HR: 0800h
AN: V51B-0566    [Abstracts]
TI: Isotope and Trace Element Characteristics of Walvis Ridge Basalts Argue Against Pelagic Sediment Involvement
AU: * Salters, V J
EM: salters@magnet.fsu.edu
AF: National High Magnetioc Field Laboratory, 1800 E Paul Dirac Drive, Tallahassee, FL 32312 United States
AU: Li, X
EM: xli@magnet.fsu.edu
AF: National High Magnetioc Field Laboratory, 1800 E Paul Dirac Drive, Tallahassee, FL 32312 United States
AB: Walvis Ridge is the "type" locality for Enriched Mantle I compositions (Zindler and Hart, 1984). We have determined the trace element and isotopic compositions of Walvis Ridge basalts to better determine the geochemical characteristics of the enriched mantle endmember which is thought to include a significant contribution from recycled pelagic sediments. Characteristic for pelagic sediment contribution is a shallow slope on a Hf-Nd isotope correlation diagram. For Walvis Ridge basalts epsilon Hf varies from -2 to +12 while epsilon Nd varies from -4 to 6. This range is larger than previously published results and results in a well-correlated array (R2=0.97) with a steeper slope than the general ocean island basalt (OIB) array. Walvis Ridge Pb-isotope variation is also large $^(206)$Pb/$^(204)$Pb and $^(207)$Pb/$^(204)$Pb range from 17.5 to 18.5 and 15.46 to 15.53 respectively. The low $^(206)$Pb/$^(204)$Pb basalts also show low Hf and Nd isotopic characteristics and in general Nd-Sr-Hf-Pb isotopic compositions are well correlated indicating two component mixing. The Walvis Ridge basalts also show a large range in trace element compositions: Ce/Yb 4.68-25.52; Sm/Hf 1.11-1.58; La/Nb 0.72-1.63, La/Sm 1.59-5.13; U/Th 0.10-0.53 all well outside the range of what is expected for variations related to degree of melting and crystallization. Isotopic and trace element characteristics of the basalts are well correlated with low epsilon-Hf basalts showing low Sm/Hf, high Ce/Yb and high U/Th. The trace element data confirm the isotopic data as most of their variations can also be explained by mixing of just two components. New data obtained on basalts from Tristan da Cunha and Gough show similar type variations in trace element and isotopic compositions as the Walvis Ridge basalts. Walvis Ridge is the "type" locality for Enriched Mantle I compositions (Zindler and Hart, 1984). The isotopic variations as well as the coupled trace element-isotope variations, especially the steep slope on the Hf-Nd isotope correlation diagram and the positive correlation between Sm/Hf and Hf-isotopic composition, argue strongly against pelagic sediment involvement. Detailed analysis of the trace element and isotope variations show that the data is hard to reconcile with recycled components and that intra mantle differentiation is a more likely process explaining the variations.
DE: 1025 Composition of the mantle
DE: 1040 Isotopic composition/chemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting