HR: 1340h
AN: V33A-1170    [Abstracts]
TI: Isotopic variations within upper oceanic crust at IODP Site 1256: Implications for crustal recycling and the formation of ocean island basalts
AU: * Duggen, S
EM: sduggen@ifm-geomar.de
AF: IFM-GEOMAR, Leibniz-Institute of Marine Sciences, Research Division "Dynamics of the Ocean Floor", Wischhofstr. 1-3, Kiel, 24148, Germany
AU: Hoernle, K
EM: khoernle@ifm-geomar.de
AF: IFM-GEOMAR, Leibniz-Institute of Marine Sciences, Research Division "Dynamics of the Ocean Floor", Wischhofstr. 1-3, Kiel, 24148, Germany
AU: Geldmacher, J
EM: geldmacher@iodp.tamu.edu
AF: Integrated Ocean Drilling Program, Texas A&M University, 1000 Discovery Drive, College Station, TX 77845-9, United States
AU: Hauff, F
EM: fhauff@ifm-geomar.de
AF: IFM-GEOMAR, Leibniz-Institute of Marine Sciences, Research Division "Dynamics of the Ocean Floor", Wischhofstr. 1-3, Kiel, 24148, Germany
AB: The origin of ocean island basalts (OIBs) is a fundamental question facing Earth scientists. It is commonly agreed that lithospheric material recycled in the mantle is involved in the magma source of OIBs. The relative importance of 1) subducted altered oceanic basaltic crust (AOC), 2) subducted marine sediments and/or 3) delaminated metasomatised subcontinental lithosphere and continental lower crust remains to be resolved. We examine the geochemical composition of a complete in situ section of oceanic crust drilled at Site 1256 during IODP Expeditions 309 and 312. It includes the extrusive layer, sheeted dikes and gabbros of ca. 15 Ma old oceanic crust of the Cocos Plate formed during a period of superfast spreading at the East Pacific Rise. Modeling in the Sr-Nd-Pb-isotope space and comparison with present day radiogenic isotope ratios of OIBs provides constraints on the significance of recycled oceanic crust in the OIB mantle source(s). Our study shows that the generation of sulphides during low- and high-temperature alteration of oceanic crust has a strong influence on U/Pb and Th/Pb ratios and whether an AOC domain evolves relatively low or high Pb-isotope ratios over geological timescales. The model suggests that AOC as the sole precursor material, modified during the subduction process, and after relatively low to moderate recycling ages of ca. 300-800 Ma, is sufficient to explain the Sr-Nd-Pb-isotopic composition of OIBs with Pb-isotopic compositions along or below the Northern Hemisphere Reference Line (NHRL) and relatively high Nd-isotope ratios (e.g. Canaries, Galapagos, Iceland, Madeira). This indicates that additional EM-components, potentially associated with recycled lithospheric material such as subducted sediments, lower continental crust or subcontinental lithosphere, are not required for an array of OIBs, but are only necessary to explain OIBs with Pb-isotope ratios above the NHRL and relatively low Nd- isotope ratios (e.g. Pitcairn, Tristan, Samoa).
DE: 1009 Geochemical modeling (3610, 8410)
DE: 1021 Composition of the oceanic crust
DE: 1038 Mantle processes (3621)
DE: 1040 Radiogenic isotope geochemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting