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首页> 外文期刊>Contributions to Mineralogy and Petrology >Experimental phase equilibria of a Mount St. Helens rhyodacite: a framework for interpreting crystallization paths in degassing silicic magmas
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Experimental phase equilibria of a Mount St. Helens rhyodacite: a framework for interpreting crystallization paths in degassing silicic magmas

机译：圣海伦斯山菱镁矿的实验相平衡：解释硅质岩浆脱气中结晶路径的框架

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摘要

We present isothermal (885 degrees C) phase equilibrium experiments for a rhyodacite from Mount St. Helens (USA) at variable total pressure (25-457 MPa) and fluid composition (XH2Ofl = 0.6-1.0) under relatively oxidizing conditions (NNO to NNO + 3). Run products were characterized by SEM, electron microprobe, and SIMS. Experimental phase assemblages and phase chemistry are consistent with those of natural samples from Mount St. Helens from the last 4000 years. Our results emphasize the importance of pressure and melt H2O content in controlling phase proportions and compositions, showing how significant textural and compositional variability may be generated in the absence of mixing, cooling, or even decompression. Rather, variations in the bulk volatile content of magmas, and the potential for fluid migration relative to surrounding melts, mean that magmas may take varied trajectories through pressure-fluid composition space during storage, transport, and eruption. We introduce a novel method for projecting isothermal phase equilibria into CO2-H2O space (as conventionally done for melt inclusions) and use this projection to interpret petrological data from Mount St. Helens dacites. By fitting the experimental data as empirical functions of melt water content, we show how different scenarios of isothermal magma degassing (e.g., water-saturated ascent, vapor-buffered ascent, and vapor fluxing) can have quite different textural and chemical consequences. We explore how petrological data might be used to infer degassing paths of natural magmas and conclude that melt CO2 content is a much more useful parameter in this regard than melt H2O.
机译：我们介绍了来自圣海伦斯山（美国）的流纹岩在等压（25-457 MPa）和流体组成（XH2Ofl = 0.6-1.0）的条件下，在相对氧化条件下（NNO至NNO）的等温（885摄氏度）相平衡实验。 + 3）。通过SEM，电子探针和SIMS对运行产物进行表征。实验相的组合和相化学与最近4000年来自圣海伦斯山的天然样品的相一致。我们的结果强调了压力和熔体中H2O含量在控制相比例和组成中的重要性，表明在不进行混合，冷却甚至减压的情况下，如何产生明显的组织和组成变异性。相反，岩浆的整体挥发物含量的变化以及流体相对于周围熔体的迁移潜力意味着，在储存，运输和喷发期间，岩浆可能会通过压力流体的组成空间而发生变化的轨迹。我们介绍了一种将等温相平衡投影到CO2-H2O空间的新方法（按常规方法对熔体夹杂物进行投影），并使用该投影来解释圣海伦斯山峰的岩石学数据。通过将实验数据拟合为熔融水含量的经验函数，我们展示了等温岩浆脱气的不同方案（例如，水饱和上升，蒸气缓冲上升和蒸气通量）如何具有完全不同的组织和化学后果。我们探讨了如何利用岩石学数据推断天然岩浆的除气路径，并得出结论，在这方面，熔融CO2含量比熔融H2O更有用。

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