Fumarolic Activity of Avachinsky and Koryaksky Volcanoes, Kamchatka, from 1993 to 1994

<p> Volcanic gas and condensate samples were collected in 1993&ndash;1994 from fumaroles of Koryaksky and Avachinsky, basaltic andesite volcanoes on the Kamchatka Peninsula near Petropavlovsk&ndash;Kamchatsky. The highest-temperature fumarolic discharges, 220&hairsp;&hairsp;&deg;C at Koryaksky and 473&hairsp;&hairsp;&deg;C at Avachinsky, are water-rich (940&ndash;985&thinsp;mmol/mol of H2O) and have chemical and isotopic characteristics typical of Kamchatka&ndash;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&hairsp;:&hairsp;1 mixture of magmatic and meteoric end members. Low-temperature, near-boiling-point discharges of Avachinsky Volcano are water poor (&asymp;880&thinsp;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 CO2 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&hairsp;&hairsp;&deg;C Koryaksky fumarole, low C isotopic ratio in CO2 at Koryaksky (&ndash;11.8&permil;), 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.</p>