Three-dimensional quantitative textural analysis of metamorphic rocks using high-resolution computed X-ray tomography : methods, techniques and application to natural samples

Quantitative three-dimensional textural analysis of metamorphic rocks using high-resolution computed X-ray tomography and automated image mensuration produces data on the size and position of thousands of crystals in a rock volume. Part I of this dissertation reviews some fundamentals of computed X-ray tomography and describes techniques of three-dimensional quantitative textural analysis that determine, from tomographic imagery, the extent of ordering, clustering, intergrowth and isolation of porphyroblasts within a rock volume. These textural data can be interpreted in light of numerical models of nucleation and growth that predict quantitative textural features of porphyroblastic rocks for different crystallization mechanisms. Comparison of the predictions of these models to natural textures, in conjunction with analysis of compositional zoning, establishes the relative importance of possible atomic-scale mechanisms of porphyroblast crystallization. In Part 11, these methods are used to assess the mechanisms governing crystallization of garnet porphyroblasts in rocks from diverse regional metamorphic environments. In every case, spatial dispositions, crystal size distributions, and compositional zoning patterns of porphyroblasts indicate the dominance of diffusionally influenced nucleation and diffusion-controlled growth mechanisms. Nine samples from three geologic areas were studied: a suite of pelitic rocks from the Picuris Mountains, New Mexico (USA); a suite of mafic samples from the Llano Uplift, Texas (USA); and a kyanite schist from Mica Dam, British Columbia (Canada). For each sample, the centers and radii of thousands of garnet crystals were located and measured in three dimensions from the tomographic images. Statistical measures of the degree of ordering and clustering of nucleation sites, and estimates of crystal isolation for each porphyroblast, were then computed from the measured spatial dispositions. These measures can be reproduced in simple numerical models only by diffusionally influenced nucleation and diffusion-controlled growth mechanisms. Normalized radius-rate relations computed from compositional zoning patterns in the garnets require thermally accelerated diffusion-controlled growth, providing independent confirmation of the conclusions based on textural analysis. The unexpected similarity of results from all samples indicates that diffusionally influenced nucleation and growth mechanisms may govern porphyroblast crystallization in many metamorphic regimes.