Evidence for dendritic crystallization of forsterite olivine during contact metamorphism of siliceous dolostones, Alta stock aureole, Utah

Abstract

Forsterite crystals produced during metamorphism of siliceous dolomites in the Alta, Utah contact aureole were investigated in-situ and with crystal separates to constrain their aureole-scale textural and morphological variability. As the contact is approached, forsterite crystals increase in abundance but decrease in average size, suggesting greater kinetic influence on forsterite nucleation and growth in the inner aureole. Crystal morphologies vary widely from dendritic to skeletal to euhedral shapes across both periclase and forsterite zones. Highly skeletal forms of forsterite preserve crystallographically-controlled internal scaffolding, reentrants and internal void boundaries. Scanning electron microscope imaging of texturally more mature forsterite crystals reveals the common occurrence of surface features such as hopper structures, ridges and terraces, features that have been linked to dendritic crystallization of olivine in igneous rocks. Near end-member forsterite crystals (> Fo99) from the middle to outer forsterite zone exhibit complex cathodoluminescence (CL) patterns which can be linked to grain-scale heterogeneities in the trace elements Li, B, P, and Ti. These grain-scale CL zoning patterns can be correlated to crystallographic forms typical of forsterite. The textural trends, morphological characteristics, surface features and CL patterns suggest that euhedral forsterite crystals grew by a process of early dendritic crystallization, hopper development, and infilling of hopper reentrants. These results caution against interpreting all grain-scale zoning profiles preserved in metamorphic forsterites as simple core-to-rim time sequences. Consideration of the possible role of dendritic crystallization in the development of forsterite morphologies is required to interpret 2-D element or isotope zoning profiles preserved in individual forsterite grains.