Basalt Alteration in High-Temperature Oxidizing-Type Fumaroles at the Tolbachik Volcano (Kamchatka, Russia). Part 2: Gas Metasomatites

Five specific types of silicate apobasaltic gas metasomatites formed in the temperature range from 850–900 to 450–500°C have been identified and characterized in oxidizing-type fumaroles at the Tolbachik volcano (Kamchatka, Russia): (1) diopside–esseneite, (2) haüyne–diopside, (3) anorthoclase/Na-sanidine, (4) sanidine, and (5) fluorophlogopite/sanidine–fluorophlogopite metasomatites. Exhalation incrustations of certain mineral and chemical composition are related to each of these types. In the vertical section of the fumarole system from bottom to top, against the background of volcanic-gas cooling, a sequential spatial change of the first four types of gas metasomatites occurs. Fluorophlogopite and sanidine–fluorophlogopite gas metasomatites do not have a clear position in the vertical section of the fumarole system and presumably are formed under the action of a gas significantly enriched with Cl and F. Highly siliceous metasomatites, consisting mainly of silica phases, form instead of silicate rocks at temperatures below 450–500°C.