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

流纹岩是地球上分化最明显的硅酸盐岩浆类型，它构成了一些最大的爆发（100-1000公里立方），包括那些发生在黄石国家公园的喷发。了解大体积流纹岩岩浆系统的起源和演化具有相当大的意义，因为它们的形成必须从根本上重建和区分大陆地壳，它们是未来“超级火山”爆发的候选者。流纹岩中的矿物相通常提供了丰富的机会来研究喷发前的温度、氧化状态和熔体水浓度，以及熔体在上地壳积累的时间尺度。然而，关于天然流纹岩熔体成分的相平衡实验却令人惊讶地缺乏，这限制了利用这些矿物相来提取最大数量信息的潜力。本方案的目的是在控制氧和水逸度（fO2和fH2O）的条件下，在冷密封和活塞缸装置中对各种天然流纹岩液体在一定温度和压力下进行水相平衡实验，这将使斜长石液体湿度计能够校准流纹岩成分。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。这种分裂行为对于理解极低锶流纹岩是如何形成的至关重要，其中一些流纹岩构成了地球上一些最大（体积最大）的爆发。最后，这些实验将极大地提高晶体-熔体平衡的广泛热力学模型的校准，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.