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).
