Improved methods for quantitative analysis of three-dimensional porphyroblastic textures

Abstract

New data-processing methods for making three-dimensional measurements in volumetric data sets, described in a companion paper1, are applied here for quantitative textural analysis of porphyroblastic rocks. In a reexamination of a suite of garnetiferous rocks from the Picuris Mountains, New Mexico, Whitt Ranch, Texas, and Mica Dam, British Columbia, signals that in earlier studies indicated ordering of porphyroblast nucleation sites and competition for nutrients are significantly altered but generally confirmed in the improved data. Better-resolution tomographic imagery greatly enhances the observation and measurement of small crystals, in some cases substantially modifying the shape of the crystal size distributions. The enhanced analysis also enables detection of instances of strong impingement among neighboring porphyroblasts that were probably previously interpreted as single crystals, which may have spuriously enhanced ordering signals. Overall the results of this study corroborate earlier findings of diffusioncontrolled nucleation and growth of garnet in the specimens examined. They also document, however, the critical importance of high-quality data, which are required to ensure that subtle mechanistic signals can emerge from statistical noise and to ensure that failure of crystal impingement to be detected or preserved does not generate a bias toward ordering. © 2005 Geological Society of America.