Exploring High PT Experimental Charges Through the Lens of Phase Maps

High pressure and temperature (PT) experimental charges are valuable systems made of minerals, often with quenched melt, and/or a fluid. They are synthesized to inform petrological solid-state, melt-forming and/or fluid-assisted processes occurring deep within the Earth. Here we explored the utility of phase mapping for analysis of high PT charges. We compared phase maps obtained with automated mineralogy software of scanning-electron-microscopes and developed an open-source software workflow. This generates phase maps from combined back-scattered-electron and 4-element maps. Phase maps were constructed for a sub-solidus assemblage, a charge containing a small percentage of melt, and a melting experiment that failed to reach thermal equilibrium. For the sub-solidus experiment, the phase map returned an accurate modal mineralogy. For the quenched melt experiments, the phase map located low abundance phases and identified best-suited targets for chemical analysis. Using modal mineralogy of sub-regions on maps and mutual neighboring relationships, the phase maps helped to establish equilibrium conditions and verify melting reactions inferred from mass balance. We propose phase maps as a valuable additional documentation tool for publishing high PT experiments, as they can help identify rare phases and discover reactions. We conclude with a set of recommended instrument settings for high-quality phase maps on small experimental charges.