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

了解速度、持续时间和地质过程之间的相互作用依赖于准确的矿物测年。在有利的情况下，通过对相同或相关的矿物应用不同的测年技术来确认这些日期的准确性是可能的。美国加利福尼亚州东部600千米的毕晓普凝灰岩是矿物测年中日益重要的基准，也是洞察大规模爆炸性喷发演化的宝贵来源。该项目将检验一种假设，即在毕晓普凝灰岩中，锆石和闪锌矿的U-Pb和Ar/Ar年龄基本上是相同的。它将确定毕晓普凝灰岩锆石中是否存在U-Pb年龄不均一性，如果存在，将更严格地量化它们。如果锆石的年龄分布是均匀的，这项研究将验证毕晓普凝灰岩作为U-Pb和Ar/Ar辐射计时之间的关键互校准点的使用。如果存在较老的锆石区域，它们的年龄和人口分布将证实先前的证据，即锆石核心的日期可能比橄榄岩的日期要早得多。在急切的资金支持下，对U-Pb和Ar/Ar测年完全相同这一假设的迅速确认或拒绝，将对提高Ar/Ar测年的准确性的努力产生直接影响。Ar/Ar测年是一种能够为新生代事件提供准确日期的技术，也是一种可能难以用其他方法确定年代的地质材料。这一结果将为研究超级火山的岩浆如何在地球下聚集的模型提供依据？S表面。该项目将探索获得毕晓普凝灰岩锆石的空间敏感和分析更可靠的离子探针年龄的新方法。分析将在锆石边缘和光学干涉校准连续切片过程中暴露的定向横截面上进行，从而导致对晶体生长的高空间敏感度。谷物将按顺序安装，以便可以同时进行边缘和内部分析，从而减少对确认谷物内年龄差异的标准的依赖。溅射将需要一个100nA的离子束，而不是通常使用的10-12nA的离子束，从而提高了放射性铅产率，减少了分析的不确定性。一旦BT锆石中的年龄分布被量化，将进行质量平衡分析，以评估这些锆石是否在几个ka中结晶，以及如果存在较老的锆石区域，它们可能在多大程度上歪曲通过单晶TIMS分析获得的年龄。这项研究探索的新技术途径可能会提高现场U-Pb测年的精度，并可被其他现场地质年代学家采用，以期更深入地了解岩浆室过程的时间尺度的年龄不均一性及其在晶体中的分布。这项拟议的研究还将为一名女研究生提供空间敏感的U-Pb锆石测年方面的研究培训。

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.