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

加利福尼亚州长谷大量含水高 SiO2 流纹岩的起源：高分辨率数值热模型和动态含水实验

基本信息

批准号：
1855751
负责人：
Rebecca Lange
金额：
$ 38.33万
依托单位：
Regents of the University of Michigan - Ann Arbor
依托单位国家：
美国
项目类别：
Standard Grant
财政年份：
2019
资助国家：
美国
起止时间：
2019-05-15 至 2022-04-30
项目状态：
已结题

来源：
https://www.nsf.gov/awardsearch/showAward?AWD_ID=1855751&HistoricalAwards=false
关键词：
Origin Voluminous Hydrous SiO2 Rhyolites

项目摘要

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.

高硅流纹岩是地球上分化最明显的硅酸盐岩浆类型，它构成了一些最大规模的爆发（100 -1000立方千米），包括黄石火山（WY）和长谷火山口（CA）在过去1亿年间的喷发。了解高sio2流纹岩的起源和运输，以及引发它们大量喷发的原因，通常是爆炸性的，是非常有趣的，因为它们的形成必须从根本上重新构成和区分大陆地壳。此外，美国西部可能是未来“超级火山”喷发的地点，因此了解喷发过程可以进一步了解与这些喷发相关的危险。虽然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，但这项提议的主要目标之一是测试流纹岩熔体形成和快速运输到地表（通过堤坝）之间的时间尺度是否可能是几周和几个月（这是人类的时间尺度），而不是几万到几万年。本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，研究含挥发物玄武岩流入大陆地壳如何驱动高sio2流纹岩熔体的形成、分离和喷发。高分辨率的数值模式将重点关注发生在短空间（米）和时间（日/年）尺度上的过程，以及在地壳距离（公里）和较长时间（公里）上运行的过程。高分辨率模式可以捕捉到四种重要现象，包括：(1)从结晶玄武岩基岩（10-100 m）向邻近围岩转移出的富h2o流体；(2)新侵位玄武岩基岩附近围岩的瞬态（100年）加热，导致周围地壳（花岗岩和先前固化的基性基岩）在H2O-present条件下显著部分熔融（固相处流体饱和状态占优势，但熔体组分超过20%处流体不存在）。(3)花岗岩类的部分熔体与前期凝固的基性岩的相互作用。(4)花岗质围岩中已存在的硅质岩脉促进了熔融充填岩脉的快速发育，从而使其快速输出围岩源区。由于热量通过平流（即岩脉）快速传递，这些短期过程影响地壳尺度的热剖面。由于这些来自花岗岩类和先前固化的基性岩质的部分熔体的运输，它们还导致了上地壳重要的和不可逆的成分再作用；因此，改造后的上地壳将带有这种混合遗产的地球化学特征。将进行动态的含水实验，模拟玄武岩岩基向邻近围岩释放H2O流体，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，以促进科学的再现性和理解。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。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。