HR: 0800h
AN: V41D-1486    [Abstracts]
TI: Coupled Trace Element and Isotope Variations in Oceanic Basalts: Recycling Re-examined
AU: * Salters, V J
EM: salters@magnet.fsu.edu
AF: National High Magnetic Field Laboratory, 1800 E Paul Dirac Drive, Tallahassee, FL 32306
AB: In 1982 Hofmann and White (Nature, 295, p.363). proposed that the enriched mantle components found their origin in recycled crustal material and this theory had found wide acceptance. However, our recent Hf-isotope data in combination with Nd- and Pb-isotopes and trace element variations shows that in detail many variations are not consistent with the origin of the enriched endmembers being related to recycled crustal materials. Although the addition of the Hf-isotopes did not result in the recognition of additional mantle endmembers, the Hf-isotopes do test their modes of origin and now question whether crustal recycling is the dominant process that causes the enriched mantle endmembers. Hf-isotopes on the enriched basalts from the Koolau volcano on Hawaii have shown that the enriched Koolau endmember contains an ancient depleted lithosphere component and indicate that a recycled crustal component is not a dominant component. The existence of this depleted component is confirmed by the recognition of this component in the Os-Hf-isotope variations in the peridotite xenoliths. Hf-isotopes on Samoan basalts (EMII endmember) have confirmed the presence of depleted lithosphere in the source and the combined trace element and isotope variation also shows the recycling of crustal material is not the dominant process causing enriched mantle signatures. Walvis Ridge is the "type" locality for Enriched Mantle I compositions. 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. However, Walvis forms one the steepest slopes being inconsistent with pelagic sediments. Furthermore, Walvis Ridge basalts have some of the highest La/Nb ratios, up to 1.6, which has been taken as evidence for the involvement of a crustal component. Isotopic and trace element characteristics of the basalts are well correlated with low epsilon-Hf basalts showing low Sm/Hf, high Ce/Yb, high U/Th and high La/Nb. 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 the more likely process explaining the variations.
DE: 1038 Mantle processes (3621)
DE: 1040 Radiogenic isotope geochemistry
DE: 1065 Major and trace element geochemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005