Compositional variations in epidote from geothermal areas

Abstract

Chemical compositions of epidote from active and ancient geothermal systems were summarized. Iron and aluminum contents of epidote from geothermal systems vary widely, ranging from about 4 to 19 wt % for Fe2O3and from 18 to 30 wt % for A1203. It was found that the iron content of epidote has roughly a positive correlation to the Fe203 content of original rocks, indicating that the Fe203 content of original rocks affects largely the variations in iron content of epidote. This correlation is consistent with the results of theoretical calculations on the water/rock interaction under the hydrothermal conditions which show that the epidote is formed under the rock-dominated conditions. Epidote associated with hematite has high iron content, while that associated with prehnite, pyrite and pyrrhotite has low iron content. These facts indicate that CO2 and O3fugacities are also important factors controlling the compositional variations in epidote. © 1984, GEOCHEMICAL SOCIETY OF JAPAN. All rights reserved.