TEXTURAL, CATHODOLUMINESCENCE, AND GEOCHEMICAL EVIDENCE FOR DENDRITIC CRYSTALLIZATION OF METAMORPHIC FORSTERITE, ALTA, UTAH

Infiltration-driven metamorphism has produced widespread development of forsterite (Fo) in dolomitic marbles of the contact aureole surrounding the Alta, Utah stock. The textural, cathodoluminescence (CL), and trace element characteristics of Fo vary systematically with distance from the igneous contact. Texturally, Fo from the inner Fo zone tends to form more euhedral polyhedrons while Fo from the outer Fo zone tends to be more tabular, with highly skeletal and dendritic forms persisting as well. Inner Fo zone olivines have up to 1.7% Fe by weight and do not show any CL signal, while Fo from the middle to outer Fo zones (<1% Fe by weight) exhibits distinct CL structural domains. These CL domains appear to be crystallographically defined, and record the early development of disequilibrium textures (i.e. dendrites, hopper morphologies, reentrants) followed by the approach to lower surface area, and thermodynamically more stable, tabular and polyhedral forms by the infilling of later Fo growth. Trace element contents determined in the Alta Fo by LA-ICP-MS demonstrate a strong positive correlation between B (10-1000 ppm) and Ti (1-200 ppm) contents and CL intensity, suggesting F^-, OH^-, Ti, or defects associated with their substitution, as the primary CL activators. P (1-1000 ppm) and Li (1-300 ppm) contents are also highly zoned on the grain-scale, anti-correlate with CL intensity, and can also be used to identify compositionally and temporally distinct Fo domains. These characteristics caution against the interpretation of grain-scale geochemical datasets within a conventional framework of core-to-rim growth of metamorphic Fo.

These observations and data suggest that kinetics play an important role in the Fo textures that develop during contact metamorphism in the Alta aureole. Even though locations close to the igneous contact might be expected to experience greater overstepping, higher temperatures persisting longer in the inner Fo zone would allow this Fo more time to texturally equilibrate to more mature, euhedral forms. In contrast, Fo far from the contact preserves significant textural disequilibrium due to the initial over-stepping of the Fo forming reaction boundary, followed by a relatively rapid return to conditions outside the Fo stability field, and hence a shorter period of textural equilibration.