Chemical zoning in the outer few 10s of microns of garnet porphyroblasts has been investigated to assess the scale of chemical equilibrium with matrix minerals in a pelitic schist. Garnet porphyroblasts from the Late Proterozoic amphibolite‐facies regional metamorphic mica schists from Glen Roy in the Scottish Highlands contain typical prograde growth zoning patterns. Edge compositions have been measured via a combination of analysis of traverses across the planar edges of porphyroblast surfaces coupled to X‐ray mapping of small areas within polished thin sections at the immediate edge of the porphyroblasts. These approaches reveal local variation in garnet composition, especially of grossular (Ca) and almandine (Fe) components, with a range at the edge from 16 mol% grs, across distances of less then 50 mm. This small‐scale patchy compositional zoning is as much variation as the core‐rim compositional zoning across the whole of a 3 mm porphyroblast. Ca and Fe heterogeneity occurs on a scale suggesting a combination of inefficient diffusive exchange across grain boundaries during prograde growth and the evolving microtopography of the porphyroblast surface control garnet composition. The latter creates haloes of compositional zoning adjacent to some inclusions, which typically extend from the inclusion towards the porphyroblast edge during further growth. The lack of a consistent equilibrium composition at the garnet edge is also apparent in the internal zoning of the porphyroblast and so processes occurring during entrapment of some mineral inclusions have a profound influence on the overall chemical zoning. Garnet compositions and associated zoning patterns are widely used by petrologists to reconstruct P‐T‐t paths for crustal rocks. The evidence of extremely localized (10‐50 mm scale) equilibrium during growth further undermines these approaches.

中文翻译：

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石榴石卟啉细胞的生长分区：晶界和微地形控制

已经研究了石榴石卟啉细胞外部几十微米的化学分区，以评估泥质片岩中基质矿物的化学平衡规模。来自苏格兰高地格伦罗伊的晚元古代角闪岩相区域变质云母片岩的石榴石卟啉母细胞包含典型的顺行生长分区模式。边缘成分的测量是通过综合分析横贯斑状细胞表面的平面边缘，结合 X 射线映射在斑状细胞的直接边缘抛光薄片内的小区域。这些方法揭示了石榴石成分的局部变化，尤其是粗粒 (Ca) 和铝镁石 (Fe) 成分，边缘范围为 16 mol% grs，距离小于 50 mm。这种小规模的斑片状成分区带与整个 3 毫米斑状细胞的核心-边缘成分区带一样多。Ca 和 Fe 异质性发生在一定范围内，表明在顺行生长过程中跨晶界的低效扩散交换和卟啉母细胞表面控制石榴石成分的演变微地形相结合。后者会在某些包裹体附近产生组成分区的晕圈，这些晕圈通常在进一步生长过程中从包裹体延伸到成卟啉边缘。石榴石边缘缺乏一致的平衡成分在卟啉母细胞的内部分带中也很明显，因此在捕获某些矿物包裹体期间发生的过程对整体化学分带具有深远的影响。石榴石成分和相关的分区模式被岩石学家广泛用于重建地壳岩石的 P-T-t 路径。生长过程中极端局部（10-50 毫米尺度）平衡的证据进一步破坏了这些方法。