Aluminum Solubility Mechanisms in Quartz: Implications for Al-in-Quartz Thermobarometry

Trace element thermobarometers in minerals are becoming increasingly important tools for studying geologic processes in many different geologic environments. The solubility of some trace-level (i.e. \u3c1000 ppmw) elements in minerals can be measured and used to estimate the pressure (P) and/or temperature (T) of mineral crystallization. To date, quartz has been useful for trace element thermobarometry, on the basis of its Ti content, due to its common occurrence in many rock types; therefore, it can provide information on a wide range of geologic processes. However, the Ti-in-quartz technique relies on an independent constraint on T (or P) to calculate P (or T), which can be difficult to obtain in some rocks. To improve utility of quartz as a thermobarometer, quartz has been experimentally co-crystallized with aluminosilicates at elevated P–T conditions to determine Al solubilities in quartz, which will allow use of the crossing isopleths method to determine a unique P and T solution from two independent techniques (using Ti and Al) in the same mineral. Experiments demonstrate that Al concentrations in quartz vary systematically with P and T, and also show that Al is soluble at greater levels than Ti. The success of an Al-in-quartz thermobarometer relies on determining both the variations in Al solubility across P–T space as well as the solubility mechanism for Al substitution into the quartz structure. Fourier transform infrared spectroscopy (FTIR) was used to quantify H+ contents as a charge-balancing ion for Al3+ to replace Si4+, electron microprobe (EPMA) to measure Al concentrations, and nuclear magnetic resonance spectroscopy (NMR) to determine the coordination environment of Al in quartz. The dominant substitution mechanism involves incorporating Al3+ onto the Si4+ tetrahedral sites in quartz coupled with H+ incorporation required for charge balancing