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

University Of California-Davis, Davis CA

Investigators

Abstract

Insights into the development of silicic magma reservoirs over space and time from crystal- scale trace-element and isotopic data and U-Th dating Intellectual 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.