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The Magma Chamber Simulator quantifies the impact of simultaneous recharge, assimilation and crystallization through mass and enthalpy balance in a multicomponent–multiphase (melt + solids ± fluid) composite system. As a rigorous thermodynamic model, the Magma Chamber Simulator computes phase equilibria and geochemical evolution self-consistently in resident magma, recharge magma and wallrock, all of which are connected by specified thermodynamic boundaries, to model an evolving open-system magma body. In a simulation, magma cools from its liquidus temperature, and crystals ± fluid are incrementally fractionated to a separate cumulate reservoir. Enthalpy from cooling, crystallization, and possible magma recharge heats wallrock from its initial subsolidus temperature. Assimilation begins when a critical wallrock melt volume fraction (0·04–0·12) in a range consistent with the rheology of partially molten rock systems is achieved. The mass of melt above this limit is removed from the wallrock and homogenized with the magma body melt. New equilibrium states for magma and wallrock are calculated that reflect conservation of total mass, mass of each element and enthalpy. Magma cooling and crystallization, addition of recharge magma and anatectic melt to the magma body (where appropriate), and heating and partial melting of wallrock continue until magma and wallrock reach thermal equilibrium. For each simulation step, mass and energy balance and thermodynamic assessment of phase relations provide major and trace element concentrations, isotopic characteristics, masses, and thermal constraints for all phases (melt + solids ± fluid) in the composite system. Model input includes initial compositional, thermal and mass information relevant to each subsystem, as well as solid–melt and solid–fluid partition coefficients for all phases. Magma Chamber Simulator results of an assimilation–fractional crystallization (AFC) scenario in which dioritic wallrock at 0·1 GPa contaminates high-alumina basalt are compared with results in which no assimilation occurs [fractional crystallization only (FC-only)]. Key comparisons underscore the need for multicomponent–multiphase energy-constrained thermodynamic modeling of open systems, as follows. (1) Partial melting of dioritic wallrock yields cooler silicic melt that contaminates hotter magma. Magma responds by cooling, but a pulse of crystallization, possibly expected based on thermal arguments, does not occur because assimilation suppresses crystallization by modifying the topology of multicomponent phase saturation surfaces. As a consequence, contaminated magma composition and crystallizing solids are distinct compared with the FC-only case. (2) At similar stages of evolution, contaminated melt is more voluminous (∼3·5×) than melt formed by FC-only. (3) In AFC, some trace element concentrations are lower than their FC-only counterparts at the same stage of evolution. Elements that typically behave incompatibly in mafic and intermediate magmas (e.g. La, Nd, Ba) may not be ‘enriched’ by crustal contamination, and the most ‘crustal’ isotope signatures may not correlate with the highest concentrations of such elements. (4) The proportion of an element contributed by anatectic melt to resident magma is typically different for each element, and thus the extent of mass exchange between crust and magma should be quantified using total mass rather than the mass of a single element. Based on these sometimes unexpected results, it can be argued that progress in quantifying the origin and evolution of open magmatic systems and documenting how mantle-derived magmas and the crust interact rely not only on improvements in instrumentation and generation of larger datasets, but also on continued development of computational tools that couple thermodynamic assessment of phase equilibria in multicomponent systems with energy and mass conservation.
Wendy A. Bohrson, Frank J. Spera, Mark S. Ghiorso, Guy A. Brown, Jeffrey B. Creamer, Aaron Mayfield, Thermodynamic Model for Energy-Constrained Open-System Evolution of Crustal Magma Bodies Undergoing Simultaneous Recharge, Assimilation and Crystallization: the Magma Chamber Simulator, Journal of Petrology, Volume 55, Issue 9, September 2014, Pages 1685–1717, https://doi.org/10.1093/petrology/egu036
Journal of Petrology
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