In situ Rb-Sr mica petrochronology: a transformative approach to characterizing tectonic processes

~(87)Rb到~(87)Sr的放射性衰变是地质学家用来测量岩石和矿物年龄的重要“时钟”。然而，这些测量传统上需要耗时的实验室工作来进行混合矿物分离。最近，通过实现就地(就地)Rb-Sr测量，几项技术进步缓解了这一问题，但它们的广泛使用仍然存在障碍。这些问题包括缺乏适当的参考材料，以及不同岩石中Rb-Sr测年记录的问题。这项提案从两个方面解决了这些问题。首先，研究人员将开发两个Rb-Sr参考玻璃，以显著扩大Rb-Sr测年的使用和应用。他们还将确定三个经过充分研究的岩套的Rb-sr年龄，为其他地方的类似数据提供关键背景。这项技术是新颖的，因为它使我们能够准确地测定云母的年代，云母是地壳中最常见的矿物之一。它还有助于在不到一小时的时间内测量岩石Rb-Sr年龄，远远短于传统方法(几周到几个月)。该项目由中早期职业科学家牵头，支持两名本科生和两名研究生的专业发展。它还促进了美国东北部两个农村机构之间的科学合作。这项提议的科学目标有两个。首先，研究人员将生产两种云母玻璃(白云母+黑云母)，并对它们的主要+微量元素和Rb-Sr同位素进行表征。这包括高精度的同位素稀释技术以及社区范围内的多仪器能力测试计划(G-Probe)。他们的云母玻璃标准物质将被自由和广泛地传播，填补了一个关键的分析空白，并推动了其他实验室的Rb-Sr技术的进一步发展，这将不可避免地导致其他重大发现。作为这项工作的一部分，将进一步开发用于处理激光烧蚀Rb-Sr数据的数据缩减计划，以包括来自多采集器仪器的质量移位的同位素和井下分馏评估。其次，他们将通过单收集器和多收集器LA-ICP-MS/MS测定来自三个代表端元构造壳体的特征良好的地点的云母Rb-Sr年龄：ME的Mooselookmegentic接触光环(美国)、加利福尼亚州的Franciscan俯冲杂岩(美国)和南阿尔卑斯山的伊夫雷亚-韦尔巴诺带(意大利)。通过在三个构造案例研究中进行运动式的云母Rb-Sr年代学，在这项工作中收集的其他结构和地球化学数据的背景下，研究人员将测试Rb-Sr系统在一系列地质相关条件下的行为。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

The radiogenic decay of 87Rb to 87Sr is an important “clock” that geologists use to measure the age of rocks and minerals. However, these measurements have traditionally required time-consuming lab work on pooled mineral separates. Several technical advances have recently alleviated this problem by enabling in-place (in situ) Rb-Sr measurements, but there are still barriers to their widespread use. These include a lack of appropriate reference materials, as well as questions about what Rb-Sr dates record in different rocks. This proposal addresses these problems in two ways. First, the researchers will develop two Rb-Sr reference glasses to significantly expand the use and application of Rb-Sr dating. They will also determine Rb-Sr dates from three well-studied rock suites, to provide key context for similar data elsewhere. This technique is novel because it allows us to accurately date mica, one of the most common minerals in Earth's crust. It also facilitates the measurement of rock Rb-Sr dates in less than an hour, far shorter than the traditional method (weeks to months). This project is led by middle and early career scientists and supports the professional development of two undergraduate and two graduate students. It additionally promotes scientific collaboration between two rural institutions in the northeastern US. The scientific goals of this proposal are two-fold. First, the researchers will produce two mica glasses (muscovite + biotite) and characterize them for major + trace elements and Rb-Sr isotopes. This includes high-precision isotope-dilution techniques as well as a community-wide, multi-instrument proficiency testing scheme (G-Probe). Their mica glass reference materials will be freely and widely disseminated, filling a critical analytical gap and spurring further development of the Rb-Sr technique in other labs, which will inevitably lead to other significant discoveries. As part of this effort, a data reduction scheme (DRS) for processing laser ablation Rb-Sr data will be further developed to include mass-shifted isotopes from multicollector instruments and downhole fractionation assessment. Second, they will determine in situ mica Rb-Sr dates by single- and multi-collector LA-ICP-MS/MS from three well-characterized locales representing endmember tectonic cases: the Mooselookmeguntic contact aureole, ME (USA); the Franciscan Subduction Complex, CA (USA); and the Ivrea-Verbano Zone, S. Alps (Italy). By performing campaign-style mica Rb-Sr geochronology in three tectonic case studies, within the context of other textural and geochemical data collected during this work, the researchers will test the behavior of the Rb-Sr system across a range of geologically relevant conditions.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.