科研之友
微信
新浪微博
Facebook
分享链接
摘要
The phase relations have been investigated experimentally at 200 and 500 MPa as a function of water activity for one of the least evolved (Indian Batt Rhyolite) and of a more evolved rhyolite composition (Cougar Point Tuff XV) from the 12.8-8.1 Ma Bruneau-Jarbidge eruptive center of the Yellowstone hotspot. Particular priority was given to accurate determination of the water content of the quenched glasses using infrared spectroscopic techniques. Comparison of the composition of natural and experimentally synthesized phases confirms that high temperatures (>900 degrees C) and extremely low melt water contents (<1.5 wt % H2O) are required to reproduce the natural mineral assemblages. In melts containing similar to 0.5-1.5 wt % H2O, the liquidus phase is clinopyroxene (excluding Fe-Ti oxides, which are strongly dependent on fO(2)), and the liquidus temperature of the more evolved Cougar Point Tuff sample (BJR; similar to 940-1000 degrees C) is at least 30 degrees C lower than that of the Indian Batt Rhyolite lava sample (IBR2; 970-1030 degrees C). For the composition BJR, the comparison of the compositions of the natural and experimental glasses indicates a pre-eruptive temperature of at least 900 degrees C. The composition of clinopyroxene and pigeonite pairs can be reproduced only for water contents below 1.5 wt % H2O at 900 degrees C, or lower water contents if the temperature is higher. For the composition IBR2, a minimum temperature of 920 degrees C is necessary to reproduce the main phases at 200 and 500 MPa. At 200 MPa, the pre-eruptive water content of the melt is constrained in the range 0-7-1.3 wt % at 950 degrees C and 0.3-1.0 wt % at 1000 degrees C. At 500 MPa, the pre-eruptive temperatures are slightly higher (by similar to 30-50 degrees C) for the same ranges of water concentration. The experimental results are used to explore possible proxies to constrain the depth of magma storage. The crystallization sequence of tectosilicates is strongly dependent on pressure between 200 and 500 MPa. In addition, the normative Qtz-Ab-Or contents of glasses quenched from melts coexisting with quartz, sanidine and plagioclase depend on pressure and melt water content, assuming that the normative Qtz and Ab/Or content of such melts is mainly dependent on pressure and water activity, respectively. The combination of results from the phase equilibria and from the composition of glasses indicates that the depth of magma storage for the IBR2 and BJR compositions may be in the range 300-400 MPa (similar to <= 13 km) and 200-300 MPa (similar to <= 10 km), respectively.
- 出版日期2012-9
