Europa likely contains an iron‐rich metal core. For it to have formed, temperatures within Europa reached1250 K. Going up to that temperature, accreted chondritic minerals — for example, carbonates and phyllosilicates — would partially devolatilize. Here, we compute the amounts and compositions of exsolved volatiles. We find that volatiles released from the interior would have carried solutes, redox‐sensitive species, and could have generated a carbonic ocean in excess of Europa's present‐day hydrosphere, and potentially an earlyatmosphere. No late delivery of cometary water was necessary. Contrasting with prior work,could be the most abundant solute in the ocean, followed by,, and. However, gypsum precipitation going from the seafloor to the ice shell decreases the dissolved S/Cl ratio, such that ClS at the shallowest depths, consistent with recently inferred endogenous chlorides at Europa's surface. Gypsum would form a 3–10 km thick sedimentary layer at the seafloor.

The ability of Rcrust software to conduct path-dependent phase equilibrium modelling with automated changing bulk compositions allows for a phase equilibrium approach to investigate an array of source controls for their effect on the bulk compositions of melts produced by sequential melting events. The following source controls of the rock system are considered: (1) initial magnesium and iron content; (2) initial sodium and calcium content; (3) pressure–temperature path followed by the system; and (4) threshold by which melt extractions in the system are triggered. These source controls are investigated in a water-restricted system and a water-in-excess system. The permutation of these cases resulted in 128 different modelled pressure–temperature bulk composition paths investigating the melting of an average pelite composition. The resultant melt compositions are compared to that of a natural granite dataset and provide a good fit for the incompatible elements Na ₂ O and K ₂ O with the allowance that granites most likely form as magmas consisting of melt and ferromagnesian-rich crystals. The fluid state of the system is shown to have the strongest control on melt compositions, with the pressure–temperature path having subordinate control on the volume and composition of melts produced.

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) the triggering of melt loss events when a critical melt volume threshold is exceeded, while allowing a portion of melt retention; (3) the entrainment of crystals during segregation and ascent of granitic magmas from its source; (4) the modification of the composition of granite magmas owing to fractional crystallization; and (5) the progressive availability (through dissolution) of slow diffusing species and their control of the effective bulk composition of a system. These cases collectively illustrate thermodynamically constrained methods for modelling systems that involve mass transfer.

One of the largest chromium deposits on Earth occurs in the Rustenburg Layered Suite (RLS) of the Bushveld Complex as laterally continuous chromitite layers. None of the hypotheses proposed for the origin of the chromitites can explain both the abundance of Cr in the RLS and the unusual enrichment in Cr and V over Ni, relative to typical depleted mantle values. This study investigates the possibility that the layering and chromitite formation are consequences of the entrainment of source components into the magmas that formed the RLS. Thermodynamic modelling results reveal a wedge-shaped domain in pressure-temperature space in the subcratonic mantle within which Cr-bearing orthopyroxene forms as a peritectic product of incongruent melting. Entrainment of this orthopyroxene produces magmas that crystallise peritectic olivine and chromite on ascent, due to the consumption of orthopyroxene by melt. The chromite- and olivine-bearing magmas intrude as sills and can produce chromite and dunite layers by density separation. This model, which interprets the RLS Sr-isotopic composition to reflect prior mantle metasomatism by crustal fluids (ideally ancient and of low volume), readily explains the formation of chromitite layers from relatively thin sills, as well as the very high ratios of Cr and V to other compatible elements relative to typical mantle compositions. The special circumstances required to produce the RLS chromitites do not relate to some oddity of repetitive crustal assimilation or magma compositions that allow chromite-only saturation. Rather, they relate to speed of melting and magma extraction which enabled peritectic orthopyroxene entrainment to the magmas.