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

从晶体尺度微量元素、同位素数据和U-Th大数据洞察硅质岩浆储层时空发育

• 批准号: 1144945
• 负责人: Kari Cooper
• 金额: $ 37.43万
• 依托单位: University of California-Davis
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2012
• 资助国家: 美国
• 起止时间: 2012-02-01 至 2016-01-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1144945&HistoricalAwards=false
• 关键词: Insights; into; Development; Silicic; Magma

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.

从晶体尺度的微量元素、同位素数据和U-Th测年资料看硅质岩浆储层的时空演化。形成火山口的火山喷发是地球上发生的最剧烈、最危险的地质事件之一。此外，大型硅质破火山口系统的融化和同化是影响大陆地壳化学特征的重要地质机制。关于硅质储层系统如何运作的重要悬而未决的问题包括：大型硅质岩浆体是如何产生的（即，现有地壳物质的再熔融与幔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层内不同岩浆体的分布来组织的，喷发前大量可喷发熔体是如何以及何时积聚的，流纹岩熔体的生成和储存过程是否因构造背景而异。该项目的结果将通过提供对硅储层中熔体成分在空间和时间上的非均质性或均匀性的见解，有助于争论。本提案的具体目标是：1)确定大型硅质岩浆体在化学成分上随时间和空间的不均匀程度，这对火山爆发前硅质岩浆是如何产生和合并/储存的有影响；2)比较不同构造环境下两个大型火山口系统的地下几何形状和非均质性的信息。我们建议通过分析美国西部的黄石火山口和新西兰的Okataina火山口复合体的样本来解决这些问题。该项目将利用一种新的分析技术组合，包括微量元素分析和锆石的年代测定，以及锆石的Hf同位素组成。岩浆系统化学变化的长期时间记录将通过两种方法获得：1)对晶面与安装介质平行的未抛光颗粒表面进行分析，这将与传统抛光颗粒的内部分析相结合，并在连续切片方法中对选定颗粒的多个内部区域进行分析。2)将锆石分析与U-Th定年和主要相矿物分离物中微量元素的分析相结合。对颗粒表面的分析将提供锆石最近生长的记录，这很难从抛光内部的斑点分析中获得，因为薄的锆石边缘比斑点尺寸小，导致年龄的混合。硅质岩浆中的锆石颗粒通常记录了火山喷发前几十到几十万年的地下条件，而主要阶段更有可能记录火山喷发前几千年岩浆体的聚集和储存信息。因此，这种综合方法提供了对长期地下历史的独特见解，以及火山爆发前硅岩浆的积累。此外，可以直接与年龄和微量元素数据相关的锆石原位Hf同位素分析是新的，将提供一种优秀的方法来识别不同的岩浆成分/体，并跟踪它们随时间的演化和混合历史。这项研究的更广泛的影响包括更好地理解硅系统的生长，这对火山危险和理解大陆地壳的生长有影响。该项目还将支持一名女性PI和一名女性研究生（将招收硕士学生）。两名研究生将获得各种尖端分析技术的宝贵经验和培训。至少有一篇本科论文将得到这项工作的支持，如果可能的话，我们将每年招收一名本科生参加这个项目。PI在指导妇女和其他代表性不足的群体方面有着良好的记录，并将继续招收代表性不足的学生参加本提案中的研究生和本科生工作。该提案还将支持大学与美国地质调查局之间的国际合作和合作。