GARNET CRYSTAL SIZE AND SPATIAL DISTRIBUTIONS IN A STAUROLITE SCHIST

The size and spatial distributions of garnet porphyroblasts have been measured in a staurolite garnet schist from the summit of Elephant Mountain, 16 miles SSW of Rangeley, Maine. This area is a classic example of a regional-contact metamorphic setting and the crystal size distributions will yield information about the mechanisms of garnet nucleation and growth in this type of thermal environment.

The three dimensional distribution of garnets in the specimen (RA59a) were imaged at the High Resolution X-ray CT Facility in the Department of Geological Sciences at the University of Texas in Austin by W.D. Carlson. Sample RA59a is a cylinder of rock 29mm in diameter and 49.5 mm high. The X-ray CT data are presented as a stack of 738 16 bit 512 x 512 pixel images that can be used to build a three dimensional model of the rock using “NIH Image” software. The "density threshold" function in NIH Image was used to highlight pixels that had values characteristic of garnet. Then the "analyze particles" function in NIH Image was used on each slice to determine the total area of each garnet cross-section and the center and axes of an ellipse fit to each crystal cross-section. These data were then processed in an Excel Visual Basic program to group the 2-D measurement data into 3-D crystals, to determine the X and Y coordinates of the center of each 3-D crystal and the mean diameter of the center slice of each crystal.

The observed size and spatial distributions for the 663 crystals in the sample will be compared with different nucleation and growth models (Foster and Dutrow 2003) to identify which of several possible thermal histories are responsible for garnet nucleation and growth in the Rangeley, Maine regional-contact metamorphic terrane.