Geothermics, an introduction: G. Buntebarth. Translated from the German by I.M. Chapman; Springer, Berlin, ix + 144 pp., DM 60.00/US $23.60 (paperback)

Abstract

An assessment of F-C1-OH partitioning between natural apatite and biotite in a variety of rocks was used to evaluate reciprocal (Mg, Fe2+, AlVI) (F, Cl, OH) mixing properties for biotite according to the reciprocal salt solution model of Wood and Nicholls (1978). Ideal mixing of F-C1-OH and Fe-Mg-Al VI in the hydroxyl and octahedral sites is assumed for biotites with dilute Cl concentrations. The reciprocal interaction parameters, in terms of Gibbs free energies, for the reactions KMg3[AlSi3O10](OH)2 + KFe3[AlSi3O10](F)2 = KMg3[AlSi3O10](F)2 + KFe3[AlSi3O10](OH)2 Phl Fann Fphl Ann and KMg3[AlSi3O10](Cl)2 + KFe3[AlSi3O10](OH)2 = KMg3[AlSi3O10](OH)2 + KFe3[AlSi3O10] (Cl)2 Clphl Ann Phl Clann are about −10 kcal/mol and −4.5 kcal/mol, respectively. These mixing properties are consistent with standard state thermodynamic properties for F and Cl endmember phases from Zhu and Sverjensky (1991). The approach of studying F-C1-OH partitioning between biotite and apatite permits distinguishing the reciprocal effects from the effects of temperature, pressure, and fluid composition. The resultant mixing properties are consistent with observations both in hydrothermal experiments and in natural mineral assemblages.The mixing properties presented in this study enable us now to predict F and Cl concentrations of hydrothermal fluids from the measured F and Cl concentrations in biotite with variable Fe-Mg-AlVI proportions. A case study of the Santa Rita porphyry copper deposits, New Mexico, shows that hydrothermal fluids responsible for the phyllic alteration had a salinity about 3 molal Cl−, in agreement with fluid inclusion studies. Our internally consistent standard thermodynamic properties and solid solution models also lead to recalibration of the apatite-biotite geothermometer. The revised geothermometer predicts temperatures that agree with those estimated from other independent geothermometers. The large reciprocal effects in biotite also point out the need to revise other geothermometers using biotite and to revise the ideal mixing model for biotite in petrologic studies.