Basalt Alteration in High-Temperature Oxidizing-Type Fumaroles at the Tolbachik Volcano (Kamchatka, Russia). Part 1. Processes and Products of Olivine Alteration

This paper is devoted to the olivine pseudomorphization processes occurring in basalt under the action of high-temperature O₂-rich gas in oxidizing-type fumaroles at the Tolbachik volcano (Kamchatka, Russia). The alteration of olivine with composition Fo_72–90 in a terraneous oxidizing-type exhalation system involves two independent processes: (1) oxidative breakdown (above 600 °C) with the formation of forsterite Fo_90–100 together with hematite, magnesioferrite, and, presumably, “magnesian laihunite”; (2) replacement by mineral aggregates in which pyroxenes [enstatite, clinoenstatite, diopside, aegirine-augite (Ca_0.5Na_0.5)(Mg_0.5Fe_0.5)Si₂O₆], low-alumina fluoromagnesian micas (yangzhumingite and fluorotetraferriphlogopite), chondrodite or cristobalite are the main components accompanied by hematite, sometimes with magnesioferrite or tenorite. In the active Arsenatnaya fumarole, the change of the main Si minerals in pseudomorphs with the gas temperature decrease occurs in a sequence that generally reflects an increase in the degree of condensation of tetrahedral Si-O motifs: olivine (forsterite) → orthopyroxene (enstatite) → clinopyroxenes → micas → cristobalite.