FixAl: A Complete Chemical Geothermometry Method for Incomplete Water Analysis with Special Reference to Aluminum

摘要

Using a synthetic geothermal water, we examine the significant effect of errors in aluminum analyses on theoretical chemical geothermometry based on multi-component chemical equilibrium calculations of mineral equilibria. A new approach named "FixAl" that entails the construction of a modified Q /K graph eliminates problems with water analyses lacking aluminum or with erroneous analyses of aluminum. This is made possible by forcing the water to be at equilibrium with a selected aluminum-bearing mineral, such as microcline. In a FixAl graph, a modified Q/K value is plotted against temperature for Al-bearing minerals. Saturation indices of non-aluminous minerals are plotted in the same way as in an ordinary Q /K graph. Examples of geothermal waters from Iceland and the US that are used to demonstrate the methods, show that errors in aluminum concentrations are common and some are severe. The proposed approach has proved useful for chemical geothermometry on geothermal waters without aluminum analysis or waters with an incorrect aluminum analysis. Hie equilibrium temperatures predicted by the FixAl approach agree well with quartz and chalcedony geothermometers. The best choice of mineral for forced equilibrium depends on pH; for most neutral pH waters, microcline works well; for more acidic waters, kaolinite or illite are good choices.