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分异的硅酸盐岩浆类型，构成了一些最大的爆炸性喷发(100‘S-1000’S立方千米)，包括最近1Myr在黄石(WY)和长谷(CA)火山口的喷发。了解高二氧化硅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，这是相当有意义的，因为它们的形成必须从根本上重新构成和区分大陆地壳。此外，美国西部可能是未来超级火山喷发的地点，因此了解喷发过程的过程可以进一步洞察与这些喷发相关的危险。尽管美国西部在不久的将来发生重大灾难性喷发的可能性很低，但这一提议的主要目标之一是测试流纹岩熔体形成和快速运移到地表(通过堤坝)之间的时间尺度是否可能是几周和几个月(因此人类的时间尺度)，而不是10‘S到100’S数千年的时间尺度。这项研究的总体目的是将动态、含水部分熔融实验与一维和二维高分辨率数值模型相结合，研究含挥发分的玄武岩流入大陆地壳如何驱动高SiO_2流纹岩熔体的形成、分离和喷发。高分辨率的数值模式将侧重于发生在短空间(米)和时间(天/年)尺度上的过程，以及那些在地壳距离(公里)和较长时间(MYR)上运行的过程。高分辨率模式捕捉到四个重要现象，包括：(1)富含水的流体从结晶玄武岩岩床(10-100m)向围岩转移；(2)与新侵位的玄武岩岩床相邻的围岩的瞬时(百年S)加热，这导致在H2O存在的条件下(固相线上流体饱和条件占优势，但熔体分数超过20%时出现无流体条件)，导致周围地壳(花岗岩类和先前固化的镁铁质岩床)显著部分熔融。(3)花岗岩类部分熔体与先前固化的镁铁质岩床的相互作用。(4)花岗岩类围岩中已有的辉长岩脉对熔体充填岩脉的快速发展，从而快速运出围岩源区的作用。这些短期过程影响到地壳尺度的热分布，因为热量通过平流(即岩脉)迅速传递。由于这些来自花岗岩类和先前固化的镁铁质岩床的部分熔体的运输，它们还导致上地壳显著和不可逆转的成分再加工；因此，重新加工的上地壳将带有这种混合遗产的地球化学特征。将进行动态的、含水的实验，模拟H2O流体从玄武岩岩床释放到邻近围岩中，以及它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，以促进科学的重复性和理解性。在这项工作的整个过程中，几名本科生和研究生将得到支持。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.