE: 1025 Composition of the mantle
DE: 1037 Magma genesis and partial melting (3619)
DE: 1038 Mantle processes (3621)
DE: 1040 Radiogenic isotope geochemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005