HR: 14:25h
AN: V52D-04    [PDF]
TI: The Deep Mantle Subduction Flux and its Implications for Mantle Th/U and Pb/Ce Ratios
AU: * Porter, K A
EM: kap32@cornell.edu
AF: Cornell University, Department of Earth \& Atmospheric Sciences, Snee Hall, Ithaca, NY 14853 United States
AU: White, W
AF: Cornell University, Department of Earth \& Atmospheric Sciences, Snee Hall, Ithaca, NY 14853 United States
AB: Subduction of oceanic crust and sediment is thought to have profoundly influenced the chemical evolution of the mantle. In particular, it has been argued that crustal recycling through subduction is responsible for the incompatible-element-enriched nature of mantle plumes; the addition of crustal material to the mantle has also been suggested as a solution to the 'kappa conundrum', i.e., the discrepancy between time-integrated mantle Th/U ratios ($\sim$3-4, as recorded by radiogenic Pb isotopes) and the present-day mantle Th/U ($\sim$2.55). These hypotheses can be tested if the composition of material subducted into the deep mantle is known. However, before oceanic crust and sediment is carried into the deep mantle, its composition is modified through extraction of fluids and/or melts that are incorporated into island arc magmas. We have attempted to account for these processes using a simple mass-balance approach to calculating deep mantle subduction fluxes. Our calculations allow us to estimate residual slab (i.e., slab material carried into the mantle) compositions for trace elements other than Th, U, Pb, and Ce. In general, the residual slabs are enriched relative to primitive mantle in all elements except Ba and Zr. The slabs themselves lose large fractions of their initial budgets of fluid-mobile elements, particularly Pb and Ba, within the subduction zone. On the other hand, less than 10% of the less fluid mobile, moderately incompatible elements, such as the intermediate and heavy rare earths, is lost from the slab in the subduction zone. Our results indicate an important role for slab-derived fluids in subduction zones, and suggest that enriched mantle sources may contain a recycled crustal component. In the six subduction systems we have examined to date (Marianas, Izu-Bonin, Tonga, Kurile, and the Southern and Northern Lesser Antilles), generally high Th/U subducting sediments (2.0-8.9) are combined with low Th/U altered oceanic crust ($\sim$0.23), and bulk subducting slabs generally have Th/U ratios similar to modern depleted mantle ($\sim$2.6 vs. $\sim$2.55). Furthermore, the similarities between the Th/U ratio of the slab component of island arc volcanics and the depleted mantle indicate that Th and U are not strongly fractionated beneath island arcs. Consequently, the average Th/U ratio of the deep mantle subduction flux (2.57 +/- 0.71) is similar to the Th/U ratio of depleted mantle. If this is true, then incorporation of altered crust into the mantle cannot be solely responsible for the low Th/U ratio of the modern mantle, i.e., it cannot be the solution to the kappa conundrum. The Pb/Ce ratio has been used as a sensitive indicator of recycled material in the mantle because of the large contrast in crust and mantle values (0.30 vs. 0.035). Most oceanic island basalts have Pb/Ce ratios similar to those of MORB (roughly 0.04). This observation is apparently inconsistent with the idea that mantle plumes, which are thought to produce oceanic island basalts, contain recycled crustal material. We find, however, that the processes occurring beneath island arcs strongly fractionate Pb from Ce. In three of the six arcs examined so far, the Pb/Ce ratio in the residual slab lies in the range 0.03-0.05 (i.e., identical to the mantle value), while in the other three it is in the range of 0.1 to 0.26. Thus, low Pb/Ce ratios in oceanic island basalts do not preclude the presence of subducted sediment in their sources. Higher Pb/Ce ratios in some slabs might explain why some oceanic islands (e.g., the Society Islands) do have elevated Pb/Ce ratios.
DE: 1010 Chemical evolution
DE: 1025 Composition of the mantle
DE: 3640 Igneous petrology
DE: 8102 Continental contractional orogenic belts
DE: 8120 Dynamics of lithosphere and mantle--general
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting