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

该奖项由极地计划办公室南极地质和地球物理计划提供，支持利用独居石中的铅(铅)扩散作为西南极马里伯德岛福斯迪克山区岩石变质热历史的指示器的研究。该地区的岩石提供了导致新西兰白垩纪中期从西南极分离的大陆裂谷早期阶段的异常清晰的记录。该项目以先前已确定的构造、变质和岩浆史的关键要素为基础，并利用现有样品进行进一步工作。这项新工作的重点是对产于高度变质的火成岩(高级正片麻岩)中的年龄分带独居岩进行U-Th-Pb测年。这项工作之所以可能，是因为最近对独居石中铅扩散的认识取得了进展，以及离子微探针技术的发展，通过10微米空间分辨率的斑点分析和在颗粒表面外1-2微米的离子钻探进行深度剖面法测定独居石的年龄。离子探针方法可以在所需的尺度和精度上测定变质独居石的年龄，以解决在高级变质事件中产生的U-Th-Pb年龄梯度。现有的实验铅扩散数据允许根据深熔(部分熔化)条件下的加热持续时间来解释结果。这种新的方法可能会在理解早期大陆裂谷所涉及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有用地质环境，因为该地区经历了与白垩纪中期地壳裂谷开始有关的重大热峰。中地壳的部分熔融伴随着高层花岗岩的侵位。中地壳岩石矿物化学峰温、峰压分别为725-780℃和5+/-1千巴。该地区的快速折返保存了顶峰级别的矿物组合。在两个正片麻岩样品中发现了年龄不一致的独居石，通过常规的同位素稀释法U-Pb测年，使用离子探针的初步研究随后证实了其中一个样品的粗独居石存在强烈的核-缘年龄变化，这证明了我们的方法的前景。本项目将对一系列变质岩中的独居石进行详细的离子探针研究，以记录颗粒内的年龄变化。对这些内部铅分布的扩散模拟，结合现有的温度和压力数据，预计将导致对再结晶条件持续时间的严格温度-时间限制。这项工作将与单晶热电离质谱U-Pb测年相结合，以评估独居石中铅扩散损失的磁区大小。此外，还将进行独居石和锆石U-Pb定年，以严格限制与区内高级花岗岩侵位相关的峰期变质条件的时间，而全岩主要和微量元素分析将有助于区分正片麻岩原岩(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).