COMPLEX PETROGENETIC LINEAGES DEFINED BY THERMOCHEMICAL MODELING: THE MAGMA CHAMBER SIMULATOR AS A TOOL FOR UNRAVELING A MAGMA FAMILY TREE

Solidified magmas preserve complex but typically incomplete histories of their origins; as a result, igneous petrogenesis can be difficult to articulate. An emerging strategy relies on iterative thermodynamic analysis of open system, energy and mass constrained forward models utilizing high-resolution data collected at multiple spatiotemporal scales. The Magma Chamber Simulator (MCS) quantifies recharge (magma mixing), assimilation and crystallization (RAFC) through mass and enthalpy exchange in a magma-recharge magma-wallrock system. MCS results establish a reference state for RAFC evolution and include compositions (major, trace elements, isotopic ratios), masses and temperatures (T) of all phases within magma, recharge magma, and wallrock. To illustrate the challenges in accurately portraying petrogenetic lineage, MCS results for a multiply recharged high alumina basalt residing in shallow andesitic crust were randomly sampled (i.e., random number generator) between the start and end of this simulation. As in field studies, this ‘sampling regimen’ does not preserve the entire evolution. Selected critical observations are: (1) the age-progressive evolutionary context is essential. In the absence of detailed relative stratigraphy or high-resolution geochronology, recharge in particular is difficult to document. (2) T does not change monotonically because of recharge, and thus opportunistic sampling will likely yield inaccurate reconstruction of the T history. (3) Opx, plagioclase and olivine are not continuously present. Thus, interpretation of mineral records may be biased, and associated petrologic and geochronologic conclusions may be flawed; this applies equally well to accessory minerals (e.g., zircon). (4) Documentation of roles and timing of magma processes requires multi-element/isotope analysis because the mass and thermodynamic responses of each to RAFC are distinct; thus deciphering a petrogenetic lineage requires application of a tool like MCS to major/trace element and isotopic data of melt±solids±fluid. We conclude that over the coming decades, computational tools must be utilized and augmented with the same vigor as analytical tools; it is through their combined use that magma family trees will be documented and associated petrogenetic questions elucidated.