HR: 17:50h
AN: V34A-07    [Abstracts]
TI: Controls on Porphyroblast Size Along a Regional Metamorphic Field Gradient
AU: * Hirsch, D M
EM: hirschd@cc.wwu.edu
AF: Western Washington University, 516 High St., Bellingham, WA 98225-9080, United States
AB: Garnet-bearing schists from the Waterville Formation of south-central Maine provide an opportunity to examine the factors governing porphyroblast size over a range of metamorphic grade. Three-dimensional sizes and locations for all garnet porphyroblasts were determined for three samples along the metamorphic field gradient spanning lowest garnet through sillimanite grade, using high-resolution X-ray computed tomography. Comparison of crystal size distributions to previous data sets obtained by stereological methods for the same samples reveals significant differences in mode, mean, and shape of the distributions. Quantitative textural analysis shows that the garnets in each rock crystallized in a diffusion-controlled nucleation and growth regime. In contrast to the typical observation of a correlation between porphyroblast size and position along a metamorphic field gradient, porphyroblast size of the lowest-grade specimen is intermediate between the high- and middle-grade specimens' sizes. Mean porphyroblast size does not correlate with peak temperatures from garnet-biotite Fe-Mg exchange thermometry, which would be expected if porphyroblast sizes were strongly affected by post-crystallization annealing (Ostwald ripening), as previously proposed for these rocks. Robust pseudosection calculations fail to reproduce the observed garnet core compositions for two specimens, indicating that these calc-pelites experienced metasomatism. For each of these two specimens, Monte Carlo calculations suggest potential pre-metasomatism bulk compositions that replicate garnet core compositions. Pseudosection analyses coupled with semi-quantitative pressure-temperature paths allow the estimation of the critical temperatures for garnet growth: ~472° C for the lowest-garnet-zone and middle-garnet-zone specimens, and ~492° C for the sillimanite-zone specimen. Porphyroblast size appears to be determined in this case by a combination of the critical temperature for the garnet forming reaction and variations in the rates of nucleation and heating. Numerical simulations of thermally accelerated, diffusion-controlled nucleation and growth for the three samples closely match measured crystal size distributions. These observations and simulations suggest that previous hypotheses linking the temperature at the onset of porphyroblast nucleation to the resulting crystal size can only partially explain the observed textures. Also important in determining porphyroblast size are the heating rate and the distribution of favorable nucleation sites.
DE: 3612 Reactions and phase equilibria (1012, 8412)
DE: 3620 Mineral and crystal chemistry (1042)
DE: 3625 Petrography, microstructures, and textures
DE: 3652 Pressure-temperature-time paths
DE: 3660 Metamorphic petrology
SC: Volcanology, Geochemistry, and Petrology [V]
MN: 2007 Joint Assembly