A New K-feldspar Liquid Hygrometer: Initial Experiments, Calibration and Application

Dissolved H₂O in magma is responsible for changing fundamental magmatic properties (e.g., viscosity and density), driving explosive eruptions and influencing magmatic liquid lines of descent on Earth. Because H₂O exsolves from melt during eruption, determining pre-eruptive magmatic H₂O contents requires analysis of melt inclusions or application of models to phenocryst phases. Because plagioclase is a common mineral in igneous rocks and has a composition that is sensitive to both changes in temperature and water content, it is a valuable mineral on which to base a hygrometer. The use of K-feldspar as the basis for a hygrometer, however, has not been fully explored. We present the thermodynamic framework and initial calibration for a new K-feldspar-liquid hygrometer based on the exchange reaction between orthoclase (KAlSi₃O₈) and albite (NaAlSi₃O₈) in crystalline sanidine and silicate melt. We utilized thermodynamic values from JANAF tables for the liquid and crystalline enthalpies, entropies, volumes, and values of thermal expansion and compressibility. The activities of the crystalline components are set as a function of temperature using a model from the literature, and the activities of the liquid components are determined through a regression of a dataset of sanidine-liquid pairs from the literature. We augment this dataset with phases equilibrium experiments conducted in a waspaloy pressure vessel at temperatures and pressures ranging from 800-900ºC and 100-200 MPa, respectively, on a phonolite, trachyte, and rhyolite. We apply the K-feldspar hygrometer to voluminous ignimbrites from the Snake River Plane, Great Basin and Yellowstone using data reported in the literature, and compare the results to those derived from the plagioclase-liquid hygrometer and melt inclusions.