Evaluation of volcanic gas analyses from Kilauea volcano

Abstract

Much of the chemical variation and disequilibrium in Kilauean volcanic gas analyses is the result of contamination by meteoric water and, to a lesser extent, organic matter. Contamination by meteoric water is extensive in some samples, causing variations of two to three orders of magnitude in atomic H/C. severely contaminated with meteoric water that did not equilibrate with the “Magmatic” gases and consequently gives the analyses a disequilibrium appearance. When the contaminating H 2O is removed, the analyses converge to equilibrium mixtures in the temperature range 1085°C to 1185°C with O 2 fugacities slightly above those for the quartz-magnetite-fayalite buffer. The restored analyses are richer in CO 2 (18–50%) and SO 2 (10–21%) and poorer in H 2O (37–70%) than are the original analyses. There is no single gas phase composition for erupting Kilauea basalts. The restored J-series gases and the higher quality gases collected from Kilauea since 1920 form a spectrum of gas compositions related by CO 2 content. A trend of decreasing CO 2 with progressive outgassing is suggested. The extremes in the spectrum of compositions, based on presently available data, range from an “early” magmatic gas containing 50% CO 2, 35% H 2O, and 15% SO 2 to a more evolved gas with 70% H 2O, 10% CO 2 and 20% SO 2. The common presence of hydrocarbons and anomalously low atomic S/C in several Kilauea gas analyses is due to contamination of erupting lavas with organic materials. A similar origin also applies to above background levels of environmentally hazardous organohalogens in volcanic gases. Thermodynamic calculations indicate these compounds would be virtually absent in gases erupted from lavas at temperatures above 500–600°C.