HR: 0800h
AN: V41D-1490    [Abstracts]
TI: Lithium Isotopic Compositions of Lavas From Samoan and Austral Volcanic Chains: Constraints on the Source Components of Mantle Reservoirs
AU: * Chan, L
EM: lchan@geol.lsu.edu
AF: Department of Geology and Geophysics, Louisiana State University, Baton Rouge, LA 70803-4101 United States
AU: Lassiter, J C
EM: lassiter1@mail.utexas.edu
AF: Department of Geological Sciences, University of Texas, Austin, TX 78712-0254 United States
AU: Hart, S R
EM: shart@whoi.edu
AF: Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543 United States
AU: Blusztajn, J
EM: jblusztajn@whoi.edu
AF: Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543 United States
AU: Hauri, E H
EM: hauri@dtm.ciw.edu
AF: Department of Terrestrial Magnetism, Carnegie Institute, Washington, DC 20015 United States
AB: Radiogenic isotope and trace elemnt studies have revealed a HIMU component, a depleted (DM) component and enriched (EM) components in lavas from Samoan and Cook-Austral volcanic chains. We have determined lithium isotopic compositions of largely fresh basaltic lavas from the Samoan chain and Austral islands (Raivavae and Rapa) to further constrain the origin and sources of the mantle reservoirs in the South Pacific. The DM endmember in Raivavae samples has δ⁷Li values of 3.2 to 4.2%_0, identical to fresh MORB. δ⁷Li of HIMU type lavas of Raivavae vary from 4.4 to 5.5%_0. The EM lavas are also enriched in ⁷Li compared to DM (δ⁷Li=3.8 to 5.9%_0). Two exceptionally high values are observed in Raivavae (10 and 11.5%_0). Analysis of mineral separates is in progress to determine the origin of these anomalous compositions. Malumalu seamount of the Samoan chain defines the most extreme composition of the EMII mantle. δ⁷Li values of this mantle endmember have been determined to be 4.8 to 5.6%_0. Available data from other seamount and islands of the chain show a range of δ⁷Li from 3.2 to 7.7%_0. Thus the south Pacific hot spots display a greater Li isotopic heterogeneity than the Hawaiian plume. Together, the Samoan and Austral samples present systematic relationships between Li and Pb isotope ratios that depict mixing of HIMU and EM materials with a DM component. The HIMU tpe samples display a positive correlation between δ⁷Li and ²⁰⁶Pb/²⁰⁴Pb, whereas the enriched samples define a generally negative correlation. The heavy Li isotopic composition of the HIMU endmember suggests an origin of recycled altered oceanic crust that has not been greatly depleted by dehydration during subduction. The high δ⁷Li of the EM member cannot be explained by addition of pelagic sediments which are typically light in Li isotopic composition but may be attributed to mantle wedge manterial that has been metasomatized by ⁷Li-rich fluids arising from the subductd slab. We have not, however, found the isotopically light complement that would remain on the dehydrated slab, as hinted by data from Koolau, an EMI type volcano.
DE: 1000 GEOCHEMISTRY
DE: 1025 Composition of the mantle
DE: 1040 Radiogenic isotope geochemistry
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005