The Magma Chamber Simulator: A Comprehensive Tool for Modeling the Evolution of Magmatic Systems

The Magma Chamber Simulator (MCS) is a computational tool for modeling the chemical and physical evolution of magmatic systems. Closed-system fractional or equilibrium crystallization as well as open-system processes such as assimilation of wallrock partial melt, equilibration of stoped country rock, magma recharge, and eruption are incorporated self-consistently by combining results from the thermodynamic model of Ghiorso &Sack (1995, i.e. MELTS) with trace element and isotope conservation equations (e.g., Spera & Bohrson, 2001, 2002; Bohrson &Spera 2001, 2003). 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. A critical aspect of the MCS is the direct coupling of phase information generated by minimization of appropriate thermodynamic potentials with partition coefficients governing trace element distribution among solid, melt, and fluid. Each MCS simulation results in a thermodynamically-based description of the chemical and energetic state of a magma body as it evolves along a complex P-T-X path. The MCS is structured so that improvements in thermodynamic and trace element partition coefficient databases can be efficiently incorporated into the code. The MCS provides for interactive visual output so that the geochemist can compare observed geochemical data with model results. Although completion of the MCS is several years away, results are presented here on a prototype version used to study the petrologic-geochemical evolution of the Campanian Ignimbrite (39.3 ka trachytic-phonolitic ignimbrite located near Naples, Italy). We show that magma evolution was dominated by crystal fractionation in a fluid-saturated environment with minor assimilation of upper crust. A trachybasaltic parent (Tliq = 1235°C) initially crystallized olivine, clinopyroxene, spinel and apatite. At 883°C, identified as a pseudo-invariant temperature, melt simultaneously saturated in alkali feldspar, plagioclase and biotite, causing a dramatic decrease in fraction of melt from ~0.5 to 0.1. Crystallization also led to a striking decrease in melt viscosity (1700 Pa s to 200 Pa s), a marked change in volume fraction of water in magma (~0.1 to 0.8), and a significant decrease in melt and magma density; these changes acted as a destabilizing eruption trigger (see Fowler et al.). Trace element modeling in the prototype MCS couples mineral and fluid abundances along the liquid line of descent with fluid-present fractional crystallization conservation equations (see Spera et al.) and estimates of mineral-melt-fluid partition coefficients. Preliminary results indicate that trace elements can be divided into 5 groups: (1) compatible elements (Ba, Sr, Cr) which are influenced by crystallizing phases; (2) REE, which show evidence of being incompatible, although behavior of Sm and Nd may reflect crystallization of apatite; (3) HFSE, which are dominated by incompatible behavior but require input from wallrock; (4) elements (e.g., As, Sb) which may owe their behavior to interaction between magma and hydrothermally altered wallrock; (5) elements such as Rb and Pb, which may be scavenged from magma by supercritical fluid.