Isotopic Geochemistry of Acid Lakes in the Uzon-Geyserny and Karymsky Geothermal Areas (KAMCHATKA)

Abstract

Conditions of formation and long existence of acid caldera lakes in areas of active volcanism are examined on the basis of study of saline and hydroisotopic composition of different water manifestations in the Uzon-Geyserny and Karymsky geothermal areas of Kamchatka. A wide spectrum of hydrochemical types of solutions with pH varying from neutral up to acid values is observed in both areas. But recent volcanic activity in caldera structures of these areas differs in degree of intensity. A short-term but powerful phreatic-magmatic eruption occurred in the Karymsky geothermal area in the Akademii Nauk caldera where the Karymskoe Lake, which until 1996 was a freshwater one, was located. Enormous quantity of erupted material was ejected. Practically instantly the lake water got acid reaction (pH = 3,2) which has already been keeping for 4 years in spite of constant inflow of fresh sub-neutral water owing to atmospheric precipitation. In this lake processes of extraction of water-soluble sulphate-chloride complexes from newly erupted rocks mainly maintain water acid reaction. Possibly, endogenic fluids are still supplying from the bottom of the lake. In the Uzon caldera (the Uzon-Geyserny geothermal area) the Fumarolnoe and Bannoe Lakes also have acid reaction (pH = 3,5-5,0) for a long time, but owing to processes of oxidation of sulfurcontaining compounds in surrounding hydrothermallychanged rocks. It has been concluded that endogenic factor intensity is determinative in acidification of water mass in caldera structures in areas of recent volcanism and acid lake existence duration depends on time of exhaustion of resources of sulfur-containing components in enclosing rocks.