HR: 1340h
AN: V13A-1437    [Abstracts]
TI: Sr-Nd-Pb Isotope Geochemistry of Melange Formation: Implications for Identification of Fluid Sources in the Mantle Wedge and the Arc
AU: Bebout, G E
AF: Earth and Env. Sciences, Lehigh Univ., 31 Williams Dr, Bethlehem, PA 18015 United States
AU: Bebout, G E
AF: PML, ISEI, Okayama Univ. at Misasa, Tottori-ken, Misasa, 682-0193 Japan
AU: * King, R L
EM: robbie@lehigh.edu
AF: Earth and Env. Sciences, Lehigh Univ., 31 Williams Dr, Bethlehem, PA 18015 United States
AU: * King, R L
EM: robbie@lehigh.edu
AF: PML, ISEI, Okayama Univ. at Misasa, Tottori-ken, Misasa, 682-0193 Japan
AU: Moriguti, T
AF: PML, ISEI, Okayama Univ. at Misasa, Tottori-ken, Misasa, 682-0193 Japan
AU: Nakamura, E
AF: PML, ISEI, Okayama Univ. at Misasa, Tottori-ken, Misasa, 682-0193 Japan
AB: Paramount to our ability to decipher the behavior of fluids and melts within the mantle wedge and the overall subduction system are the chemical compositions of rocks adjacent to the slab-mantle interface. Profound metamorphic and metasomatic alteration of pre-subduction lithologies to form melange along the slab-mantle interface may yield rock types inheriting mixed chemical compositions of diverse pre-subduction lithologies. Early work on melange geochemistry indicates competitive effects between mechanical mixing, metasomatism by fluids or melts, and mineral stabilities imposed by the resulting bulk composition. We have explored the Sr-Nd-Pb isotope geochemistry of low- to high-grade melange zones in the Catalina Schist, CA, to address this crucial missing component in studies of subduction-zone mass flux. The Catalina Schist contains lawsonite-albite (LA), lawsonite-blueschist (LB), and amphibolite (AM) facies melange zones, all with mineralogy dominated by talc, chlorite, and Na-Ca amphiboles, with additional minerals such as micas, rutile, zircon, and apatite stabilized based on bulk sample chemistry. Major element compositions vary, from strongly ultramafic in the AM melange, to more crustal-like compositions (i.e., more reminiscent of basaltic to sedimentary protoliths) for LA and LB melange. However, initial Sr and Nd isotope ratios for all grades of melange are essentially indistinguishable, displaying a wide variation from $^{87}$Sr/$^{86}$Sr=0.703-0.709 and $\epsilon$Nd= +15 to -15. Covariations are generally negative, similar to that of the mantle array, but with some samples extending to higher Sr ratios at constant $\epsilon$Nd that probably reflect inheritance of seawater Sr. No clear mixing relationships between $^{87}$Sr/$^{86}$Sr and 1/Sr exist, suggesting either localized buffering of Sr isotope ratios or that mixing relations are obscured by secondary devolatilization. However, a clear mixing trend for Nd indicates two end-members, one a high-concentration, positive $\epsilon$Nd source (AOC?), the other with low-concentration and negative $\epsilon$Nd (devolatilized sediments?). Likewise, initial Pb isotope ratios for all grades of melange form a single array independent of rock type or inferred protolith. Melange matrix of the Catalina Schist preserves initial $^{206}$Pb/$^{204}$Pb of 18.95-19.59, $^{207}$Pb/$^{204}$Pb of 15.61-15.68, and $^{208}$Pb/$^{204}$Pb of 37.85-39.05. Such elevated Pb ratios are typical of subducting oceanic sediments, but not of MORB-like oceanic crust or peridotites of the depleted mantle. The similarity of these initial ratios suggests pervasive alteration of Pb isotope signatures within diverse rock types by fluids during subduction. As Pb concentrations decline from LA/LB to AM melange, this suggests devolatilization of Pb from the ultramafic AM melange will transfer crustal-like Pb isotope ratios. Sr-Nd-Pb isotope systematics for arc volcanic rocks are commonly used as indicators of fluid sources from the subducting slab to the arc magma source region. Our results suggest such an assumption is extremely dangerous, as hybridization processes common to melange zones are more likely to occur along the slab-mantle interface than is preservation of a pre-subduction section. Such metamorphic mediation and buffering of "slab" compositions is essentially unknown, yet our data support an interpretation where these processes impart a fundamental control on the chemistry of fluids passed to the mantle wedge.
DE: 3660 Metamorphic petrology
DE: 1040 Isotopic composition/chemistry
DE: 1010 Chemical evolution
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting