"THERMODYNAMICS OF A MAGMATIC GAS PHASE" 50 YEARS LATER: COMMENTS ON A PAPER BY JOHN VERHOOGEN (1949)

Abstract

In 1949 John Verhoogen published a paper entitled Thermodynamics of a Magmatic Gas Phase in The University of California Publications in the Geological Sciences. This paper documented the first sophisticated application of thermodynamics to a problem in igneous petrology. In that paper, Verhoogen examined many aspects of the behavior of a magmatic gas phase: development and composition of a gas phase, deposition from a gas phase, variation of vapor pressure over a cooling magma. He also discussed two other possible processes in magmas: distillation and crystallization of indifferent states. Distillation occurs when a constituent has a higher concentration in the vapor than in the melt. The volatile constituents of magmas, H2O, CO2, and H2S, obviously fit the definition of constituents that undergo distillation. Verhoogen 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. Indifferent states occur during processes that change the masses of the phases of a system but leave their compositions unchanged. Univariant systems and azeotropes are familiar examples of indifferent states. At the time Verhoogen wrote his paper, there were limited amounts of experimental data on NaAlSi3O8–H2O and not much else. Thermodynamic databases constructed in the last decade provide the tools needed to explore processes described by Verhoogen 50 years ago.