Investigation of Lead Diffusion in Monazite as a Potential Monitor of Thermal Perturbations Associated with Continental Rifting, Fosdick Mountains, Antarctica

该奖项由极地项目办公室的南极地质和地球物理项目提供，支持利用铅（Pb）在独居石中的扩散作为西南极洲玛丽伯德地福斯迪克山区岩石变质热历史指标的研究。该地区的岩石提供了早期大陆裂谷的异常清晰的记录，导致了白垩纪中期新西兰与西南极洲的分离。该项目以先前确定的构造、变质和岩浆历史的关键要素为基础，并利用现有的样品进行进一步的工作。这项新工作的重点是发生在高度变质的火成岩（高等级正长石）中的年龄带独居石的U-Th-Pb定年。这项工作之所以成为可能，是因为铅在独居石中的扩散知识的最新进展，以及离子探针技术的发展，这些技术可以通过10微米空间分辨率的斑点分析和在颗粒表面外1-2微米的离子钻孔进行深度剖面分析来确定独居石的年代。离子探针法可以在所需的尺度和精度上测定变质独居石的年龄，以解决在高变质事件中引起的U-Th-Pb年龄梯度。现有的实验Pb扩散数据允许根据在不溶性（部分熔融）条件下加热的持续时间来解释结果。这种新方法可以在理解早期大陆裂谷的时间尺度方面取得重大进展，并提供目前无法获得的信息。福斯迪克山区是应用和测试这些新的热年代学技术的一个有用的地质环境，因为该地区经历了与白垩纪中期地壳裂陷开始相关的主要热峰值。中地壳的部分熔融伴随着高位花岗岩的侵位。基于矿物化学的中地壳岩石峰值温度和压力分别为725 ~ 780℃和5 +/- 1千巴。该地区的快速发掘保存了峰级矿物组合。通过常规同位素稀释U-Pb测年，在两个正长岩样品中发现了年龄不一致的独居石，随后使用离子探针进行的初步研究证实了其中一个样品中粗独居石的核心到边缘年龄的强烈变化，证明了我们的方法的前景。该项目将对一系列变质岩中的独居石进行详细的离子微探针研究，以记录粒内年龄的变化。这些内部Pb分布的扩散模型结合可用的温度和压力数据，预计将导致对析氧条件持续时间的严格温度-时间限制。这项工作将得到单晶热电离质谱U-Pb定年的支持，以评估独居石中扩散Pb损失的范围。另外，还将进行铀-铅单殖石和锆石定年，以严格限制与该地区高水平花岗岩就位相关的变质峰条件的时间，而全岩主量元素和微量元素分析将有助于区分正长性原岩（烃源岩）。

This award, provided by the Antarctic Geology and Geophysics Program of the Office of Polar Programs, supports research to use lead (Pb) diffusion in monazite as an indicator of the thermal history of metamorphism of rocks in the Fosdick Mountain region of Marie Byrd Land, West Antarctica. Rocks in this region provide an exceptionally clear record of the early stages of continental rifting leading to the mid-Cretaceous separation of New Zealand from West Antarctica. The project builds on key elements of the structural, metamorphic and magmatic history that have been determined previously and utilizes existing samples for further work. The focus of this new work is on U-Th-Pb dating of age-zoned monazites that occur in highly metamorphosed igneous rocks (high-grade orthogneiss). This work is possible because of recent advancements in knowledge of Pb diffusion in monazite and the development of ion microprobe techniques for dating of monazite by both spot analysis at 10 micron spatial resolution and depth profiling by ion drilling in the outer 1-2 microns of grain surfaces. The ion microprobe method permits the determination of metamorphic monazite ages at the required scale and precision to resolve U-Th-Pb age gradients induced during high-grade metamorphic events. Existing experimental Pb diffusion data allow the results to be interpreted in terms of duration of heating at anatectic (partial melting) conditions. This new approach could lead to significant advancements in understanding time scales involved in incipient continental rifting, and provides information that is currently not otherwise obtainable.The Fosdick Mountain region is an useful geologic setting to apply and test these new thermochronological techniques because the area experienced a major thermal spike related to the initiation of mid-Cretaceous crustal rifting. Partial melting of the middle crust was accompanied by emplacement of high-level granites. Peak temperature and pressure of middle crustal rocks based on mineral chemistry are 725-780 degrees C and 5 +/- 1 kilobar. Rapid exhumation of the region preserved peak-grade mineral assemblages. Monazites with discordant ages were discovered in two orthogneiss samples by conventional isotope dilution U-Pb dating, and a pilot study using an ion probe subsequently confirmed a strong core to rim age variation in coarse monazite from one of these samples demonstrating the promise of our approach.This project will undertake a detailed ion microprobe study of monazite from a range of metamorphic rocks to document intra-grain age variations. Diffusion modeling of these internal Pb distributions combined with available temperature and pressure data is anticipated to lead to firm temperature-time constraints on the duration of anatectic conditions. This work will be supported by thermal ionization mass spectrometry U-Pb dating of single crystals to evaluate the domain size of diffusive Pb loss in monazite. Additional U-Pb monazite and zircon dating will be carried out to tightly constrain the timing of peak metamorphic conditions relative to emplacement of high level granites in the region while whole-rock major and trace element analyses will aid in differentiating orthogneiss protolith (source rocks).