Diagnosing open-system magmatic processes using the Magma Chamber Simulator (MCS): part I—major elements and phase equilibria

Abstract

The Magma Chamber Simulator (MCS) is a thermodynamic tool for modeling the evolution of magmatic systems that are open with respect to assimilation of partial melts or stoped blocks, magma recharge + mixing, and fractional crystallization. MCS is available for both PC and Mac. In the MCS, the thermal, mass, and compositional evolution of a multicomponent–multiphase composite system of resident magma, wallrock, and recharge reservoirs is tracked by rigorous self-consistent thermodynamic modeling. A Recharge–Assimilation (Assimilated partial melt or Stoped blocks)–Fractional Crystallization (R_nAS_nFC; n_tot ≤ 30) scenario is computed by minimization or maximization of appropriate thermodynamic potentials using the family of rhyolite- and pMELTS engines coupled to an Excel Visual Basic interface. In MCS, during isobaric cooling and crystallization, resident magma thermally interacts with wallrock that is in internal thermodynamic equilibrium. Wallrock partial melt above a user-defined percolation threshold is homogenized (i.e., brought in to chemical potential equilibrium) with resident magma. Crystals that form become part of a cumulate reservoir that remains thermally connected but chemically isolated from resident melt. Up to 30 instances (n ≤ 30) of magma mixing by recharge and/or bulk assimilation of stoped wallrock blocks can occur in a single simulation; each recharge magma or stoped block has a unique user-defined composition and thermal state. Recharge magmas and stoped blocks hybridize (equilibrate) with resident melt, yielding a single new melt composition and temperature. MCS output includes major and trace element concentrations and isotopic ratios (Sr, Nd, Hf, Pb, Os, and O as defaults) of wallrock, recharge magma/stoped blocks, resident magma melt, and cumulates. The chemical formulae of equilibrium crystalline phases in the cumulate reservoir, wallrock, and recharge magmas/stoped blocks are also output. Depending on the selected rhyolite-MELTS engine, the composition and properties of a possible supercritical fluid phase (H₂O and/or CO₂) are also tracked. Forward modeling of theoretical magma systems and suites of igneous rocks provides quantitative insight into key questions in igneous petrology such as mantle versus crustal contributions to terrestrial magmas, the record of magmatism preserved in cumulates and exsolved fluids, and the chronology of RASFC processes that may be recorded by crystal populations, melt inclusions, and whole rocks. Here, we describe the design of the MCS software that focuses on major element compositions and phase equilibria (MCS-PhaseEQ). Case studies that involve fractional crystallization, magma recharge + mixing, and crustal contamination of a depleted basalt that resides in average upper crust illustrate the major element and phase equilibria consequences of these processes and highlight the rich array of data produced by MCS. The cases presented here, which represent an infinitesimal fraction of possible RASFC processes and bulk compositions, show that the records of recharge and/or crustal contamination may be subtle and are not necessarily those that would be predicted using conventional intuition and simple mass balance arguments. Mass and energy constrained thermodynamic tools like the MCS quantify the open-system evolution of magmas and provide a systematic understanding of the petrology and geochemistry of open system magmatic processes. The trace element and isotope MCS computational tool (MCS-Traces) is described in a separate contribution (part II).