HR: 11:20h
AN: V42A-04    [Abstracts]
TI: Recycled Oceanic Mantle Lithosphere in Hawaii: The Samples and the Models
AU: * Bizimis, M
EM: bizimis@magnet.fsu.edu
AF: NHMFL and Dept. of Geological Sci., FSU, 1800, E. Paul Dirac Dr, Tallahassee, Fl 32310 United States
AU: Sen, G
EM: seng@fiu.edu
AF: Dept. of Earth Sci., FIU, 11200 SW 8th St., Miami, Fl 33199 United States
AU: Lassiter, J
EM: lassiter1@mail.utexas.edu
AF: Dept. Of Geological Sci., UT Austin, 1 University Station C1100, Austin, TX 78712 United States
AU: Salters, V
EM: salters@magnet.fsu.edu
AF: NHMFL and Dept. of Geological Sci., FSU, 1800, E. Paul Dirac Dr, Tallahassee, Fl 32310 United States
AU: Keshav, S
EM: s.keshav@gl.ciw.edu
AF: Geophysical Lab. CIW, 5251 Broad Branch Rd, Washington, DC 20015 United States
AB: Subduction of basaltic oceanic crust is a key mechanism for introducing chemical heterogeneities into the Earth's mantle. Together with the crust however, the associated depleted lithosphere is also subducted. In the mantle, basaltic crust has a lower solidus while the depleted lithosphere has a higher solidus temperature than the ambient mantle. Thus, if a recycled depleted lithosphere is part of an upwelling plume, it could survive melting and be more easily recognized in mantle xenoliths found in plume volcanism, than its basaltic counterpart. Some spinel peridotite xenoliths from Salt Lake Crater, Oahu, Hawaii, have coupled highly radiogenic Hf (ε_Hf= 3 to 114) and unradiogenic Os isotopes (¹⁸⁷Os/¹⁸⁶Os = 0.1291-0.1134). Os and Hf isotopes correlate with indices of depletion (e.g. Mg&35;) suggesting that their extreme values are related to melting and not to an exotic mantle component never before sampled. Both Lu-Hf systematics and Re-depletion ages in these samples give ages between 500 million to 2 billion years; those are hard to reconcile with the 90 million year old Pacific lithosphere in Oahu. We suggest that these peridotites represent ancient recycled depleted lithosphere brought to the surface by the Hawaiian plume, and represent a plume component previously not recognized in lavas. However, the lack of extreme Hf-Os isotope compositions in the erupted lavas suggests that although this recycled lithosphere is part of the plume, it does not significantly contribute to plume volcanism. It has been suggested that recycled metasomatized lithosphere can be the source of OIB volcanism. Therefore, we can use the trace element compositions of peridotite and pyroxenite xenoliths from Oahu, representing the metasomatized oceanic lithosphere, to test this hypothesis. Assuming an origin from a MORB or primitive mantle source and an age of 1.5 to 2.5 Ga, this lithosphere develops highly variable compositions in any combination of Hf-Nd-Sr-Pb isotopes that hardly overlap or extent towards any proposed mantle endmembers. Pyroxenites (representing fertile veins in the mantle) develop compositions that fall below the terrestrial array in Hf-Nd space, while the peridotite compositions fall both above and below the array. In Sr-Nd space, only some pyroxenites with phlogopites and carbonates will develop compositions falling within or at the extension of the OIB field, while peridotites evolve with unradiogenic Sr, for a given Nd. In any Pb isotope space, peridotites will develop radiogenic ²⁰⁶Pb/²⁰⁴Pb while pyroxenites are highly variable and can develop both radiogenic and unradiogenic Pb values. We show that recycled metasomatized lithosphere is too heterogeneous to develop the well-defined OIB arrays in the different isotope correlation diagrams. We suggest that some other mechanism in the mantle is needed that either homogenizes the heterogeneous recycled lithosphere, or only a small portion of that lithosphere participates in OIB melting. The survival of recycled lithospheric peridotites with extreme isotopes at Salt Lake Crater however, suggests that long storage and stirring in the mantle alone does not erase such heterogeneities.
DE: 1025 Composition of the mantle
DE: 1040 Isotopic composition/chemistry
DE: 1065 Trace elements (3670)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2005 Joint Assembly