Collaborative Research: Crystal-Scale Histories of Magma Evolution from Zircon T-X-t

含有高浓度SiO2的岩浆是地球上一些最具灾难性的火山爆发的原因，但它们很少在地壳中被探测到。因此，人们对这些岩浆在喷发前的寿命和动态知之甚少。就像树木年轮一样，这些岩浆中晶体的化学分带包含了岩浆演化的顺序记录；与树木年轮不同，揭示这种分区的实际意义被证明是难以捉摸的。特别是对于硅质岩浆，晶体尺度的分带剖面可能为岩浆历史提供重要的见解。该项目将利用一种基于锆石钛含量的新校准的地温计（Watson和Harrison， 2005），目的是利用晶体分区来确定晶体的大部分历史是悬浮在大部分液体岩浆中，还是在堆积堆中被隔离，还是作为花岗岩类的一部分被固化。这项工作最初将集中在三个重要的硅火山活动中心（长谷、多巴和黄石）喷发的流纹岩上，它们的U-Th-Pb晶体年龄已经确定。由于这种新的地温计在许多地质应用中具有很高的潜力，因此将对锆石进行离子探针微量元素分析，以评估晶体生长动力学、晶体-熔体分配和岩浆混合如何影响温度计。当考虑到所获得的年龄数据时，将有可能划定破火山口火山硅岩浆的时间-温度-成分演化。预计这一结果将有助于更好地了解火山喷发前的硅岩浆演化、岩体生长机制以及地球大陆地壳的演化。这项研究将促成两所院校的研究人员和学生之间的合作，这两所院校为大量少数民族学生提供服务，并将支持两名硕士生和几名本科生的研究项目。

Magmas containing high concentrations of SiO2 are responsible for some of the most catastrophic eruptions on Earth yet they are rarely detected in the crust. Consequently, little is known about the longevity and dynamics of such magmas prior to their eruption. Much like tree-rings, the chemical zoning of crystals in such magmas contain a sequential record of magmatic evolution; unlike tree-rings, unraveling the practical significance of this zoning has proved to be elusive. For silicic magmas in particular, crystal-scale zoning profiles may provide essential insights into magmatic history. This project will exploit a newly calibrated geothermometer that is based on the titanium content of zircons (Watson and Harrison, 2005) with the goal of using crystal zoning to determine whether crystals spend much of their history suspended in mostly liquid magma, isolated in cumulates piles, or solidified as a part of granitoids. The work will initially focus on rhyolites erupted at three important centers of silicic volcanism (Long Valley, Toba, & Yellowstone) for which U-Th-Pb crystal ages have already determined. Because this new geothermometer has a high potential for many geologic applications, ion microprobe trace element analyses of the zircons will be performed in order to evaluate how crystal growth kinetics, crystal-melt partitioning, and magma mixing might affect the thermometer. When considered in the context of the age data obtained, it will be possible to delimit the time-temperature-compositional evolution of silicic magmas at caldera volcanoes. It is expected that the results will provide a better understanding of how silicic magmas evolve prior to eruption, the mechanisms of pluton growth, and the evolution of Earth's continental crust. The research will engender collaboration between researchers and students at two institutions that serve significant numbers of ethnic minority students, and will support the research projects of two M.Sc. students and several undergraduates.