HR: 08:30h
AN: V51E-03 INVITED     [Abstracts]
TI: Physical and Temporal Controls on Lower Crustal Melting and Mixing: Mass and Enthalpy Transport in Actively Growing Arcs
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: Bergantz, G W
EM: bergantz@u.washington.edu
AF: Department of Earth and Space Science, University of Washington, Box 351310, Seattle, WA 98195
AB: The growth of continental crust in arc settings, as well as the thermal and compositional character of the crust, is ultimately dictated by the flux of basaltic magma from the mantle and the interaction between crustal and basaltic material. We present a quantitative assessment of the thermal and dynamic response of the lower crust to the intrusion of basaltic dike swarms in a two-dimensional, stochastic computational framework. We will examine the physical and temporal controls on crustal melting, mingling, and mixing as well as some of the major element, trace element, and U-series consequences of these lower crustal interactions. Distinct melting and mixing environments are predicted as a result of the crustal thickness, flux of basalt, and age of the arc system. Shallow crustal (approx. 30 km) environments and arc settings with low fluxes of mantle basalt are likely repositories of isolated pods of mantle and crustal melts in the lower crust, both converging on dacitic to rhyodacitic composition. These may be preferentially rejuvenated in subsequent intrusive episodes. Mature arc systems with thicker crust (approx. 50 km) produce higher crustal and residual basaltic melt fractions reaching approx. .4 for geologically reasonable basalt fluxes. The basaltic to basaltic-andesite composition of both crustal and mantle melts will readily mix as the network of dikes collapses and Reynolds numbers reach 10$^{-4}$ to 1.0 in the interiors of dikes that have been breached by ascending crustal melts. This may provide one mechanism for MASH-like processes. Residual mineral assemblages of the crust thickened by repeated intrusion are predicted to be garnet pyroxenitic, which are denser than mantle peridotite and also generate convective instabilities where some of the crustal material is lost to the mantle. This reconciles the thinner than predicted crust in regions that have undergone flux of mantle basalt for a prolonged period of time, and helps explain the enrichment of incompatible elements such as K$_{2}$O typical of mature arc settings without the associated mass balance problem. Sr/Y and La/Yb ratios in the associated melts are predicted to be elevated and accompanied by significant $^{226}$Ra-excesses.
DE: 8439 Physics and chemistry of magma bodies
DE: 3640 Igneous petrology
DE: 3210 Modeling
DE: 1020 Composition of the crust
DE: 1040 Isotopic composition/chemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting