Experimental calibration of cotectic melt compositions coexisting with quartz and feldspar in rhyolitic systems - Application to Snake River Plain rhyolites,Yellowstone hotspot

The depth of magma storage prior to eruption is difficult to constrain for rhyolitic systems and remains often unknown because the typical mineral assemblages in rhyolite magmas are not suitable for geobarometry. As an alternative to mineral compositions, the silica content of rhyolitic melts can be used to constrain pressure, provided that the silicate melts have cotectic compositions (melts coexisting with quartz and feldspar). From studies in synthetic haplogranitic systems, it is well known that the silica content of cotectic melts decreases with increasing pressure and that it may be used as barometer. However, the evolution of silica content with pressure is not calibrated for natural rhyolitic systems containing up to 2 wt% CaO and 4 wt% FeO. Crystallization experiments in Fe-, Ca- and Ti-bearing rhyolitic systems have been performed at 200 MPa and a temperature range of 790 to 850°C to calibrate the position of cotectic compositions in natural rhyolitic systems and the results have been projected onto the haplogranitic quartz-albite-orthoclase diagram (Qz-Ab-Or). The starting materials were glasses with various proportions of the Qz, Ab and Or components; all glasses contained 1 wt% FeO, 3.4 wt% An component and 0.2 wt% TiO2. The glasses were pre-hydrated with 3 wt% H2O in an internally heated pressure vessel. The crystallization experiments were conducted in cold seal pressure vessels. The water contents of the pre-hydrated starting glasses were checked by Fourier Transform Infrared Spectroscopy (FTIR) and Karl Fischer titration. The experimental products (glasses and crystals) were analyzed via electron microprobe. Results at 200 MPa show that the cotectic line separating the quartz primary field from the feldspar primary field is shifted towards the Qz apex by 6 wt% when compared with the water-undersaturated haplogranitic system (containing 3 wt% water). The minimum composition is shifted towards the Qz-Or sideline when compared with the haplogranitic system. The results also indicate that the approach proposed by Blundy and Cashman (2001) to account for the effect of An content on the position of the cotectic lines in the system Qz-Ab-Or is qualitatively correct, but that the shift of the cotectic line is more pronounced than predicted by Blundy and Cashman (2001). The Ca, Fe and Ti concentrations of the rhyolitic compositions were chosen so that the results can be applied to rhyolites erupted from the Yellowstone hotspot along the central Snake River Plain. Using the composition of glasses (matrix glass and glass inclusions) from selected eruptive products in which quartz and feldspar co-exist, the preliminary experimental results are applied to constrain the pressure prevailing prior to eruption with particular attention to rhyolites collected from Kimberly drill core (Project Hotspot). Magma storage conditions in the pressure range 250-400 MPa determined experimentally for the Indian Batt Rhyolite, Bruneau-Jarbidge eruptive center, are confirmed by the projection of the glass compositions in the Qz-Ab-Or system. Reference: Blundy J, Cashman K (2001) Contrib Mineral Petrol, 140, 631-650