Determination of the depth of rhyolitic magma chambers in the Snake River Plain province, USA,: an experimental calibration

在最近的ICDP钻探中从蛇河平原(SRP)火山岩省(美国西部)采集到的高硅质流纹岩以及邻近流纹岩杂岩的岩石调查为追踪黄石热点陆内火山岩省岩浆储存条件的时空演变提供了独特的机会。可用于确定喷发前温度的各种地温计的应用表明，在过去16 Ma期间，温度总体上呈下降趋势。然而，岩浆房的深度(或压力)很难限制，而且主要是未知的，因为流纹岩系统中的矿物组合不适合进行地球重力测量。作为矿物组成的替代，流纹岩熔体中的二氧化硅含量可以用来限制压力，只要硅酸盐熔体具有共晶成分(熔体与石英和长石共存)，就像大多数SRP流纹岩一样。在合成体系中的研究表明，共晶熔体中的二氧化硅含量随着压力的增加而减少，在~1000~50兆帕的压力范围内可用作晴雨表。然而，对于含有2wt%CaO和4wt%FeO的自然体系，二氧化硅含量随压力的变化没有进行校准。在这项研究中，我们计划确定压力对与SRP成分相关的共晶熔体成分中二氧化硅含量的作用。初步结果表明，CaO(或钙长石含量)对该位置的影响被低估了。实验数据对于解释ICDP岩芯样品中分析的天然玻璃成分(基质玻璃和玻璃包裹体)至关重要，并被用来提取有关喷发前岩浆储存深度的定量信息。从各种喷发事件(ICDP钻探和其他SRP流纹岩样品)获得的数据集将用于检查黄石热点在过去16 Ma期间是否存在岩浆储存深度的演化，以及喷发前压力、喷发物质体积、温度(或分异程度)和岩浆水活动之间是否存在相关性。这项研究是与负责分析ICDP流纹岩样品的其他美国组织合作进行的。强调指出，本项目所获得的实验数据库也可应用于其他案例研究(高硅流纹岩、A型花岗岩)。

The investigation of high-silica rhyolitic rocks collected in the recent ICDP drilling from the Snake River Plain (SRP) volcanic province (western United States) as well as rocks from the adjacent rhyolitic complexes offers a unique opportunity to track the evolution of magma storage conditions in time and space in the Yellowstone hotspot intracontinental volcanic province. The application of various geothermometers which can be used to determine pre-eruptive temperatures show a general trend indicating a general decrease of temperature over the last 16 Ma. However, the depth (or pressure) of the magma chambers is difficult to constrain and remains mainly unknown because the mineral assemblage in the rhyolitic systems is 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), which is the case for most SRP rhyolites. From studies in synthetic systems, it is well known that the silica content of cotectic melts decreases with increasing pressure and that it may be used as barometer in pressure ranges of ~ 1000 to 50 MPa. However, the evolution of silica content with pressure is not calibrated for natural systems containing up to 2 wt% CaO and 4 wt% FeO. In this study, we plan to determine the role of pressure on the silica content of cotectic melts compositions relevant for SRP compositions. Preliminary results show that the effect of CaO (or Anorthite content) on the position has been underestimated. The experimental data are crucial to interpret the natural glass compositions (matrix glass and glass inclusions) analyzed in the ICDP core samples and are used to extract quantitative information on the depth of magma storage prior to eruption. The dataset obtained from various eruptive events (samples from ICDP drillings and other SRP rhyolites) will be used to check if there is an evolution of the depth of magma storage over the lifetime of the Yellowstone hotspot in the last 16 Ma and if there is a correlation between the pre-eruptive pressure, the volume of erupted material, the temperature (or differentiation level) and the water activity of magmas. This study is conducted in cooperation with other U.S. groups who are in charge of the analysis of ICDP rhyolitic samples. It is emphasized that the experimental database obtained in this project can also be applied to other case studies (high silica rhyolites, A-type granites).