HR: 1340h
AN: V13A-1453    [Abstracts]
TI: Petrology and Geochemistry of Eclogite Xenoliths from the Colorado Plateau: Implications for the Evolution of Subducted Oceanic Crust
AU: * Usui, T
EM: tusui@misasa.okayama-u.ac.jp
AF: The Pheasant Memorial Laboratory for Geochemistry and Cosmochemistry, Institute for Study of the Earth's Interior, Okayama University, Yamada 827, Misasa, Tottori, 682-0193 Japan
AU: Nakamura, E
EM: eizonak@misasa.okayama-u.ac.jp
AF: The Pheasant Memorial Laboratory for Geochemistry and Cosmochemistry, Institute for Study of the Earth's Interior, Okayama University, Yamada 827, Misasa, Tottori, 682-0193 Japan
AU: Helmstaedt, H
EM: helmstaedt@geol.queensu.ca
AF: Department of Geological Sciences and Geological Engineering, Queen's University, Kingston, Ontario, K7L 3N6 Canada
AB: Eclogite xenoliths from the Colorado Plateau, interpreted as representing fragments of the subducted Farallon plate, are used to infer the evolution of trace element and isotopic compositions of oceanic crust subducted into the subarc mantle. The xenoliths consist of almandine-rich garnet, Na-clinopyroxene, lawsonite and zoisite with minor amounts of phengite, rutile, pyrite and zircon. They have essentially basaltic bulk-rock major element compositions, except for significantly higher Na$_{2}$O, but similar K$_{2}$O, with increasing SiO$_{2}$ contents, compared to altered MORB. These major element characteristics are explained by spilitization/albitization during hydrothermal alteration at a mid-ocean ridge and by subduction zone metasomatism in the forearc region. Whole-rock trace elements and Sr, Nd, and Pb isotopic compositions of the xenoliths are variable and exhibit significantly enriched characteristics compared to altered MORB, except for similar Zr/Hf ratios of 36.9 to 37.6. Mass balance for the Colorado Plateau eclogite xenoliths can be achieved for 26 trace elements, Rb, Cs, Sr, Ba, Y, REE, HFSE (Zr, Hf, Nb, and Ta), Pb, Th and U. The mass balance calculations and mineralogical observations corroborate that whole-rock chemistry of the xenoliths were contaminated by near-surface processes after eruption and limited interaction with the serpentinized ultramafic microbreccia host magma. Thus, Sr, Nd and Pb isotopic compositions of separated minerals from the xenoliths were measured to avoid these secondary effects; these separates yield distinctively enriched isotopic compositions in the range of 0.70502 to 0.70590 for $^{87}$Sr/$^{86}$Sr, -1.5 to -3.1 for $\epsilon$Nd and 18.928 to 19.052 for $^{206}$Pb/$^{204}$Pb. This suggests that the xenoliths were metasomatized by a fluid equilibrated with the sedimentary layer probably covering the Farallon plate in the forearc region. This metasomatism resulted in the xenoliths acquiring distinctively enriched isotopic compositions compared with those of altered MORB. Some of the distinct isotopic signatures observed in OIBs compared to those from MORBs have been interpreted as a result of oceanic sediment subducted deep into the mantle. Our results, on the contrary, suggest an alternative possibility that these anomalous isotopic reservoirs in the mantle are formed by the subduction of oceanic crust modified by the metamorphic fluid from the covering sedimentary rocks.
DE: 3660 Metamorphic petrology
DE: 1040 Isotopic composition/chemistry
DE: 1010 Chemical evolution
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting