Compositional variations in epidote from geothermal areas

抄録

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 Fe₂O₃ and from 18 to 30 wt % for Al₂O₃. It was found that the iron content of epidote has roughly a positive correlation to the Fe₂O₃ content of original rocks, indicating that the Fe₂O₃ 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 O₂ fugacities are also important factors controlling the compositional variations in epidote.