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首页> 外文期刊>Contributions to Mineralogy and Petrology >Compositional controls on ~(40)Ar/~(39) Ar ages of zoned mica from a rare-element pegmatite
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Compositional controls on ~(40)Ar/~(39) Ar ages of zoned mica from a rare-element pegmatite

机译：稀有伟晶岩对云母带状区〜（40）Ar /〜（39）Ar年龄的成分控制

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摘要

A high spatial resolution Ar-Ar dating study of compositionally zoned micas using UV laser extraction has been used to investigate the effect of composition and compositional boundaries upon argon diffusion in mica. The crystals are characterised by muscovite cores and Li-F-Rb-Cs-rich lepidolite rims produced by the interaction of a residual melt-vapour phase with the original muscovite during the late stages of pegmatite crystallisation. Single crystals exhibit dramatic intra-grain age variations, with a maximum range of 2,880-2,117 Ma. Backscattered electron images combined with UV-laser analyses reveal that apparent ages younger than 2,600 Ma are directly associated with the compositionally distinct rims, suggesting that Ar diffusion was faster in the lepidolite than in muscovite. Although it is not possible to be fully quantitative using the present data, it seems that argon diffusion rates in lepidolite are similar to those in biotite. Major rearrangements of the crystal structure needed to accommodate high concentrations of lithium, fluorine and large interlayer cations (Rb~+ , Gs~+ ) in muscovite account for lower argon retentivity of the crystal rims. In most cases the age profiles show no disturbance at the compositional boundary, despite the transition from a decahedral to tri-octahedral mica, indicating that in general the boundaries do not act as fast diffusion pathways. However, in some cases there is a clear drop in ages at the compositional boundary, which acts as an effective sub-grain boundary. The difference between the two types may be related to the internal structure of the compositional boundaries or their degree of contact with the grain boundary network of the rock.
机译：使用紫外激光提取技术对组成区带的云母进行了高空间分辨率的Ar-Ar约会研究，以研究组成和组成边界对氩气在云母中扩散的影响。晶体的特征在于白云母结晶后期，残留的熔融蒸气相与原始白云母的相互作用产生了白云母核和富含Li-F-Rb-Cs的锂云母岩圈。单晶具有明显的晶粒内年龄变化，最大范围为2880-2117 Ma。反向散射电子图像结合紫外激光分析表明，年龄小于2600 Ma的明显年龄与成分独特的边缘直接相关，这表明锂云母中Ar的扩散比白云母中的Ar扩散快。尽管使用目前的数据不可能完全定量，但似乎锂云母中氩气的扩散速率与黑云母中的氩气扩散速率相似。在白云母中容纳高浓度的锂，氟和大的层间阳离子（Rb〜+，Gs〜+）所需的晶体结构的重大重排导致晶体边缘的氩气保持力较低。在大多数情况下，尽管从十面体云母向三八面体云母过渡，但年龄分布在组成边界处没有显示出任何干扰，这表明边界通常不充当快速扩散途径。但是，在某些情况下，成分边界处的年龄明显下降，这是有效的次粒子边界。两种类型之间的差异可能与组成边界的内部结构或它们与岩石的晶界网络的接触程度有关。

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