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分异的硅酸盐岩浆类型，构成了一些最大的爆炸性喷发(100-1000‘S千米)，包括发生在黄石国家公园的那些喷发。了解大量流纹岩岩浆系统的起源和演化是相当有意义的，因为它们的形成必须从根本上重建和区分大陆地壳，而且它们是未来“超级火山”喷发的候选对象。流纹岩中的矿物相通常为研究喷发前的温度、氧化态、熔融水浓度以及熔体在上地壳堆积的时间尺度提供了丰富的机会。然而，关于天然流纹岩熔体成分的相平衡实验令人惊讶地稀少，这限制了利用这些矿物相提取最大信息量的潜力。该方案的目的是在冷封和活塞-圆柱体装置中，在受控的氧逸度和水逸度(fO2和fH2O)条件下，在一定温度和压力范围内对各种天然流纹岩液体进行水合相平衡实验，这将使斜长石-液体湿度计能够校准流纹岩成分。这些实验还将被用来确定微量元素在矿物和熔体之间的分配系数，特别是对锶和钡。划分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(最大)的爆炸性喷发。最后，这些实验将大大加强晶体-熔体平衡的广泛热力学模型的校准，这些模型与地球物理模型相结合，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炸，这是一种重大的地质灾害。

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.