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

EAGER：通过地质天文台校准和岩浆洞察的连续切片对主教凝灰岩锆石进行 U-PB 定年

• 批准号: 1336012
• 负责人: Mary Reid
• 金额: $ 1.21万
• 依托单位: Northern Arizona University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2013
• 资助国家: 美国
• 起止时间: 2013-03-15 至 2015-02-28
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1336012&HistoricalAwards=false
• 关键词: EAGER; PB; DATING; BISHOP; TUFF

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.

了解地质过程的速率、持续时间和相互作用依赖于矿物的准确定年。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测年技术来确认这些测年的准确性。美国加州东部600 km ~ 3的主教凝灰岩是矿物测年中一个越来越重要的基准，也是了解大量爆发性喷发演化的一个有价值的来源。该项目将检验这一假设，即锆石和透长石的U-Pb和Ar/Ar年龄分别在主教凝灰岩中基本相同。它将确定主教凝灰岩锆石中是否存在U-Pb年龄不均匀性，如果存在，将对其进行更严格的量化。如果锆石的年龄分布是均匀的，这项研究将验证使用主教凝灰岩作为U-Pb和Ar/Ar辐射计时钟之间的关键互校点。如果存在较老的锆石域，它们的年龄和人口分布将验证以前的证据，即锆石核的年龄可能比透长石年龄大得多。快速确认或拒绝，如EAGER基金，假设U-Pb和Ar/Ar年龄是相同的，将有一个直接的影响，努力提高Ar/Ar定年的准确性，一种技术，能够提供精确的新生代事件的日期，并为地质材料，可能难以通过其他方法。这一结果将为超级火山的岩浆如何在地球下面积累的模型提供信息。该项目将探索新的方法来获取空间敏感和分析更强大的离子微探针主教凝灰岩锆石的日期。分析将进行锆石边缘和光学干涉校准的连续切片过程中暴露的定向横截面，从而在相对于晶体生长的高空间灵敏度。谷物将按顺序安装，以便可以同时进行边缘和内部分析，减少对确认谷物内年龄差异（如果存在）的标准的依赖。溅射将需要100 nA的离子束，而不是通常采用的10-12 nA的光束，从而提高放射性铅产量和减少分析的不确定性。一旦对BT锆石的年龄分布进行了量化，将进行质量平衡分析，以评估锆石是否在几ka内结晶，如果存在较老的锆石区域，则评估它们可能在多大程度上偏离单晶TIMS分析获得的年龄。新的技术方法，在这项研究中探索，可以提高精度的原位U-Pb测年，并通过其他原位地质年代学家寻求获得更多的洞察年龄不均匀性和它们的分布在晶体内的岩浆房过程的时间尺度。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。