Active volcanoes of Nicaragua, including Momotombo volcano, were monitored in 1982–1985 to determine volcanic hazard and to establish criteria for prediction of eruptions. In 1982 the chemical composition and isotopic ratios of fumarolic gases were found to depend on temperature. In the products of fumarolic activity the materials of crustal origin are of great significance. Sharp variations in the gas composition depending on exogenic factors were noted in 1983; the H2S/SO2 ratio was influenced the least. The temperature of fumarolic gases at Momotombo gradually increased from 740°C in 1974 to 895°C in 1984. During the same period the contents of H2S, SO2, CO, and H2 and the values of S/Cl, S/C, H2S/SO2, H2/H2O, and CO/CO2 ratios have risen, and the isotope ratios of hydrogen, sulfur, and especially carbon have changed. Future work in the crater of Momotombo will show whether the changes in the temperature and composition of the fumarolic gases are the precursors to a forthcoming eruption.

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Volcanic gas and condensate samples were collected in 1993-1994 from fumaroles of Koryaksky and Avachinsky, basaltic andesite volcanoes on the Kamchatka Peninsula near Petropavlovsk-Kamchatsky. The highest-temperature fumarolic discharges, 220 7C at Koryaksky and 473 7C at Avachinsky, are water-rich (940-985 mmol/mol of H 2 O) and have chemical and isotopic characteristics typical of Kamchatka-Kurile, high-and medium-temperature volcanic gases. The temperature and chemical and water isotopic compositions of the Koryaksky gases have not changed during the past 11 years. They represent an approximate 2 : 1 mixture of magmatic and meteoric end members. Lowtemperature, near-boiling-point discharges of Avachinsky Volcano are water poor (;880 mmol/mol); Their compositions have not changed since the 1991 eruption, and are suggested to be derived from partially condensed magmatic gases at shallow depth. Based on a simple model involving mixing and single-step steam separation, low water and high CO 2 contents, as well as the observed Cl concentration and water isotopic composition in low-temperature discharges, are the result of near-surface boiling of a brine composed of the almost pure condensed magmatic gas. High methane content in low-temperature Avachinsky gases and the 220 7C Koryaksky fumarole, low C isotopic ratio in CO 2 at Koryaksky (-11.8‰), and water isotope data suggest that the "meteoric" end member contains considerable amounts of the regional methane-rich thermal water discovered in the vicinity of both volcanoes.

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