GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 119-2
Presentation Time: 9:00 AM-6:30 PM


RUTHVEN, Rachel C. and KETCHAM, Richard A., Jackson School of Geosciences, The University of Texas at Austin, 2305 Speedway Stop C1160, Austin, TX 78712-1692

Combined textural and chemical analyses pose novel tests that can challenge and enhance our understanding of metamorphic crystallization. Electron microprobe data for cores and core-to-rim traverses reflect porphyroblast nucleation and growth rates as well as the evolution of sample composition during crystallization. Manganese oxide (MnO) concentrations in garnet cores serve as proxies for the relative timing of nucleation, and rim concentrations test the hypothesis that the MnO content of crystallizing garnet is equal sample-wide during the final stages of crystallization and verify that concentrations have not been greatly altered by intracrystalline diffusion. Crystal-size distributions, central section locations, and other textural information acquired using High Resolution X-ray Computed Tomography (HRXCT), combined with compositional data, can potentially be used to reveal the crystallization process in space and time, or 4D. Previous studies on rocks with homogeneously distributed garnet have shown evidence of diffusion-controlled crystallization rates, meaning that they are limited by diffusion of nutrients to growing porphyroblasts.

This study focuses on a pelitic quartzite schist from the Picuris Mountains with a heterogeneous garnet distribution consisting of mm- to cm-scale aluminum (Al) -rich and -poor layers. These compositionally distinct domains evolved in direct proximity under identical P-T conditions, providing an ideal setting to assess scales of disequilibrium and transport for different cations, and their consequences for quantitatively understanding garnet crystallization. 3D data show Al-poor layers have smaller, less euhedral garnets, and petrographic observations show Al-poor layers have larger quartz crystals and less mica than Al-rich layers. Due to the extreme layering, textural analyses were unable to determine if garnet textures exhibit diffusion-controlled nucleation and growth. Chemical analyses reveal delayed nucleation and off-center MnO peaks in the Al-poor layers, suggesting irregular growth histories or local disequilibrium, likely due to transfer of fast-diffusing cations from Al-rich regions. Considering these chemical differences, we plan to construct and compare model pressure-temperature paths for Al-rich and -poor layers.