ALUMINUM SOLUBILITY IN QUARTZ AND APPLICATION OF THE SINGLE MINERAL CROSSING ISOPLETHS METHOD

Quartz is both common and abundant in crustal rocks, and experimental determination of Ti solubility (Wark and Watson, 2006; Thomas et al. 2010) and diffusivity (Cherniak et al. 2007) in quartz has opened new possibilities for determining the pressure-temperature-time conditions of metamorphic and geodynamic processes. Since its development the Ti-in-quartz technique has been applied to reconstruct conditions of crystallization and recrystallization in a wide range of geologic settings and has proven to be a useful independent constraint on the conditions of petrogenesis.

In this study we present results of experiments in which quartz grew in equilibrium with aluminosilicate (kyanite or sillimanite) to evaluate the pressure-temperature dependence of Al solubility in quartz. Experiments conducted at conditions ranging from 575-925 °C and 8-15 kbar reveal a systematic and reproducible relationship of Al substitution into quartz. Equilibrium Al solubilities in quartz are 2-3 times greater than Ti concentrations at the same condition and exhibit a similar positive dependence on temperature and negative dependence on pressure. Aluminum diffusion in quartz has also been investigated and is shown to be similarly sluggish under most crustal conditions (Tailby et al. 2018).

By combining measurements of Ti and Al from the same volume of quartz, it is possible to use the crossing isopleths method to determine the unique pressure-temperature condition of formation. Paired Al and Ti measurements of quartz from several natural field localities are used to test the crossing isopleths method for quartz, and P-T estimates from quartz are compared with results from other petrologic and microstructural thermometers and barometers.