HR: 1340h
AN: V53A-0614    [Abstracts]
TI: ($^{226}$Ra)/($^{230}$Th) Excess Generated in the Lower Crust: Implications for Magma Transport Rates in Arc Settings
AU: * Dufek, J D
EM: dufek@u.washington.edu
AF: Department of Earth and Space Science, University of Washington, Box 351310, Seattle, WA 98195
AU: Cooper, K M
EM: kmcooper@u.washington.edu
AF: Department of Earth and Space Science, University of Washington, Box 351310, Seattle, WA 98195
AB: $^{226}$Ra-excesses in arc magmas have been interpreted to result from flux melting of the mantle above subducting slabs and subsequent fast ascent rates of magma from slab to surface, up 1000 m/yr and higher. Implied by this hypothesis is that the magma has not stalled at any depth for long periods of time, limiting the amount of time for assimilation of crustal material and homogenization of magmas. However, mixing of mantle melts with mid to lower crustal melts has been inferred in numerous localities, especially in continental arc settings. In addition, very short residence times may be in conflict with crystal residence of thousands to tens of thousands of years in arc lavas. We present calculations that demonstrate that incongruent melting of the lower crust could either maintain or augment mantle-derived Ra-excesses and so reduce vertical transport rates to 10 m/yr or less. In particular, we found that dehydration melting of amphibolite can produce modeled $^{226}$Ra-excesses greater than 300 percent with corresponding $^{230}$Th-excesses of approx. 14 percent. Both the modal percentage of garnet and melt fraction contribute to the degree of Ra-excess, and a wide range of garnet compositions can produce significant disequilibria. Mixtures of such an amphibolite dehydration melt with mantle melts will likely retain a $^{238}$U-excess (subducted slab) signature, although lower crustal melting without such mixing could help explain some of the enigmatic silicic magmas observed that have $^{230}$Th-excesses. This amphibolite dehydration melting process will also produce elevated Sr/Y and La/Yb ratios, similar to those observed in several arc settings, that may distinguish these magmas from those with $^{226}$Ra-excesses produced by slab-dewatering alone.
DE: 8400 VOLCANOLOGY
DE: 8434 Magma migration
DE: 8439 Physics and chemistry of magma bodies
DE: 3600 MINERALOGY AND PETROLOGY (replaces
DE: 1000 GEOCHEMISTRY (New field, replaces Rock Chemistry)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting