Scales of equilibrium and disequilibrium during cleavage formation in chlorite and biotite‐grade phyllites, SE Vermont
Scales of equilibrium and disequilibrium during cleavage formation in chlorite and biotite‐grade phyllites, SE Vermont
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佛蒙特州东南部绿泥石和黑云母级千枚岩中解理形成过程中的平衡和不平衡尺度
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发表时间：
2007
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通讯作者：
M. Kunk
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作者：
C. McWilliams;R. Wintsch;M. Kunk
对佛蒙特州东南部具皱纹的千叶岩中的层状硅酸盐进行了详细的电子探针分析，结果表明，颗粒尺度的分带很常见，相邻矿物中的共生分带几乎是普遍的。我们将其解释为反映了解理发展的压力-溶解机制，从一个非常小的流体储集层中的降水使该流体分馏。沿单个白云母、黑云母和绿泥石(长30-200 μm)的多次分析表明，白云母中的硅质晶置换和黑云母和绿泥石中的双八面体和双八面体置换都表明了白云母中的硅质晶取代作用。利用横切关系和矿物化学研究表明，这些模式在绿泥石级、绿泥石级叠加黑云母级和黑云母级变质条件下产生的解理中持续存在。在所有三种环境中，分带模式都是可比较的，需要类似的解理形成机制，而不依赖于变质级别。此外，40Ar/39Ar年代学的使用表明，无论年龄如何，这一点都是正确的。此外，绿泥石级解理被黑云母斑岩覆盖的样品表明，铝、硅、镁和铁离子扩散的闭合温度高于黑云母等轴线的闭合温度(>；∼400 °C)。成分图上白云母-绿泥石、白云母-黑云母对形成的平行而平缓的扇形连线显示了有效的瞬时化学平衡，可能指示了同时结晶。
Detailed electron microprobe analyses of phyllosilicates in crenulated phyllites from south‐eastern Vermont show that grain‐scale zoning is common, and sympathetic zoning in adjacent minerals is nearly universal. We interpret this to reflect a pressure‐solution mechanism for cleavage development, where precipitation from a very small fluid reservoir fractionated that fluid. Multiple analyses along single muscovite, biotite and chlorite grains (30–200 μm in length) show zoning patterns indicating Tschermakitic substitutions in muscovite and both Tschermakitic and di/trioctahedral substitutions in biotite and chlorite. Using cross‐cutting relationships and mineral chemistry it is shown that these patterns persist in cleavages produced at metamorphic conditions of chlorite‐grade, chlorite‐grade overprinted by biotite‐grade and biotite‐grade. Zoning patterns are comparable in all three settings, requiring a similar cleavage‐forming mechanism independent of metamorphic grade. Moreover, the use of 40Ar/39Ar geochronology demonstrates this is true regardless of age. Furthermore, samples with chlorite‐grade cleavages overprinted by biotite porphyroblasts suggest the closure temperatures for the diffusion of Al, Si, Mg and Fe ions are greater than the temperature of the biotite isograd (>∼400 °C). Parallel and smoothly fanning tie lines produced by coexisting muscovite–chlorite, and muscovite–biotite pairs on compositional diagrams demonstrate effectively instantaneous chemical equilibrium and probably indicate simultaneous crystallization.