Collaborative Research: Tectonic Rates From Garnet Differential Geochronology

摘要：合作研究：基于石榴石差年代学的构造速率研究项目拟通过Sr同位素差年代学研究石榴石生长速率和P-T变化。过去二十年来，测量矿物中Sr同位素的分析能力迅速发展，因此，可以用比20年前提高了几乎一个数量级的空间分辨率来解决成分变化，并且可以更有效地测量石榴石的生长速率。通过差示Sr同位素分析，对三个地区变质岩套的石榴石生长速率和P-T变化进行了表征。知识价值。提出的研究解决了关于矿物生长和变质作用的两个基本问题：(1)石榴石在区域变质环境中的生长速度有多快？(2)区域变质作用和构造作用是间断的、短暂的，还是渐进的、长期的？样本将包括3种经典环境：新英格兰中部的推覆体堆，阿尔卑斯-基克拉迪斯带的高压/超高压岩石，以及科迪勒拉达尔文（智利最南端）的巴罗维岩石。石榴石的生长速率将在10个样品中测量，这些样品最能代表它们各自的地质体和变质演化。这些岩石的地球化学特征、岩石学和构造解释已经发表，大大简化了拟议的研究。大多数样品在与邻近岩性的流体交换方面具有封闭系统行为。每个岩石中的特定颗粒将使用x射线图（电子显微探针，RPI）和微量元素剖面（LA-ICP-MS， WSU）进行筛选，以确保分区模式符合已公布的数据。微样品（1mm）石榴石将分析Rb/Sr和87Sr/86Sr比率。差异地质年代学将确定每块石榴石生长所需的时间，并用于测试基于每块岩石的P-T路径的构造模型。这些数据将检验最近提出的建议的有效性，即石榴石在巴罗维亚地体中的生长是短暂的（1兆热当量），并与不同的热脉冲有关，即石榴石在构造引起的热扰动下生长的经典模型，这意味着石榴石的生长超过兆热当量到10兆热当量。更广泛的影响。这项工作将支持地球化学、岩石学和地质年代学研究生的教育。此外，pi以及时和适当的方式向其他地球科学家发表和展示结果，因此科学界将迅速获得结果。

Summary: Collaborative research: Tectonic rates from garnet differential geochronology Funding is requested to investigate rates of garnet growth and P-T changes through Sr isotope differential geochronology. The last two decades have witnessed rapid growth in analytical capabilities for measuring Sr isotopes in minerals, such that compositional changes can be resolved with a spatial resolution almost an order of magnitude better than 20 years ago, and rates of garnet growth can be measured more efficiently. Rates of garnet growth and P-T changes through differential Sr isotope analysis will be characterized in metamorphic suites from three regions. Intellectual Merit. The proposed research addresses two basic questions regarding mineral growth and metamorphism: (1) How fast do garnets grow in regional metamorphic settings? (2) Are regional metamorphism and tectonism punctuated and brief, or gradual and prolonged? Samples will include 3 classic settings: the nappe pile of central New England, HP/UHP rocks of the Alpine-Cyclades belt, and Barrovian rocks of Cordillera Darwin (southernmost Chile). Garnet growth rates will be measured in 10 samples total that are most representative of their respective terranes and metamorphic evolution. Geochemical characterization, and petrologic and tectonic interpretations have already been published for these rocks, vastly simplifying the proposed research. Most samples have experienced closed-system behavior with respect to fluid exchange with adjacent lithologies. Specific grains in each rock will be screened using X-ray maps (electron microprobe, RPI) and trace element profiles (LA-ICP-MS, WSU) to ensure that zoning patterns conform with published data. Microsamples ( 1mm) of garnets will be analyzed for Rb/Sr and 87Sr/86Sr ratios. Differential geochronology will establish how long each garnet took to grow, and used to test tectonic models based on each rock's P-T path. These data will test the validity of the recent proposal that garnet growth in Barrovian terranes is brief (1 Myr) and associated with distinct thermal pulses, viz a viz classic models of garnet growth in response to tectonically-induced thermal perturbations, which imply that garnets grow over Myrs to 10s of Myrs. Broader Impacts. This work will support the education of a graduate student in geochemistry, petrology, and geochronology. Also, the PIs publish and present results in a timely and appropriate fashion to other geoscientists, so results will become rapidly available to the scientific community.