Coupled compositional and three-dimensional textural analysis of garnet porphyroblasts




Chernoff, Carlotta Bridget

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Integration of garnet compositional data with quantitative textural data for garnet positions acquired from high-resolution computed X-ray tomography (X-ray CT) allows the growth zoning in a crystal to be assessed with a full knowledge of the locations and sizes of all other garnets in the specimen. This study of a specimen of garnetiferous quartzite from the Picuris Range of north-central New Mexico is the first to evaluate garnet compositional profiles for hundreds of crystals with a clear knowledge of the degree of isolation of each garnet. Using the X-ray CT imagery as a guide, a 47.9 cm³ volume of rock containing 1248 garnet crystals was serially sectioned by grinding to expose the central section of each garnet. Electron microprobe analyses through the centers of 470 garnets include profiles of Mn, Mg, Fe, Ca, Al, and Si at 10-20 μm spacing across 372 of the crystals and analyses of the cores of the remaining 98. Part I of this thesis presents results from several textural and compositional analysis techniques applied to the data set in order to assess the mechanisms that governed crystallization of garnet porphyroblasts. Textural data enable determination of statistical measures of the extent of ordering, clustering, and impingement of adjacent porphyroblasts and characterization of the crystal size distribution and degree of isolation of each porphyroblast. These data may be compared with predictions of quantitative textural features for porphyroblastic rocks resulting from different crystallization mechanisms. Likewise, normalized radius-rate relations computed from compositional zoning patterns in the garnets are compared with predicted relations for several growth-rate laws. Interpretation of these results indicates the dominance of diffusionally-influenced nucleation and diffusion-controlled growth mechanisms. In Part II, the effects of competition among adjacent porphyroblasts for garnet-forming nutrients during a diffusion-controlled nucleation-and-growth process are investigated. Measures of crystal isolation when compared with crystal sizes fail to provide a measure of diffusional competition, although detailed analysis of sizes of closely adjacent crystals reveals clear evidence for the effects of competition during growth. These data lead to the conclusion that the evidence for competitive growth is obscured by the effects of inhomogeneities in the precursor. Part III presents evidence for Ca disequilibrium during garnet growth. Compositional data for Mn, Mg and Fe indicate that their compositions evolved systematically during crystallization and achieved equilibrium in the intergranular medium at hand-sample scale, whereas Ca did not. Instead, Ca concentrations appear to reflect the local extent of reaction around each individual garnet crystal, yielding equilibrium for Ca at scales no larger than that of an individual porphyroblast. These observations are consistent with the speculation that the relative rate of diffusion of Mn, Mg and Fe through the intergranular medium is more rapid than that of Ca, permitting rock-wide equilibration for these species. Slower relative diffusion of Ca prevents equilibration at scales much larger than that of a single garnet crystal. These unexpected results challenge the current thinking about garnet crystallization and call into question the accuracy of thermobarometers based on cation-exchange and phase equilibria involving garnet