doi:10.1016/j.epsl.2005.03.005 
Copyright © 2005 Elsevier B.V. All rights reserved.
Subduction cycling of U, Th, and Pb
aDepartment of Earth Sciences, Boston University, 685 Commonwealth Ave., Boston, MA 02215, USA
bCRPG/CNRS Vandoeuvre-lès-Nancy, BP 20, France
cInstitute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92093-0225, USA
Received 11 February 2004;
revised 29 January 2005;
accepted 4 March 2005.
Editor: K. Farley.
Available online 3 May 2005.
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Abstract
Many studies argue, based partly on Pb isotopic evidence, that recycled, subducted slabs reside in the mantle source of ocean island basalts (OIB) [1], [2] and [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 801, 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 208Pb/206Pb compositions unobserved in the modern 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. Gülen, NATO ASI 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 208Pb/206Pb isotopic array of OIB, although these conditions fail to simultaneously satisfy the 207Pb/206Pb system. Lower growth rates would require additional U loss (29%) at depths beyond the zones of arc and back-arc magmagenesis, which would decrease upper mantle κ (232Th/238U) over time, consistent with one solution to the “kappa conundrum” [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 μ [238U/204Pb]) and subduction (doubling of ω [232Th/204Pb]) are sufficient to create the Pb isotopic signatures of oceanic basalts.
Keywords: uranium; thorium; lead; altered oceanic crust; mantle; subduction
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Fig. 1. Growth curves for (a) 208Pb/204Pb and (b) 207Pb/204Pb vs. 206Pb/204Pb used to construct the Pb isotope evolution models [4] in Fig. 2. Thick black line is a simple, two-stage growth curve [33], beginning the first stage at T1 = 4.5 Ga and the second stage at T2 = 4.0 Ga. The endpoint at t = 0 Ga on the two-stage curve represents the model position of the least radiogenic MORB at the present day (DMM). Conditions for stages 1 and 2 are provided in Table 1. Three-stage growth curves are shown as thin black lines, which follow the two-stage curve until initiation of the third stage at T3 in 0.5 Ga intervals. Each three-stage curve represents the growth curve followed by oceanic crust derived from the MORB mantle at some time in the past (T3), geochemically modified through alteration and/or subduction, and then sequestered until the present (t = 0) to allow ingrowth of radiogenic Pb [4]. The dashed line connects the two- and three-stage curves at t = 0 (present day) to show the trend of the isotopic array of crust of a given composition continually subducted throughout Earth history, as in Fig. 2. All three-stage curves derive from constant μ and ω (from the 417/418 Super composite [4]; Table 1). Gray circles are MORB and OIB [5].
Fig. 2. Pb isotope evolution model [4] (see Fig. 1 for detail). Tick marks indicate the sequestration age of the oceanic crust (T3). Small gray circles are MORB and OIB [5]. Models of bulk altered oceanic crust in (a) 208Pb/204Pb and (b) 207Pb/204Pb vs. 206Pb/204Pb. Line A is as for Fig. 1, for the DSDP Site 417/418 Super composite (500 m) [4], line B is the DSDP/ODP Hole 504B volcanic zone (500 m) [32], line C is the ODP Site 801 Super composite (500 m), line D is ODP Site 801 (6 km; see Table 1). ODP Site 801 AOC (6 km) after subduction zone processing in (c) 208Pb/204Pb and (d) 207Pb/204Pb vs. 206Pb/204Pb. Line E is the residual slab after processing through the arc at maximum crustal growth (lost 75% Pb, 10% U), line F is the residual slab after processing through the arc and back-arc at maximum crustal growth (lost 98% Pb, 50% U), line G is from model OIB reservoir parameters [5] (equivalent to slab losses of 58% Pb and 54% U).
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Fig. 3. (a) U concentration vs. depth at Site 801, modified from Kelley et al. [36]. Shaded area traces the general shape of continuous, downhole U concentration recorded by the natural gamma logging tool [34]. Gray circles are discrete, whole-rock samples from the Site 801 core, from Kelley et al. [36]. Samples highlighted with stars are discrete, whole-rock samples from which thin sections were used for laser ablation ICP-MS analyses of alteration phases (see panel b). Zones related to U enrichment (hydrothermal deposits, hyaloclastites, breccia) are highlighted by dark, horizontal bands. Vertical lines indicate bulk U concentrations of average Site 801 glass (0.08 ppm) and the 801 Super composite (0.39 ppm). (b) U concentrations in secondary carbonates vs. depth. Laser-ablation ICP-MS analyses of individual carbonates (open circles), and average carbonates within samples (filled stars), and down-core distribution (guided by logging data; light gray shading). Data, averages, and depth intervals are given in Supplementary Table B. These sample averages are weighted by the length weights given in Supplementary Table B to determine the average for the site (1.46 ppm U on average in carbonates; dark vertical band). Error bars for each sample are 2σ of the mean of the analyses. The uncertainty on the site average was calculated using a Monte Carlo scheme where each sample average was allowed to vary randomly within its 2σ prior to weighting and calculating site average. The uncertainty quoted (± 0.42 ppm) represents the full range in 90% of the trials. Box calculation shows how we estimate the proportion of carbonate-bound U that is added by alteration. CU and CCaCO3 are concentrations of U and CaCO3 in different phases, subscript SC is 801 Super composite, Gl is average 801 glass, Carb is average 801 carbonate. Value for CaCO3 in the 801 Super composite from [64].
Fig. 4. Nb, Pb, and U vs. Th in ODP Site 801 samples. Data for discrete samples, composites, and glass from Kelley et al. [36]. Gray lines are NMORB ratios from Sun and McDonough [65].
Fig. 5. Pb/U vs. Th/U in arc lavas (all ICP-MS data). Dark gray triangles are Honshu [66], light gray squares are Aleutians [67], black circles are Marianas [45], open diamonds are Mariana back-arc [55]. Thin black lines are RMA linear regressions through data. Thick black line illustrates the difference between bulk AOC and the AOC-derived fluid.
Fig. 6. Slab losses of Pb and U vs. depth in the subduction zone. Gray boxes indicate the full range of possible losses permitted by crustal growth rates. Heavy black line traces slab losses at preferred growth rates (Table 1).
Table 1.
Composition of subducting crust and mass balance results
Losses reported as % of original 6 km crustal input. Values in parenthesis indicate preferred values for crustal growth rates of 32 and 83 km3/Ma/km for the arc and back-arc. Preferred arc growth rate is average of Reymer and Schubert [52] rates, preferred back-arc rate constrained to make ω = 60 in slab residue.
a Many of the 417/418 samples suffered Pb contamination, thus Hart and Staudigel
[4] used a Pb value (underlined) derived from a subset of samples inferred to have avoided contamination.
b U value is from Site 504 “precursor” composition, Th and Pb (italicized) are assumed to be unchanged in bulk upper 500 m during alteration.
c Calculated by summing 2 km of 801 dikes and 4 km of gabbro to a Fo
89 liquid back-calculated from the primitive glass composition (see text).
d Sum of 0.5 km of 801 Super, 1.5 km of 801 dikes and 4 km of 801 gabbro (see text).
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