"Thermodynamics of a magmatic gas phase" 50 years later: Comments on a paper by John Verhoogen (1949)

In 1949 John Verhoogen published a paper entitled Thermodynamics of a Magma tic Gas Phase in The University of California Publications in the Geologica l Sciences. This paper documented the first sophisticated application of th ermodynamics to a problem in igneous petrolopy. In that paper, Verhoogen ex amined many aspects of the behavior of a magmatic gas phase: development an d composition of a gas phase, deposition from a gas phase, variation of vap or pressure over a cooling magma. He also discussed two other possible proc esses in magmas: distillation and crystallization of indifferent states. Di stillation occurs when a constituent has a higher concentration in the vapo r than in the melt. The volatile constituents of magmas, H2O, CO2, and H2S, obviously fit the definition of constituents that undergo distillation. Ve rhoogen did not discuss the reverse process, partitioning of a constituent into the melt rather than the gas phase. HF is a constituent that undergoes reverse distillation; its concentration is higher in a melt than in a gas phase. Volatile constituents that undergo reverse distillation cause magmas to vesiculate at shallower depths than those that undergo distillation. in different states occur during processes that change the masses of the phase s of a system but leave their compositions unchanged. Univariant systems an d azeotropes are familiar examples of indifferent states. At the time Verho ogen wrote his paper, there were limited amounts of experimental data on Na AlSi3O8-H2O and not much else. Thermodynamic databases constructed in the l ast decade provide the tools needed to explore processes described by Verho ogen 50 years ago.