The Origin of Voluminous, Hydrous, High-SiO2 Rhyolites at Long Valley, CA: High-resolution Numerical Thermal Models and Dynamic, Hydrous Experiments

高硅流纹岩是地球上最具差异化的硅酸盐岩浆类型，并构成了一些最大的爆炸性喷发（100 -1000立方公里），包括黄石公园（WY）和长谷（CA）在过去100万年的破火山口。 了解高SiO2流纹岩的起源和运输，以及是什么触发了它们的大量喷发，通常是爆炸性的，是相当有趣的，因为它们的形成必须从根本上重新构成和区分大陆地壳。因此，了解喷发过程的过程可以进一步了解与这些喷发有关的危险。 虽然美国西部在不久的将来发生重大灾难性火山爆发的可能性很低，这项提议的主要目标之一是测试流纹岩熔体形成和快速迁移到地表之间的时间尺度是否（通过堤坝）可能是几周或几个月的时间，（因此人类的时间尺度），而不是10年代至100年代的数千年。这项研究的总体目的是联合收割机动态，含水部分熔融实验与1D和2D高分辨率数值模型，以研究如何流入挥发性玄武岩进入大陆地壳驱动的高SiO2流纹岩熔体的形成，分离和喷发。高分辨率数值模型将侧重于在短空间（米）和时间（天/年）尺度上发生的过程，以及在地壳距离（公里）和较长时间（百万年）上发生的过程。 高分辨率模型捕捉到四种重要现象，包括：（1）从结晶玄武质岩床中出溶的富H2O流体的转移（2）瞬变电磁波（100年代）邻近新侵位玄武岩岩床的围岩受热，导致周围地壳的部分熔融（花岗岩和先前固化的镁铁质岩床）在H2O存在的条件下（其中在固相线处流体饱和条件占优势，但在超过20%的熔体分数处出现不存在流体的条件）。(3)花岗岩类部分熔体与先前凝固的镁铁质岩床的相互作用。(4)花岗岩类围岩中先存细晶岩脉在促进熔体充填岩脉快速发育，从而快速迁移出围岩源区中的作用。 这些短期过程影响地壳尺度的热剖面，因为热量通过平流快速传递（即，堤防）。它们还导致显着的和不可逆转的组成改造的上地壳由于这些部分熔体从花岗岩和以前固化的镁铁质岩床的运输，因此，改造的上地壳将携带这种混合遗产的地球化学特征。将进行动态含水实验，模拟H2O流体从玄武岩岩床释放到邻近围岩中，及其在驱动部分熔融中的作用。 新的数值模型和软件代码将广泛提供，以促进科学的可重复性和理解。该奖项反映了NSF的法定使命，并通过使用基金会的智力价值和更广泛的影响审查标准进行评估，被认为值得支持。

High-silica rhyolite is the most differentiated silicate magma type on Earth and makes up some of the largest explosive eruptions (100's-1000's cubic km), including those at Yellowstone (WY) and Long Valley (CA) calderas in the last 1 Myr. Understanding the origin and transport of high-SiO2 rhyolites and what triggers their eruption in large volumes, often explosively, is of considerable interest because their formation must fundamentally re-constitute and differentiate continental crust Additionally, the western U.S. may be the site of future "supervolcano" eruptions, and so understanding the process of the eruptive process can yield additional insight into the hazard associated with these eruptions. Although the chances of a major catastrophic eruption in the western U.S. in the immediate future are low, one of the primary goals of this proposal is to test whether the timescales between rhyolite melt formation and rapid transport to the surface (via dikes) may be on the order of weeks and months (and thus human time scales) and not 10's to 100's of thousands of years.The overall purpose of this study is to combine dynamic, hydrous partial melting experiments with 1D and 2D high-resolution numerical models to examine how the influx of volatile-bearing basalt into continental crust drives the formation, segregation and eruption of high-SiO2 rhyolitic melts. The high-resolution, numerical models will focus on processes that occur over short spatial (meter) and temporal (days/years) scales, in addition to those that operate over crustal distances (kms) and longer times (Myrs). There are four important phenomena that are captured by high-resolution models, including: (1) The transfer of exsolved H2O-rich fluid from crystallizing basaltic sills (10-100 m) to adjacent wall rock, (2) The transient (100's of years) heating of the wall rock adjacent to newly emplaced basaltic sills, which leads to significant partial melting of surrounding crust (both granitoid and previously solidified mafic sills) under H2O-present conditions (where fluid-saturated conditions prevail at the solidus, but fluid-absent conditions develop at melt fractions over 20%). (3) The interaction of partial melts of granitoid and previously solidified mafic sills. (4) The role of pre-existing aplite dikes in granitoid wall rock in facilitating the rapid development of melt-filled dikes, and thus rapid transport out of the wall-rock source region. These short-term processes impact crustal-scale thermal profiles because of the rapid transfer of heat via advection (i.e., dikes). They also lead to significant and irreversible compositional re-working of the upper crust due to the transport of these partial melts from both granitoid and previously solidified mafic sills; thus the re-worked upper crust will carry the geochemical signature of this mixed heritage. Dynamic, hydrous experiments will be performed that simulate the release of H2O fluid from the basaltic sills into adjacent wall rock, and its role in driving partial melting. The new numerical models and software codes will be made widely available to facilitate scientific reproducibility and comprehension. Throughout the course of this work, several undergraduate and graduate students will be supported.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.