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

该奖项由极地方案办公室南极地质和地球物理方案提供，支持研究使用独居石中的铅（Pb）扩散作为南极洲西部玛丽伯德地福斯迪克山区岩石变质热历史的指标。该地区的岩石提供了一个非常清晰的记录，记录了导致新西兰从西南极洲分离的白垩纪中期大陆裂谷的早期阶段。 该项目建立在以前确定的结构、变质和岩浆历史的关键要素基础上，并利用现有样品进行进一步的工作。这项新工作的重点是U-Th-Pb测年的年龄带独居石，发生在高度变质的火成岩（高级正片麻岩）。 这项工作是可能的，因为最近的进步，在独居石中的铅扩散的知识和离子微探针技术的发展，独居石的测年现场分析在10微米的空间分辨率和深度剖析的离子钻孔在外部1-2微米的颗粒表面。 离子探针法可以在要求的尺度和精度上测定变质独居石的年龄，以解决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的结果来解释在anaerobic（部分熔融）条件下加热的持续时间。 这种新的方法可能会导致重大的进步，在理解的时间尺度参与初期大陆裂谷，并提供信息，是目前没有其他方式available.The Fosdick山区是一个有用的地质环境，应用和测试这些新的热年代学技术，因为该地区经历了一个重大的热峰有关的启动中白垩纪地壳裂谷。 中地壳的部分熔融伴随着高位花岗岩的侵位。 根据矿物化学，中地壳岩石的峰值温度和压力为725-780摄氏度和5 +/- 1毫巴。 该地区的快速折返保存了顶级矿物组合。独居石与不一致的年龄被发现在两个正片麻岩样品的常规同位素稀释U-Pb测年，和一个试点研究，使用离子探针随后证实了一个强大的核心，从这些样品中的粗独居石的边缘年龄变化，证明了我们的prompty.This项目将进行详细的离子探针研究独居石从一系列的变质岩，以记录晶粒内的年龄变化。这些内部铅分布的扩散模型结合现有的温度和压力数据预计会导致公司的温度-时间约束的持续时间的anasthenic条件。 这项工作将得到单晶热电离质谱U-Pb测年的支持，以评估独居石中扩散铅损失的畴尺寸。 将进行额外的U-Pb独居石和锆石测年，以严格限制峰变质条件相对于该地区高级花岗岩侵位的时间，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).