Insights into the Development of Silicic Magma Reservoirs over Space and Time from Crystal-scale Trace-element and Isotopic Data and U-Th Datin

从晶体尺度的微量元素和同位素数据以及U-Th定年揭示硅质岩浆储集层在时空上的发育。形成火山口的喷发是地球上发生的最戏剧性和最危险的地质事件之一。此外，大型硅质破火山口系统的熔融和同化作用是影响大陆地壳化学特征的重要机制。关于硅质储集层系统如何运作的重要悬而未决的问题包括：如何产生大量硅质岩浆(即现有地壳物质的重熔和地幔来源物质的分异之间的平衡)；硅质岩浆系统是如何根据其地下几何形状和储集层内不同岩浆体的分布而组织起来的；在喷发之前如何以及何时积累大量可喷发熔体；流纹岩熔体的产生和储存过程是否因构造环境而异。该项目的结果将有助于深入了解硅质储集层中熔体成分在空间和时间上的异质性或同质性。这一建议的具体目标是：1)确定大型硅质岩浆体在化学成分上在空间和时间上的不均一程度，这对硅质岩浆在喷发前如何产生和融合/储存具有意义，以及2)比较不同构造环境下两个大型破火山口系统之间的地下几何形状和不均一性信息。建议通过分析美国西部黄石火山口和新西兰奥卡泰纳火山口建筑群的样品来解决这些问题。该项目将利用一种新的分析技术组合，包括对锆石和主要物相的微量元素分析和测年，以及锆石的Hf同位素组成。将通过两种方法获得岩浆系统化学变化的较长期的时间记录：1)分析与安装介质平行的晶面安装的未抛光颗粒表面，这将与常规抛光颗粒的内部分析相结合，并以连续切片的方法与选定颗粒的多个内部区域相结合；2)将锆石分析与U-Th年龄测定和主相矿物分离中的微量元素分析相结合。对颗粒表面的分析将提供最近锆石生长的记录，这是很难从抛光内部的斑点分析中获得的，因为薄的锆石边缘小于斑点尺寸，导致年龄混合。硅质岩浆中的锆石颗粒往往在将其带到地表的喷发之前记录了地下数万年至数十万年的条件，而主要相更有可能记录了岩浆体在喷发前几千年内的积累和储存信息，因此综合方法提供了对长期地下历史以及硅质岩浆在喷发前积累的独特见解。此外，可以直接与年龄和微量元素数据相关的锆石原位Hf同位素分析是一种新的方法，将提供一种很好的方法来指纹识别不同的岩浆成分/矿体，并跟踪它们随时间的演化和混合历史。这项研究的广泛影响包括更好地理解硅质系统的增长，这对火山灾害和大陆地壳的增长具有意义。该项目还将支持一名女性PI，并可能支持一名女性研究生(硕士研究生将被招募)。两名研究生将在各种尖端分析技术方面获得宝贵的经验和培训。这项工作将至少支持一篇本科生论文，如果可能的话，我们将每年多招收一名本科生参与这一项目。该协会有指导妇女和其他任职人数不足群体的记录，并将继续招募任职人数不足的学生参加这项提案中的研究生和本科生工作。这项提案还将支持该大学和美国地质调查局之间的国际合作和合作。

Insights into the development of silicic magma reservoirs over space and time from crystal- scale trace-element and isotopic data and U-Th datingIntellectual merit. Caldera-forming eruptions are among the most dramatic and hazardous geologic events to occur on Earth. In addition, melting and assimilation at large silicic caldera systems represent over geologic time important mechanisms that influence chemical characteristics of the continental crust. Important outstanding questions about how silicic reservoir systems operate include how large bodies of silicic magmas are generated (i.e., the balance between re-melting of existing crustal material and differentiation of mantle-derived material), how silicic magma systems are organized in terms of their subsurface geometry and the distribution of distinct magma bodies within the reservoir, how and when large volumes of eruptible melt are accumulated prior to eruptions, and whether the processes of rhyolitic melt generation and storage differ with tectonic setting. Results of this project will contribute to the debate by providing insights into the heterogeneity or homogeneity of melt compositions in silicic reservoirs over space and time. Specific goals of this proposal are: 1) to determine the degree to which large silicic magma bodies are chemically heterogeneous in composition over space and time, which has implications for how silicic magmas are generated and amalgamated/stored prior to eruptions, and 2) to compare this information about subsurface geometry and heterogeneity between two large caldera systems in different tectonic settings. It is proposed to address these questions by analyzing samples from Yellowstone Caldera, Western US, and Okataina Caldera Complex, New Zealand. The project will utilize a novel combination of analytical techniques, including trace-element analyses and dating of zircon and major phases along with Hf isotopic compositions of zircon. A longer-term temporal record of chemical changes in magmatic systems will be obtained through two approaches: 1) analysis of surfaces on unpolished grains mounted with the crystal face parallel to the mounting medium, which will be combined with interior analyses of conventional polished mounts and with multiple interior regions of selected grains in a serial-sectioning approach, and 2) combining zircon analyses with U-Th dating and analyses of trace elements in mineral separates of major phases. The analysis of surfaces of grains will provide a the record of the most recent growth of zircon, which is difficult to obtain from spot analyses of polished interiors because thin zircon rims are smaller than the spot dimensions, leading to mixing of ages. Zircon grains in silicic magmas often record conditions in the subsurface tens to hundreds of thousands of years prior to the eruptions that brought them to the surface, whereas major phases are more likely to record information about the accumulation and storage of magma bodies within a few thousands of years prior to their eruption, thus the combined approach provides unique insights into the long-term subsurface history as well as the accumulation of silicic magmas in the lead-up to eruptions. Furthermore, the addition of in-situ Hf isotopic analyses of zircon that can be directly related to age and trace-element data is new and will provide an excellent method of fingerprinting different magma compositions/bodies and tracking their evolution and mixing history over time.Broader impacts of this study include a better understanding of the growth of silicic systems, which has implications for volcanic hazards and for understanding the growth of the continental crust. This project will also support a female PI and possibly a female graduate student (MS student to be recruited). Two graduate students will gain valuable experience and training in a wide variety of cutting-edge analytical techniques. At least one undergraduate thesis will be supported by this work, and if possible we will recruit an additional undergraduate student each year to participate in this project. The PI has a track record of mentoring women and other underrepresented groups, and will continue to recruit underrepresented students for the graduate and undergraduate work in this proposal. This proposal will also support international collaborations and collaborations between the university and USGS.