THERMODYNAMIC EVALUATION AND RESTORATION OF VOLCANIC GAS-ANALYSES - AN EXAMPLE-BASED ON MODERN COLLECTION AND ANALYTICAL METHODS

Thermodynamic evaluation and restoration procedures are applied to a set of 10 volcanic gas analyses obtained by modern collection and analytical methods. The samples were collected from a vigorously fuming fissure during episode 1 of the Puu Oo eruption of Kilauea Volcano in 1983. Collections were made through titanium and mullite tubing at 895-935 degrees C in evacuated bottles containing 4N NaOH solution. A variety of analytical techniques were used to determine the gas compositions. In most samples, the combined amounts of N-2 + Ar + O-2 are far less abundant than H-2, CO, or H2S, suggesting little or no contamination or reaction with atmospheric gases. Analyses of samples collected through titanium and mullite tubing are similar, indicating chemical alterations from gas-titanium interactions were insignificant. Thermodynamic evaluation shows that 6 of the 10 analyses are equilibrium compositions, and 4 analyses are disequilibrium compositions. Three of the disequilibrium analyses involve samples affected by minor spilling of NaOH solution from the sample bottles during collection. The deviation of these analyses from equilibrium is dominated by the effects of disequilibrium water-loss. The fourth disequilibrium analysis, is contaminated with meteoric water. In all 4 cases, the restoration procedures retrieve the original equilibrium compositions. Minor loss of sulfur by elemental S precipitation is restored for all samples. All gas samples approach closely to equilibrium compositions at temperatures up to 117 degrees C above collection temperatures. The equilibrium preserved in the gases, however, actually reflects oxygen buffering of the gases by coexisting lavas, not just simple internal homogeneous equilibrium among the gas species. The oxygen buffering process is not evident if oxygen fugacities or R(H) redox parameter values are calculated and plotted directly from the analytical data. It is only evident after thermodynamic evaluation and restoration of the analyses. Direct use of gas analytical data in thermodynamic calculations and diagrams without prior evaluation of equilibrium/disequilibrium and restoration is not recommended, despite the undeniable improvements in collection and analytical techniques.