Chemical and isotopic composition of fumarolic gases and the SO2 flux from Volcلn de Colima, México, between the 1994 and 1998 eruptions

Volcán de Colima is the most active volcano in Mexico, including a significant effusive eruption in 1991 and a weak explosive event in 1994. Geochemical monitoring of the volcano started in January 1996 with collection of fumarolic gases approximately every 3 months from two sites in the summit crater (Z3, ∼800°C, and Z2, ∼400°C). Chemical compositions of volcanic gases from the Z2 fumarole showed a peak in S/Cl in the middle of 1997, 14–15 months before the start of the 1998 lava eruption. High-temperature volcanic vapors from Z3 changed progressively in water isotopic composition and showed a weak trend of increasing HCl/HF and a peak in S/Cl at the same time. Starting from the summer of 1997, the volcanic water became gradually enriched in deuterium, which probably indicated an increasing contribution from deep, less degassed magma. A negative correlation between δD and Cl content for the Z3 fumarolic gases is explained by shallow degassing of the Colima hot lava plug. This hypothesis is consistent with published data on Cl contents and solubilities in magmas and supported by the observed positive trends of S/Cl and HCl/HF with δD for the Z3 fumarole. Until September 1998, the SO2 flux from the volcano was at a low level (<100 tons/day), and often below the detection limit of the instrument (∼30 tons/day). A sharp increase in the SO2 flux was recorded 1 month before the eruption started. It jumped from 400±50 tons/day to 1600 tons/day 2 days before new lava emerged on 20 November 1998. The SO2 flux correlated well with seismicity.