PETROLOGICAL MODELING VS PETROLOGICAL MODELING: A COMPARATIVE STUDY OF TWO PHASE EQUILIBRIUM MODELING TOOLS

Phase equilibria modeling is a powerful petrological tool to address both forward and inverse geological problems over a broad range of crustal and upper mantle conditions of pressure (P), temperature (T), composition (X) and redox (f_O2). The development of thermodynamic databases, relatively complex activity-composition (a-X) relations for solids, melts and fluids, pressure-volume-temperature (PVT) equations of state (EOS), and efficient numerical algorithms represent an inflection point in our ability to understand the nexus between tectonics and petrogenesis. Some of the published thermodynamic models have overlapping P-T-X calibration ranges, which enables comparisons of model outcomes for similar conditions within the range of applicability and with experimental data not used in the model calibrations.

Here, we systematically compare the results of two such models that are routinely used for modeling phase equilibria in melt-bearing systems: rhyolite-MELTS and the metabasite set of Green et al. (2016) using the thermodynamic database ds62 (hereafter denoted as “MG16”). We selected a N-MORB composition and modeled closed system equilibrium phase relations as a function of temperature at 0.25 GPa and 1 GPa for N-MORB with 0.5 wt% and 4 wt% H₂O. Our results show that phase relations exhibit some key differences that, in some instances, impact geological inferences. High-T liquids are generally similar in SiO₂ contents but diverge at lower temperatures; in these cases, MG16 liquids are SiO₂-depleted compared to those produced by rhyolite-MELTS. Liquids are also systematically and substantially more mafic in MG16, and alumina and the alkali concentrations are relatively different and show different trends as a function of temperature at constant pressure. Overall, liquid compositions show the greatest differences near the solidus. Differences in modal abundances of phases and liquid compositions influence liquid trace-element signatures, and these differences can affect geological interpretations. Finally, a comparison between melting experiments of basaltic bulk composition and both thermodynamic models show that rhyolite-MELTS better reproduces the higher temperature experiments, whereas MG16 better reproduces the lower temperature experiments.