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.

中文翻译：

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犹他州 Alta stock aureole 硅质白云岩接触变质过程中镁橄榄石枝晶结晶的证据

对犹他州 Alta 硅质白云岩变质过程中产生的镁橄榄石晶体进行了原位研究，并使用晶体分离物来限制其光环尺度的结构和形态变异性。随着接触的接近，镁橄榄石晶体的丰度增加，但平均尺寸减小，表明对镁橄榄石成核和内部光环中的生长有更大的动力学影响。晶体形态在方镁石和镁橄榄石区域中从树枝状到骨架再到自形形状变化很大。镁橄榄石的高度骨架形式保留了晶体学控制的内部支架、折返体和内部空隙边界。结构更成熟镁橄榄石晶体的扫描电子显微镜成像揭示了常见的表面特征，如漏斗结构、山脊和阶地，这些特征与火成岩中橄榄石的树枝状结晶有关。从中间到外部镁橄榄石区域的近端元镁橄榄石晶体 (> Fo99) 表现出复杂的阴极发光 (CL) 模式，这可能与微量元素 Li、B、P 和 Ti 的晶粒尺度异质性有关。这些晶粒尺度的 CL 分区模式可以与镁橄榄石的典型结晶形式相关联。纹理趋势、形态特征、表面特征和 CL 图案表明自形镁橄榄石晶体通过早期树枝状结晶、料斗发育和料斗折返物的填充过程生长。这些结果警告不要将变质镁橄榄石中保存的所有晶粒尺度分区剖面解释为简单的核到边缘时间序列。