Detrital Garnet Sm/Nd Geochronology: A New Window into Earth's Tectonic Past

古今沉积物中碎屑矿物的年代学记录被用来研究地球历史和构造变质过程。在许多情况下，沉积物中的碎屑矿物是唯一保存下来的长期被侵蚀的山脉和大陆碎片的记录。然而，现有的碎屑矿物年代学档案具有重要的局限性，包括这些矿物直接制约构造(即造山和俯冲)过程的能力。该项目首次使用碎屑石榴石Sm-Nd年代学作为了解地球--S构造历史的一个全新窗口。石榴石生长是对进退变质作用的响应，并记录了造山事件(通常与大陆碰撞有关)的形成部分，而目前可确定年代的碎屑矿物档案中完全没有这些形成部分。此外，通过分析众所周知的石榴石颗粒的化学成分，有可能将其年龄置于特定的压力-温度范围和生长的构造环境中(例如，俯冲带、区域或接触变质作用)。该项目的目标是探索、定义和推动通过简化过去几年开发的应用于单颗粒碎屑石榴石年龄测定的现有分析和样品制备方法而达到的样品大小和精确度的极限。该方法将使用从新英格兰采集的样品以及来自三个不同油田环境的经过充分研究的碎屑矿物套件进行测试，每个环境都有自己一套潜在的未来研究应用：1.太古代沉积，如杰克山砾岩；2.古生代造山砂岩侵蚀苏格兰经典的巴罗纪类型；3.北卡罗来纳州阿巴拉契亚山脉的现代水系沉积物。通过开发和利用古代碎屑石榴石年龄(及其矿物化学)档案，有可能挖掘地球上最古老的构造变质过程的直接记录，探索造山事件的全球年龄组，以检验从其他碎屑矿物年龄群体得出的关于全球大陆增长速度的推断，并解决涉及整个造山事件中进变质作用的开始、进展和事件的区域规模问题。

The geochronologic record of detrital minerals in ancient and modern sediments is used to study Earth history and tectonometamorphic processes. In many cases, detrital minerals in sediments represent the only preserved record of long since eroded mountains and continental fragments. However, the available detrital mineral geochronologic archive has important limitations including, importantly, the ability for these minerals to directly constrain tectonic (i.e. mountain building and subduction) processes. This project pioneers the use of detrital garnet Samarium-Neodymium (Sm-Nd) geochronology as a brand new window into Earth?s tectonic past. Garnets grow in response to prograde metamorphism and record the formative portions of mountain building events (typically related to continental collision) that can be completely missed in the currently dateable detrital mineral archive. Furthermore, by analyzing the well-understood chemistry of a garnet grain, it is possible to place its age into a specific pressure-temperature range and tectonic context wherein it grew (e.g. subduction zone, regional, or contact metamorphism). The goal of this project is to explore, define, and push the limits on sample size and precision attainable by streamlining existing analytical and sample preparation methods developed over the past several years for application to single grain detrital garnet age dating. The method will be tested using samples collected from New England as well as well-studied detrital mineral suites from three diverse field settings, each with its own set of potential future research applications: 1. Archean sediments such as the Jack Hills conglomerate; 2. Paleozoic orogenic sandstones eroding the classic Barrovian type-locality in Scotland; 3. modern stream sediments draining the Appalachian Mountains in North Carolina.By developing and utilizing an archive of ancient detrital garnet ages (and their mineral chemistry) there is the potential to tap the very oldest direct record of tectonometamorphic processes on the planet, explore global age clusters of mountain building events to test inferences about the pace of global continental growth drawn from other detrital mineral age populations, and address regional scale questions involving the onset, progression, and episodicity of prograde metamorphism throughout a mountain building event.