Thermobarometry Gone Astray
Quantitative thermobarometers require a univariant reaction, consideration of dilutions in all phases, and application to fully buffered systems. Many empirical thermobarometers, e.g., Al-in-chlorite, Al-in-hornblende, b0-of-phengite, as well as clay and organic transformations, were not evaluated with buffering phases and solid solutions, and they are not adequately calibrated. Reactions proposed for Al-in-chlorite involve SiO2 and H2O, such that the presence of quartz and the dilution of aqueous fluid by salts are important. The Al-in-chlorite with epidote-quartz-calcite-fluid should decrease with T, whereas with muscovite/illite-quartz-fluid or epidote-actinolite-quartz-fluid the Al should increase with T for fixed compositions of the other phases. The Al-in-hornblende barometer is often applied to obtain pressures of granitoid emplacement without consideration of the composition of plagioclase (or of biotite and K-feldspar) or reduced activities of SiO2/H2O. Experimental calibrations were obtained only in the presence of H2O fluid and did not evaluate the effect of reducing its activity. Reactions with clays and organic materials are kinetically controlled, depending on heating rates and time at peak T as well as T. Raman measurements of carbonaceous materials in metamorphic rocks do not obviate these concerns.
Several new thermometers have been proposed recently in CaO-TiO2-ZrO2-SiO2 involving Ti in zircon or quartz, and Zr in rutile or sphene. Original calibrations assumed that pressure had no influence on the results despite a significant volume change in buffering reactions. Calibrations and applications included experiments and natural assemblages in the absence of a full buffering reaction, e.g. with rutile. A high TiO2 activity (0.5-0.8) was assumed based on garnet-bearing assemblages at high pressures and in application to granitoids lacking both garnet and rutile. Studies of Ti in zircon from Archean granitoids in support of an early wet/cool earth have large systematic errors and are not recommended without explicit correction for all reduced activities.