NUCLEATION-DOMINATED CRYSTALLIZATION OF FORSTERITE IN THE UBEHEBE PEAK CONTACT AUREOLE, CALIFORNIA

The morphology and number of forsterite crystals in the Ubchebe Peak, California, contact aureole vary systematically as a function of metam orphic grade. From their first appearance to as close as similar to 15 0 m from the intrusive contact, forsterite crystals are large (5-20 mm ) and have a tabular habit (a approximate to c >> b). In contrast, for sterite near the contact is equigranular and much smaller (<1 mm in di ameter). The number of crystals per mole of forsterite increases from 3.5 x 10(4) at the forsterite-in isograd to more than 1.5 x 10(8) near the contact, This trend is interpreted to result from an increase in the ratio of nucleation rates with respect to growth rates with proxim ity to the intrusion, The change in morphology from tabular to equigra nular is explained by kinetic surface roughening. The nucleation and g rowth information gained from this study highlights the important role of nucleation kinetics in the crystallization of forsterite at the Ub ehebe Peak aureole.