TEXTURAL ASPECTS OF FORSTERITE IN THE ALTA AUREOLE, UTAH

Intrusion of the Alta Stock into siliceous dolomites resulted in the formation of an extensive contact aureole, where the development of forsterite (Fo) occurs from the igneous contact to the Fo reaction front (~700 m into the aureole). Quantitative textural studies of a suite of samples collected across the Fo zone have been started in order to investigate the mechanisms and rates of nucleation and growth during contact metamorphism.

The morphology, abundance, and size of Fo crystals vary between sites (across grade) and within sites (at constant grade) in the Fo zone. Overall, there is an upgrade change from relatively large, sparse tabular (aspect ratio 1:1-4:4-10) Fo to small, abundant, more sub-rounded Fo (~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 roughly 0.2 mm and the calculated number of Fo crystals per mole of Fo increases from ~2x10⁵ to ~2x10⁷.

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). Variations in Fo morphology, abundance and size within sites are likely the result of localized bulk composition and infiltrating fluid effects. Future work including 3-D textural analysis using X-Ray Computed Tomography, isotopic analysis, and quantitative trace element analysis will assist in resolving the effects of temperature, bulk composition, and infiltrating fluids on the nucleation and growth of minerals in contact metamorphic terranes.