Subduction cycling of U, Th, and Pb

Many studies argue, based partly on Pb isotopic evidence, that recycled, subducted slabs reside in the mantle source of ocean island basalts (OIB) [1-3] [A.W. Hofmann, W.M. White, Mantle plumes from ancient oceanic crust. Earth Planet. Sci. Lett. 57 (1982) 421-436; B.L. Weaver, The origin of ocean island basalt end-member compositions: trace element and isotopic constraints. Earth Planet. Sci. Lett. 104 (1991) 381-397; J.C. Lassiter, E.H. Hauri, Osmium-isotope variations in Hawaiian lavas: evidence for recycled oceanic lithosphere in the Hawaiian plume, Earth Planet. Sci. Lett. 164 (1998) 483-496]. Such models, however, have remained largely untested against actual subduction zone inputs, due to the scarcity of comprehensive measurements of both radioactive parents (Th and U) and radiogenic daughter (Pb) in altered oceanic crust (AOC). Here, we discuss new, comprehensive measurements of U, Th, and Pb concentrations in the oldest AOC, ODP Site 80 1, and consider the effect of subducting this crust on the long-term Pb isotope evolution of the mantle. The upper 500 m of AOC at Site 801 shows > 4-fold enrichment in U over pristine glass during seafloor alteration, but no net change to Pb or Th. Without subduction zone processing, ancient AOC would evolve to low Pb-208/Pb-206 compositions unobserved in the modem mantle [4] [S.R. Hart, H. Staudigel, Isotopic characterization and identification of recycled components, in: Crust/Mantle Recycling at Convergence Zones, Eds. S.R. Hart, L. Gulen, NATO AST Series. Series C: Mathematical and Physical Sciences 258, pp. 15-28, D. Reidel Publishing Company, Dordrecht-Boston, 1989]. Subduction, however, drives U-Th-Pb fractionation as AOC dehydrates in the earth's interior. Pacific arcs define mixing trends requiring 8-fold enrichment in Pb over U in AOC-derived fluid. A mass balance across the Mariana subduction zone shows that 44-75% of Pb but < 10% of U is lost from AOC to the arc, and a further 10-23% of Pb and 19-40% of U is lost to the back-arc. Pb is lost shallow and U deep from subducted AOC, which may be a consequence of the stability of phases binding these elements during seafloor alteration: U in carbonate and Pb in sulfides. The upper end of these recycling estimates, which reflect maximum arc and back-arc growth rates, remove enough Pb and U from the slab to enable it to evolve rapidly (<< 0.5 Ga) to sources suitable to explain the Pb-208/Pb-206 isotopic array of OIB, although these conditions fail to simultaneously satisfy the Pb-208/Pb-206 system. Lower growth rates would require additional U loss (29%) at depths beyond the zones of arc and back-are magmagenesis, which would decrease upper mantle kappa (Th-232/U-238) over time, consistent with one solution to the kappa conundrurrf' [5] [T. Elliott, A. Zindler, B. Bourdon, Exploring the kappa conundrum: the role of recycling in the lead isotope evolution of the mantle. Earth Planet. Sci. Lett. 169 (1999) 129-145]. The net effects of alteration (doubling of mu [U-238/Pb-204]) and subduction (doubling of omega [Th-232/Pb-204]) are sufficient to create the Pb isotopic signatures of oceanic basalts. (c) 2005 Elsevier B.V. All rights reserved.