Investigations of Trace Element Titanium in Silica Minerals

Quartz is a common mineral in many igneous, metamorphic, and sedimentary rocks because it is stable over a wide range of pressure (P) and temperature (T) conditions; therefore, quartz is an excellent mineral to target for thermobarometric uses. Thermobarometry is the quantitative determination of the pressure (baro-) and temperature (thermo-) conditions at which a natural rock sample reached chemical equilibrium. This dissertation contains three chapters describing mineral synthesis experiments that quantify and define the trace-element solubility behavior of titanium (Ti) in the silica minerals quartz and coesite, to advance the understanding and application of trace-element thermobarometry.Chapter 1 details the experiments that were conducted to quantify the pressure and temperature effects of Ti-in-coesite (TitaniC) solubility to develop the first Ti-in-coesite thermobarometer. The Ti-in-coesite thermobarometer was applied to coesite samples from two UHP (ultra high-pressure) metamorphic locales, the western Alps, Italy, (Dora-Maira) and Papua New Guinea; the estimates for each respective sample were in excellent agreement with previously reported P–T conditions. Chapter 2 involves experiments which Ti solubility in quartz was investigated at previously unexplored conditions, focused on extending and improving the Ti-in-quartz (TitaniQ) solubility model (Thomas et al. 2010). Additional data enabled characterization of Ti-in-quartz solubility across much of the α- and β-quartz stability fields and demonstrated that Ti concentrations in β-quartz decrease linearly with increasing pressure, while Ti-in-quartz concentrations in α-quartz decrease nonlinearly with increasing pressure at the conditions studied. New thermobarometric model equations for Ti solubility in α-quartz, β-quartz, and α–β quartz combined were developed; remarkable agreement between calculated and measured experimental P−T conditions using the Zr-in-rutile and Ti-in-quartz solubility models demonstrated that the solubility models accurately describe the equilibrium solubility of titanium in quartz. Chapter 3 describes experiments that synthesized quartz in the presence of titanite at sub-unity titania activity (????2) conditions. Ti-in-quartz concentrations and known experiment P–T conditions were entered into the TitaniQ solubility models to estimate ????2 values for the experimental system. The estimated ????2values were in excellent agreement with those calculated by using the mineral reactions and thermodynamic parameters from internally consistent datasets (Berman 1988; Holland and Powell 2011); this provides another source of validation of the Ti-in-quartz solubility models (Thomas et al. 2010; Osborne et al. 2021)