HR: 1340h
AN: V43A-1120    [Abstracts]
TI: The Magma Chamber Simulator: An Interactive Computer Program for Modeling the Chemical and Physical Evolution of Complex Magmatic Systems
AU: * Bohrson, W A
EM: bohrson@geology.cwu.edu
AF: Dept. Geol. Sci., Central Washington University, Ellensburg, WA 98926, United States
AU: Spera, F J
EM: spera@geol.ucsb.edu
AF: Dept. Earth Sci., UCSB, Santa Barbara, CA 93106, United States
AU: Ghiorso, M S
EM: ghiorso@ofm-research.org
AF: OFM Researchâ€"West, 7336 24th Ave NE, Seattle, WA 98115, United States
AB: The Magma Chamber Simulator (MCS) is an interactive graphical computational tool for modeling the chemical and physical evolution of a complex magmatic system defined here as a magma body-wallrock couple. The tool seamlessly integrates the phase equilibria capabilities of MELTS (Ghiorso & Sack 1995) with trace element and isotope conservation equations based on the energy constrained (EC-RAFC) approach of Spera & Bohrson (2001, 2002, 2004) and Bohrson & Spera (2001, 2003). Evaluation of trace element behavior includes partitioning among coexisting solid, melt and fluid phases (Spera et al. 2007). Each MCS simulation provides a thermodynamically based description of the chemical and energetic state of a magma body and its surrounding wallrock as it evolves along a complex P-T-X path. The MCS is designed to address closed-system fractional or equilibrium crystallization as well as open-system processes such as assimilation of wallrock partial melt, stoping of wallrock blocks, recharge, and eruption. Input includes initial compositions and masses of magma, wallrock, and recharge magma and mineral-melt-fluid partition coefficients for trace elements. Compositions (major, trace element, isotopes) and abundances of solids and melt, and thermodynamic and physical properties of the system (e.g., viscosity, density, volume fraction fluid) are output. Because users may be interested in a subset of the large database generated for each simulation, the MCS offers an efficient method of presenting results tailored to the user's interest. The MCS also includes a graphing tool that allows direct comparison between model results and data for a particular natural system. Because the graphical display is updated after each step, the user can compare model vs. natural data in real-time and modify MCS input. Modifications are accommodated through an interactive module that allows the user to stop the simulation, return to previous states, modify a subset of input parameters (e.g., recharge magma composition), and continue the simulation. The potential of the MCS will be highlighted by examining results of a case study in which mafic magma undergoes assimilation-fractional crystallization in typical upper crust. Data for compositions of initial magma and wallrock provide the starting point for a MELTS analysis that incorporates energy conservation, such that, for each step of the simulation, energy liberated from the magma provides constraints on the degree of melting in wallrock; thus, the compositions of associated anatectic melt and residual solids are known. Some user-defined proportion of anatectic melt is incorporated into the magma, and the MCS provides physical and chemical information about the contaminated magma and the wallrock. Trace element analysis involves calculation of bulk partition coefficients for elements of interest using fluid-solid-melt partition coefficients estimated from experimental or theoretical data. Because the mineral assemblage and mass fraction of fluid for each step are output, calculated bulk partition coefficients reflect compositional and other dependencies, and the behavior of elements that are partitioned into a separate fluid phase is also quantified. Addition of the stoping, recharge, and eruption functions will expand the capabilities of the MCS, thereby providing a comprehensive modeling tool to address mechanistic questions about crustal magmatic processes.
DE: 3610 Geochemical modeling (1009, 8410)
DE: 3611 Thermodynamics (0766, 1011, 8411)
DE: 3618 Magma chamber processes (1036)
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting