Performing process-oriented investigations involving mass transfer using Rcrust: a new phase equilibrium modelling tool

Modern quantitative phase equilibria modelling allows the calculation of the stable phase assemblage of a rock system given its pressure, temperature and bulk composition. A new software tool (Rcrust) has been developed that allows the modelling of points in pressure–temperature–bulk composition space in which bulk compositional changes can be passed from point to point as the system evolves. This new methodology enables quantitative process-oriented investigation of the evolution of rocks. Procedures are outlined here for using this tool to model: (1) the control of the water content of a subsolidus system based on available pore space; (2) triggering of melt loss events when a critical melt volume threshold is exceeded, while allowing a portion of melt retention; (3) entrainment of crystals during segregation and ascent of granitic magmas from its source; (4) modification of the composition of granite magmas due to fractional crystallization; and (5) progressive availability (through dissolution) of slow diffusing species and their control on the effective bulk composition of a system. These cases collectively illustrate thermodynamically constrained methods for modelling systems that involve mass transfer.