HR: 0800h
AN: V11B-1424    [Abstracts]
TI: Coupling Thermal and Chemical Signatures of Crustal Magma Bodies: Energy-Constrained Eruption, Recharge, Assimilation, and Fractional Crystallization (E'RA$\chi$FC)
AU: * Bohrson, W A
EM: bohrson@geology.cwu.edu
AF: Dept. of Geological Sciences, Central Washington University, 400 E University Way, Ellensburg, WA 98926 United States
AU: Spera, F J
EM: spera@geol.ucsb.edu
AF: Dept. of Geological Sciences, University of California, Santa Barbara, CA 93106 United States
AB: Energy-Constrained Eruption, Recharge, Assimilation and Fractional Crystallization (E'RA$\chi$FC) tracks the evolution of an open-system magmatic system by coupling conservation equations governing energy, mass and species (isotopes and trace elements). By linking the compositional characteristics of a composite magmatic system (host magma, recharge magma, wallrock, eruptive reservoir) to its mass and energy fluxes, predictions can be made about the chemical evolution of systems characterized by distinct compositional and thermal characteristics. An interesting application of E'RA$\chi$FC involves documenting the influence distinct thermal regimes have on the chemical evolution of magmatic systems. Heat transfer between a magma-country rock system at epizonal depths can be viewed as a conjugate heat transfer problem in which the average country rock-magma boundary temperature, Tb, is governed by the relative vigor of hydrothermal convection in the country rock vs. magma convection. For cases where hydrothermal circulation is vigorous and magmatic heat is efficiently transported away from the boundary, contact aureole temperatures (~Tb) are low. In cases where magmatic heat can not be efficiently transported away from the boundary and hydrothermal cells are absent or poorly developed, Tb is relatively high. Simultaneous solution of the differential equations governing momentum and energy conservation and continuity for the coupled hydrothermal-magmatic conjugate heat transfer system enables calculation of the characteristic timescale for EC-RAFC evolution and development of hydrothermal deposits as a function of material and medium properties, sizes of systems and relative efficiency of hydrothermal vs. magmatic heat transfer. Characteristic timescales lie in the range 10$^{2}$-10$^{6}$ yr depending on system size, magma properties and permeability among other parameters. In E'RA$\chi$FC, Tb is approximated by the user-defined equilibration temperature, Teq, which is the temperature at which all parts of the composite magmatic system achieve thermal equilibrium. Comparison of the results of three EC-AFC simulations at different Teq ($1150\deg$C, $1050\deg$C, $1000\deg$C) for a mafic magma intruding middle-upper crust of mafic-intermediate composition illustrate the distinctions that can be imparted by a range of thermal regimes. Model parameters relevant to the following results include: initial Sr concentration, isotope composition and bulk D for host magma are 700 ppm, 0.7035, and 1.5, respectively; those for wallrock are 230 ppm, 0.7100, 0.05. The $1150\deg$C case (i.e., high Tb) yields the least crust-like Sr isotope signatures. The mass of wallrock that reaches thermal equilibrium is relatively small (0.26, normalized to the mass of initial host magma), although the degree of melting is high (97%). In contrast, the $1000\deg$C case (i.e., low Tb) yields the most crust-like Sr isotope signatures. This case is also characterized by the largest mass of wallrock (0.98, normalized to the mass of initial host magma) that achieves thermal equilibrium, but the degree to which this wallrock melts is small (10%). A fundamental issue that derives from these results is the relationship between the chemical evolution of the hydrothermal system and the chemical evolution of associated melt and cumulates. In particular, to what extent can predictions be made from the thermal interactions between magma and wallrock on the chemical signatures of the associated magmatic rocks and hydrothermal deposits?
DE: 8439 Physics and chemistry of magma bodies
DE: 3640 Igneous petrology
DE: 1040 Isotopic composition/chemistry
DE: 1010 Chemical evolution
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting