HR: 1330h
AN: V12D-0613    [PDF]
TI: Open-System Magma Chamber Evolution: an Energy-Constrained Geochemical Model Incorporating the Effects of Concurrent Eruption, Recharge, Imperfect Assimilation and Fractional Crystallization (EC-E'RA$\chi$FC)
AU: * Spera, F J
EM: spera@geol.ucsb.edu
AF: Dept of Earth Sciences and Institute for Crustal Studies, University of California, Santa Barbara, CA 93106
AU: Bohrson, W A
EM: bohrson@caliente.geology.cwu.edu
AF: Dept of Geology, Central Washington University, Ellensburg, WA 98926
AB: Geochemical data for many plutonic and volcanic cogenetic suites provide overwhelming evidence for open system behavior. Significant petrogenetic processes influencing the geochemical evolution of magma bodies include magma Recharge, heating and partial melting (Assimilation) of country rock, formation and separation of cumulates by Fractional Crystallization and the formation of enclaves by rapid chilling of newly-intruded recharge magma. We have extended the EC-RAFC trace element and isotopic geochemical model (Spera and Bohrson, 2002; Bohrson and Spera, 2002) to include the effects of magma Eruption and variable amounts of assimilation, A$\chi$. The EC-E'RA$\chi$FC model tracks the geochemical path (trace element and isotopic composition) of magma body melt (host melt), eruptive magma, cumulates and enclaves for magma undergoing simultaneous eruption, recharge, assimilation and fractional crystallization as a function of magma temperature. EC-E'RA$\chi$FC is formulated as a set of $4+t+i+s$ coupled nonlinear ordinary differential equations, where the number of trace elements, radiogenic and stable isotope ratios simultaneously modeled are t, i and s, respectively. There are no limitations on the values of t, i or s. Solution of the EC-E'RA$\chi$FC equations provides values for the average temperature of wall rock (Ta), mass of host melt within the magma body (Mm), masses of cumulates (Mct), enclaves (Men) and wall rock involved in the thermal interaction (Mao), the mass of anatectic melt generated (Ma$\star$) and assimilated ($\chi$Ma$\star$), the concentration of t trace elements and i+s isotopic ratios in host melt (Cm), eruptive magma, cumulates (Cct), enclaves (Cen) and anatectic melt (Ca) as a function of magma temperature (Tm). Input parameters include the (user-defined) equilibration temperature (Teq), the efficiency of mixing of anatectic melt factor ($\chi$), the initial temperature and composition of pristine host melt (Tmo, Cmo, $\epsilon$mo), recharge melt (Tro, Cro) and wall rock (Tao, Cao), distribution coefficients (Dm, Dr, Da) and their temperature dependencies ($\Delta$Hm, $\Delta$Hr, $\Delta$ Ha), heat of transition for wall rock ($\Delta$ha), pristine ($\Delta$hm) and recharge ($\Delta$hr) and the isobaric specific heat capacity of assimilant (Cp,a), pristine (Cp,m) and recharge melt (Cp,r). The magma recharge mass and eruptive magma mass functions Mr(Tm) and Me(Tm), respectively, are specified a priori and allow one to predict the effects of recharge and magma eruption on the geochemical evolution of the system. Melt productivity functions, which prescribe the relationship between melt mass fraction and temperature, are defined for end-member bulk compositions characterizing the local geologic site. EC-E'RA$\chi$FC makes testable quantitative predictions for relative masses and compositions for volcanic and plutonic products of magma system evolution which can be can be compared to information gathered from field studies to critically test petrogenetic hypotheses. The 'systems' approach to understanding magma chamber evolution promises to enhance the ability to describe the efficacy of petrologic processes in various petrotectonic settings. Illustrative examples of complex EC-E'RA$\chi$FC simulations are presented to highlight the salient features of the model.
UR: http://magma.geol.ucsb.edu/
DE: 3230 Numerical solutions
DE: 3640 Igneous petrology
DE: 3670 Minor and trace element composition
DE: 3939 Physical thermodynamics
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting