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首页> 外文期刊>Contributions to Mineralogy and Petrology >Phase-equilibrium geobarometers for silicic rocks based on rhyolite-MELTS. Part 1: Principles, procedures, and evaluation of the method
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Phase-equilibrium geobarometers for silicic rocks based on rhyolite-MELTS. Part 1: Principles, procedures, and evaluation of the method

机译：基于流纹岩-MELTS的硅质岩相平衡气压计。第1部分：方法的原理，步骤和评估

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Constraining the pressure of crystallization of magmas is an important but elusive task. In this work, we present a method to derive crystallization pressures for rocks that preserve glass compositions (either glass inclusions or matrix glass) representative of equilibration between melt, quartz, and 1 or 2 feldspars. The method relies on the well-known shift of the quartz-feldspar saturation surface toward higher normative quartz melt compositions with decreasing pressure. The critical realization for development of the method is the fact that melt, quartz and feldspars need to be in equilibrium at the liquidus for the melt composition. The method thus consists of calculating the saturation surfaces for quartz and feldspars using rhyolite-MELTS over a range of pressures, and searching for the pressure at which the expected assemblage (quartz+1 feldspar or quartz+2 feldspars) is found at the liquidus. We evaluate errors resulting from uncertainties in glass composition using a series of Monte Carlo simulations for a quartz-hosted glass inclusion composition from the Bishop Tuff, which reveal errors on the order of 20-45 MPa for the quartz+2 feldspars constraint and on the order of 25-100 MPa for the quartz+1 feldspar constraint; we suggest actual errors are closer to the lower bounds of these ranges. We investigate the effect of fluidsaturation in two ways: (1) By applying our procedure over a range of water contents for three glass compositions; we show that the effect of fluid saturation is more important at higher pressures (~ 300 MPa) than at lower pressures (~ 100 MPa), but reasonable pressure estimates can be derived irrespective of fluid saturation for geologically relevant H_2O concentrations >3 wt% and (2) by performing the same type of pressure determinations with a preliminary version of rhyolite-MELTS that includes a H_2O-CO_2 mixed fluid phase; we use a range of H_2O and CO_2 concentrations for two compositions characteristic of early-erupted and late-erupted Bishop Tuff glass inclusions and demonstrate that calculated pressures are largely independent of CO_2 concentration (for CO_2 <1,000 ppm), at least for relatively high H_2O contents, as expected in most natural magmas, such that CO_2 concentration can be effectively neglected for application of our method. Finally, we demonstrate that pressures calculated using the rhyolite-MELTS geobarometer compare well with those resulting from H_2O-CO_2 glass inclusion barometry and Al-in-hornblende barometry for an array of natural systems for which data have been compiled from the literature; the agreement is best for quartz-hosted glass inclusions, while matrix glass yields systematically lower rhyolite-MELTS pressures, suggestive of melt evolution during eruptive decompression.
机译：限制岩浆结晶的压力是一项重要但难以捉摸的任务。在这项工作中，我们提出了一种方法来导出岩石的结晶压力，该岩石保留玻璃成分（玻璃夹杂物或基体玻璃），代表熔体，石英和1或2个长石之间的平衡。该方法依赖于随着压力降低石英-长石饱和表面向更高规范的石英熔体成分的转移。开发该方法的关键认识是以下事实：对于熔体成分，熔体，石英和长石需要在液相线处于平衡状态。因此，该方法包括使用流纹岩-MELTS在一定压力范围内计算石英和长石的饱和面，并搜索在液相线处发现预期组合（石英+1长石或石英+2长石）的压力。我们使用Bishop Tuff的一系列石英承载玻璃夹杂物成分的一系列蒙特卡洛模拟评估了玻璃成分不确定性引起的误差，该模拟揭示了石英+2长石约束和20-45 MPa量级的误差。石英+1长石约束的量级为25-100 MPa;我们建议实际误差应接近这些范围的下限。我们以两种方式研究流体饱和的影响：（1）通过对三种玻璃组合物在一定范围的水含量上应用我们的程序;我们表明，在较高压力（〜300 MPa）下，流体饱和度的影响比在较低压力（〜100 MPa）下更为重要，但对于地质相关的H_2O浓度> 3 wt％和（2）通过使用包括H_2O-CO_2混合液相的流纹岩-MELTS的初步版本进行相同类型的压力确定;我们对早期喷出和晚期喷出的Bishop Tuff玻璃夹杂物的两种成分使用了一定范围的H_2O和CO_2浓度，并证明了计算压力很大程度上与CO_2浓度无关（对于CO_2 <1,000 ppm），至少对于相对较高的H_2O如大多数天然岩浆中所预期的那样，其含量可以有效地忽略CO_2的浓度，从而适用于我们的方法。最后，我们证明了使用流纹岩-MELTS气压计计算出的压力与H_2O-CO_2玻璃夹杂物气压计和Al-hornblende气压计对于一系列自然系统的压力比较，这些自然系统已从文献中收集了数据。该协议最适用于石英基玻璃夹杂物，而基质玻璃的流纹岩-MELTS压力有系统地降低，这表明喷发减压过程中熔体会析出。

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