The effect of anorthite content and water on quartz–feldspar cotectic compositions in the rhyolitic system and implications for geobarometry

Since moving to Hannover, I have become involved in a number of exciting new projects. One project, lead by François Holtz and carried out by Sören Wilke, involved carrying out a large number of experiments to determine how variations in the anorthite (i.e. calcium) and water contents of rhyolites affects the position of quartz–feldspar cotectics. This is important because the position of quartz–feldspar cotectics can be used as a geobarometer, especially is systems lacking pressure sensitive minerals such as amphibole, but only if the effects of anorthite and water contents are appropriately accounted for.

Our experiments allowed us to define thermal minima and quartz–sanidine–plagioclase triple points on quartz–feldspar cotectics at various pressures, water contents and anorthite contents. This information was then used calibrate an empircal barometer (DEtermination of Rhyolite Pressures; DERP) to esimtate the storage pressure of rhyolitic glasses in equilibrium with quartz and at least one feldspar. DERP is calibrated in the range 50–500 MPa and for any H2O content. Importantly, our findings suggest that rhyolite-MELTS may underestimate the storage pressures of rhyolitic magmas. Bringing emprical and thermodynamic geobarometers into alignment thus represents a key next step in the investigation of rhyolitic magmas.

Publication

Wilke, S., Holtz, F., Neave, D. A. & Almeev, R. R. 2017. The effect of anorthite content and water on quartz–feldspar cotectic compositions in the rhyolitic system and implications for geobarometry. Journal of Petrology 58, 789–818.