The minimum T-XCO₂ location of the reaction boundary calcite + quartz + rutile = sphene + CO₂ has been experimentally determined in H₂0-CO₂-NaCl fluids at 2 kbar. Extent and direction of reaction were monitored by the CO₂ weight change method using natural materials. All bulk compositions contain 23 wt. % NaCI, relative to NaCl-H₂0. Synthetic fluid inclusions trapped at the T-XCO₂ conditions of several points along the curve suggest unmixing of the fluid phase, as evidenced by coexisting CO₂ vapor-rich and aqueous, halite-bearing inclusions. Results from 450-520 °C are listed below. / Master of Science