HR: 11:35h
AN: V22A-06    [Abstracts]
TI: The Zoo of Mantle Components Seen Through the Lens of Stable Isotope Geochemistry
AU: * Eiler, J M
EM: eiler@gps.caltech.edu
AF: Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125 United States
AB: Solid earth geochemistry has imposed order on the radiogenic isotope diversity exhibited by basalts and mantle rocks, but struggles to determine when this diversity reflects crustal recycling vs. intra-mantle differentiation. Stable isotope geochemistry can help make such distinctions because surface processes lead to large isotopic variations whereas mantle processes are imagined not to. Subducted components of the sources of arc lavas lead to obvious 'signals' in isotopic compositions of H, Li, B, and O, suggesting similar signals should identify subducted components in MORB and intraplate lavas. The reality is a hash of positive, negative and enigmatic results: ¹⁸O-enrichments are clearly associated with the 'EM'-type ocean-island-basalts, re-enforcing long-held hypotheses that these lavas sample sources rich in recycled sediment and/or upper oceanic crust. Similar observations suggest recycled crust contributes to 'background' geochemical variations in the sources of normal MORBs. On the other hand, the ocean-island-basalt end member often associated with the largest amounts of subducted crust ('HIMU') is essentially invariant and normal in oxygen isotope composition, lending weight to heretical hypotheses that this component is a consequence of intramantle differentiation. The Li and H isotope systems are proving to have much in common: Subducted materials are ⁷Li and D enriched relative to the mantle and subduction-zone metamorphism likely drives some deeply subducted rocks to very ⁷Li and D-depleted compositions. Despite this great diversity in subducted components, Li and H isotopic compositions of most MORBs and intraplate lavas vary little. The exceptions, while real, would not have been predicted based on their radiogenic and O isotope and trace element compositions. Mantle xenoliths exhibit highly variable Li and H isotope compositions, including values lower in ⁷Li and D than any fresh basalt. While both of these isotopic systems suffer from the complication of large high-temperature fractionations, their first-order features can be reconciled with a model in which subduction-zone modification of asthenospheric wedge is the dominant mechanism for 'recharging' the mantle in its highly volatile constituents.
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 1033 Intra-plate processes (3615, 8415)
DE: 1038 Mantle processes (3621)
DE: 1041 Stable isotope geochemistry (0454, 4870)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005