Chemical patterns of erupting silicic magmas and their influence on the amount of degassing during ascent

We present a chemical model of magma degassing based on nine volatile species part of S-O-H-C-Fe-bearing rhyolitic melts. It is based on equilibrium, closed-system degassing, and does not take in account the crystallization of mineral phases. For given initial conditions at depth, the model calculates the gas composition as pressure decreases, as well as physical variables controlling conduit flow. We conducted a parametric study of degassing by varying initial conditions at depth within the range typical of arc magmas and characterizing the chemical evolution of the volatile components during the ascent to the surface. The resulting complex patterns of chemical changes are controlled by four groups of interrelated parameters: (1) redox state, fH(2), and H2S/SO2 ratio in the gas phase, (2) gas amount, (3) melt water content and fH(2)O, and (4) relative melt sulfur content, fCO(2), and C/S ratio in the gas phase. The other degrees of freedom (temperature, initial pressure, and melt iron content) alter the amplitude of the chemical fluctuations during ascent, but do not change the general degassing patterns. When degassing starts at depth, volatile chemistry has little effect on the amount of degassing as measured by the evolution of porosity. Magma expansion at shallow levels, however, can be greatly affected by degassing chemistry.