TEXTURAL ASPECTS OF FORSTERITE IN THE ALTA STOCK AUREOLE, UTAH

Infiltration-driven metamorphism in the south Alta Aureole resulted in the development of forsterite (Fo) in siliceous dolomites from the igneous contact to the Fo reaction front (700 m into the aureole). The morphology and size of Fo crystals vary with increasing grade across the Fo zone, from relatively large, sparse tabular (aspect ratio 1:1-4:4-10) Fo to small, abundant, more sub-rounded Fo (at < 250 m from the igneous contact). Initial results from quantitative textural analysis of Fo in 2-D using image analysis of element X-Ray maps (Ca, Mg, and Si) support these observations and reveal other trends. Upgrade, the average grain size decreases from more than 1 mm to approximately 0.2 mm and the calculated number of Fo crystals per mole of Fo increases from ~2x10⁵ to ~3x10⁷. The increase in Fo crystals per mole Fo suggests a progressive increase in the rate of crystal nucleation relative to the rate of crystal growth toward the contact. This is consistent with the greater impact of increased temperature on crystal nucleation rates compared to crystal growth rates as observed in other contact aureoles (e.g., Ubehebe Peak: Roselle et al., 1997).

Tests of the random distribution of crystal centers in three mutually perpendicular planes from one lower-grade sample (600 m from the igneous contact) yield mixed results; two planes have non-random distributions, the remaining plane random distribution. All three planes pass statistical tests for homogeneity, so the highly anisotropic geometry of the Fo crystals in this low-grade sample may be introducing artifacts into the statistical analysis of 2-D sections. The non-random distributions of crystal centers in two of the three planes may provide some evidence of an important role for diffusion-controlled nucleation and growth mechanisms in this lower grade sample. Tests in a statistically homogeneous, higher-grade sample (300 m from the igneous contact) indicate a random distribution of crystal centers, suggesting an increased role for interface-controlled reaction mechanisms. This change is consistent with the measured increases in Fo crystals per mole Fo with grade. Three-dimensional analysis using X-Ray Computed Tomography is in progress to attempt to define more clearly the spatial dispositions of these anisotropic Fo crystals.