Extension of the Plagioclase-liquid Hygrometer to Rhyolites and Sr and Ba Partitioning Studies: New Phase Equilibrium Experiments on Hydrous Rhyolite

流纹岩是地球上最具差异的硅酸盐岩浆类型，并构成了一些最大的爆炸性喷发（100-1000平方公里），包括发生在黄石国家公园的那些。 了解大体积流纹质岩浆系统的起源和演化是相当有趣的，因为它们的形成必须从根本上重新组成和区分大陆地壳，它们是未来“超级火山”爆发的候选人。 流纹中的矿物相往往提供了一个丰富的机会，以检查喷发前的温度，氧化状态和融水浓度，以及在上地壳熔体积累的时间尺度。 然而，有一个令人惊讶的缺乏天然流纹岩熔体组合物的相平衡实验，这限制了潜在的使用这些矿物相提取的最大量的信息。本建议的目标是进行含水相平衡实验在一个冷密封和活塞缸装置控制下的氧和水逸度（fO 2和fH 2 O）的条件下对各种天然流纹岩液体在一定的温度和压力范围内，这将使校准的斜长石液体湿度计流纹岩组合物。 实验还将用于确定矿物和熔体之间的微量元素分配系数，特别是Sr和Ba。 这种分配行为对于理解极低锶流纹是如何形成的至关重要，其中一些流纹构成了地球上最大（体积最大）的爆炸性喷发。 最后，这些实验将大大提高广泛的晶体-熔体平衡热力学模型的校准，当与地球物理模型相结合时，可以更深入地了解大体积流纹岩岩浆体是如何形成的，以及为什么它们有时会爆发，这是一个重大的地质灾害。

Rhyolite is the most differentiated silicate magma type on Earth and makes up some of the largest explosive eruptions (100-1000's km3), including those that have occurred at Yellowstone National Park. Understanding the origin and evolution of large-volume rhyolitic magmatic systems is of considerable interest because their formation must fundamentally reconstitute and differentiate continental crust, and they are candidates for future "supervolcano" eruptions. The mineral phases in rhyolites often provide a rich opportunity to examine pre-eruptive temperatures, oxidation states, and melt water concentrations, as well as time scales for melt accumulation in the upper crust. However, there is a surprising paucity of phase-equilibrium experiments on natural rhyolite melt compositions, which limits the potential to use these mineral phases to extract the maximum amount of information. The goal of this proposal is to perform hydrous phase-equilibrium experiments in a cold-seal and piston-cylinder apparatus under controlled oxygen and water fugacity (fO2 and fH2O) conditions on a variety of natural rhyolite liquids over a range of temperature and pressure, which will enable calibration of the plagioclase-liquid hygrometer to rhyolite compositions. The experiments will also be used to determine trace element partition coefficients between mineral and melt, particularly for Sr and Ba. The partitioning behavior is critical to understanding how extremely low-Sr rhyolites form, some of which constitute some of the largest (most voluminous) explosive eruptions on Earth. Finally, these experiments will greatly enhance the calibration of broad thermodynamic models of crystal-melt equilibrium, which when combined with geophysical models can provide a deeper understanding of how large-volume rhyolite magma bodies form and why they sometimes erupt explosively, which is a significant geohazard.