EAGER: U-PB DATING OF BISHOP TUFF ZIRCON BY SERIAL SECTIONING FOR GEOCHRONOMETER CALIBRATION AND MAGMATIC INSIGHTS

了解地质过程之间的速率、持续时间和相互作用依赖于矿物的准确年代测定。在有利的情况下，可以通过对相同或相关的矿物应用不同的年代测定技术来确认这些年代的准确性。美国加利福尼亚州东部600平方公里的主教凝灰岩是矿物测年的一个日益重要的基准，也是了解大量爆炸性喷发演化的宝贵来源。该项目将检验毕晓普凝灰岩中矿物锆石的U-Pb和Ar/Ar日期基本相同的假设。它将确定主教凝灰岩锆石中是否存在U-Pb年龄非均质性，如果存在，则更严格地量化它们。如果锆石的年龄分布是均匀的，本研究将验证Bishop凝灰岩作为U-Pb和Ar/Ar辐射时钟之间的关键intercalibration点的有效性。如果存在更古老的锆石域，它们的年龄和种群分布将证实之前的证据，即锆石岩心的年代可能比锆石的年代要早得多。在EAGER的资助下，对U-Pb和Ar/Ar年代相同的假设的快速确认或拒绝将对提高Ar/Ar定年准确性的努力产生直接影响，Ar/Ar定年技术能够为新生代事件和其他方法难以确定年代的地质材料提供精确的日期。这一结果将为研究导致超级火山的岩浆如何在地下积聚的模型提供信息。年代的表面。该项目将探索获取Bishop凝灰岩锆石空间敏感性和分析可靠性更高的离子微探针日期的新方法。分析将在锆石边缘和定向截面上进行，这些截面是在光学干涉校准的连续切片中暴露出来的，这导致了晶体生长的高空间灵敏度。谷物将按顺序安装，以便边缘和内部分析可以同时进行，减少对确认谷物（如果存在）年龄差异的标准的依赖。溅射将需要100 nA离子束而不是通常使用的10-12 nA离子束，从而提高放射性铅产量并降低分析不确定度。一旦BT锆石的年龄分布被量化，将进行质量平衡分析，以评估锆石是否在几ka内结晶，如果存在更古老的锆石域，它们对单晶TIMS分析获得的年龄有多大程度的偏差。本研究探索的新技术方法可以提高原位U-Pb定年的精度，并为其他原位地质年代学家在岩浆室过程的时间尺度上寻求更深入地了解年龄非均质性及其在晶体中的分布所采用。该研究还将为一名女研究生提供空间敏感U-Pb锆石测年的研究培训。

Understanding the rates, durations, and interplay between geologic processes relies on accurate dating of minerals. Under favorable circumstances, it is possible to confirm the accuracy of these dates by applying different dating techniques to the same, or related, minerals. An increasingly important benchmark in mineral dating as well as a valuable source of insights into the evolution of voluminous explosive eruptions is the 600 km3 Bishop Tuff of eastern California, USA. This project will test the hypothesis that U-Pb and Ar/Ar dates for the minerals zircon and sanidine, respectively, are essentially identical in the Bishop Tuff. It will determine whether there are U-Pb age heterogeneities within Bishop Tuff zircon and, if present, to more rigorously quantify them. If the age distribution with zircon is homogeneous, this study will validate use of the Bishop Tuff as a key intercalibration point between the U-Pb and Ar/Ar radiometric clocks. If older zircon domains are present, their ages and their population distributions will validate previous evidence that dates for zircon cores can be considerably older than the sanidine dates. Rapid confirmation or rejection, as enabled by EAGER funding, of the hypothesis that the U-Pb and Ar/Ar dates are identical will have an immediate impact on efforts to improve the accuracy of Ar/Ar dating, a technique capable of providing precise dates for events in the Cenozoic Era, and for geologic materials that may be difficult to date by other methods. The outcome will inform models for how magmas responsible for supervolcanoes accumulate beneath Earth?s surface.The project will explore novel approaches to acquiring spatially sensitive and analytically more robust ion microprobe dates for Bishop Tuff zircons. Analyses will be performed on zircon rims and on oriented cross-sections exposed during optical interferometry-calibrated serial sectioning, resulting in a high spatial sensitivity with respect to crystal growth. Grains will be mounted sequentially so that rim and interior analyses can be performed in tandem, reducing the reliance on a standard for confirming age differences within grains, if present. Sputtering will entail a 100 nA ion beam instead of the normally employed 10-12 nA beam, resulting in enhanced radiogenic Pb yields and reduced analytical uncertainties. Once the age distribution within the BT zircons has been quantified, mass balance analyses will be performed in order to assess whether the zircons crystallized in a few ka and, if older zircon domains are present, the extent to which they could skew the ages obtained by single crystal TIMS analyses. The new technical approaches to be explored in this study could advance the precision of situ U-Pb dating and be adopted by other in situ geochronologists seeking to gain more insight into age heterogeneities and their distributions within crystals for timescales of magma chamber processes. The proposed study will also provide research training in spatially sensitive U-Pb zircon dating for a female graduate student.