HR: 1340h
AN: V13A-1460    [Abstracts]
TI: Subduction Zone Mass Fluxes
AU: * Porter, K A
EM: porter@geology.geo.cornell.edu
AF: Department of Earth & Atmospheric Sciences, Snee Hall Cornell University, Ithaca, NY 14853 United States
AU: White, W
EM: white@geology.geo.cornell.edu
AF: Department of Earth & Atmospheric Sciences, Snee Hall Cornell University, Ithaca, NY 14853 United States
AB: Recycled crustal material is often cited as a source for various mantle reservoir characteristics. These inferences are often based on ingoing slab compositions, although processes within the subduction zone could significantly change the chemistry of the slab before it is mixed into the mantle. Few studies have examined the composition of "residual" slab material. Using a mass-balance approach, we have estimated the chemical compositions of residual slabs at nine oceanic subduction zones (Aleutians, Central America, Izu-Bonin, Kurile, Mariana, Northern Lesser Antilles, Southern Lesser Antilles, Sunda-Java, and Tonga), representing approximately 30% of the total length of subduction zones worldwide. We remove an inferred mantle wedge component from fractionation-corrected average island arc volcanics (IAV) in order to estimate the composition and flux of the "slab component" in the IAV. We then calculate the residual slab composition by subtracting the "slab component" flux from the ingoing slab flux. We have also calculated slab-to-backarc fluxes at the Marianas and Tonga island arcs in order to more accurately constrain residual slab fluxes. Our results allow us to investigate the feasibility of various mantle evolution models. The HIMU mantle reservoir gives rise to basalts with very high $^{206}$Pb/$^{204}$Pb and somewhat low $^{87}$Sr/$^{86}$Sr. It has often been suggested that this reservoir represent anciently subducted oceanic crust. We use our calculated residual slab U/Pb, Rb/Sr, and Sm/Nd values to estimate the present isotopic composition of material subducted 1.8 Ga ago. Only two of the arcs (Izu-Bonin and the Marianas) have high enough residual slab U/Pb to produce HIMU-like Pb isotope signatures. However, Rb/Sr ratios in these arcs are too high to be consistent with HIMU characteristics, and $\epsilon_{Nd}$ is also inconsistent with the HIMU reservoir. Galer and O'Nions (1985) argued that to account for the difference between present $^{232}$Th/$^{238}$U ratios (kappa) and the time-integrated $^{232}$Th/$^{238}$U calculated from Pb isotope ratios ($\kappa _{Pb}$) in MORB, the residence time of Pb in the depleted mantle must be short (about 600 Ma). Others have argued instead that low present-day kappa in the depleted mantle reflects preferential recycling of U via subduction of low-Th/U altered crust. Our results indicate that subduction processing does not fractionate Th from U; therefore, a residual slab will only contribute low Th/U material to the mantle if the ingoing slab has a low Th/U ratio. Low crustal Th/U is only expected in oxidizing ocean conditions. However, recent studies indicate that the oceans may have remained fairly anoxic for much of Earth's history (until about 600 Ma). If this was the case, it is unlikely that oceanic crust could have caused the lowering of mantle Th/U ratios. The Pb/Ce ratio has been used as an indicator of recycled material in the mantle because of the large contrast in crust and mantle values (0.30 vs. 0.035). Most basalts derived from mantle plumes have Pb/Ce ratios similar to those of MORB (about 0.04). This observation is apparently inconsistent with the idea that mantle plumes contain recycled crustal material, since subducted slabs tend to have high Pb/Ce (greater than 0.20) due to their sedimentary component. Our results show that, although subduction zone processing can lower slab Pb/Ce ratios, residual slab compositions are still not consistent with the observed mantle plume compositions: the lowest residual slab Pb/Ce we observe is roughly 0.07. It therefore seems likely that other processes must contribute to the lowering of slab Pb/Ce ratios.
DE: 8125 Evolution of the Earth
DE: 1030 Geochemical cycles (0330)
DE: 1040 Isotopic composition/chemistry
DE: 1099 General or miscellaneous
DE: 1010 Chemical evolution
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting