Nucleation-dominated crystallization of forsterite in the Ubehebe Peak contact aureole, California

Abstract

The morphology and number of forsterite crystals in the Ubehebe Peak, California, contact aureole vary systematically as a function of metamorphic grade. From their first appearance to as close as ∼150 m from the intrusive contact, forsterite crystals are large (5–20 mm) and have a tabular habit ( a ≈ c ≫ b ). In contrast, forsterite near the contact is equigranular and much smaller (<1 mm in diameter). The number of crystals per mole of forsterite increases from 3.5 × 104 at the forsterite-in isograd to more than 1.5 × 108 near the contact. This trend is interpreted to result from an increase in the ratio of nucleation rates with respect to growth rates with proximity to the intrusion. The change in morphology from tabular to equigranular is explained by kinetic surface roughening. The nucleation and growth information gained from this study highlights the important role of nucleation kinetics in the crystallization of forsterite at the Ubehebe Peak aureole.