Unraveling the presence of multiple plagioclase populations and identification of representative two-dimensional sections using a statistical and numerical approach
Unraveling the presence of multiple plagioclase populations and identification of representative two-dimensional sections using a statistical and numerical approach
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DOI：
10.2138/am-2017-5929ccbyncnd
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发表时间：
2017-09-01
影响因子：
3.1
通讯作者：
Carniel, Roberto
中科院分区：
文献类型：
--
作者：
Cheng, Lilu;Costa, Fidel;Carniel, Roberto
许多火山岩和深成岩中的斜长石斑晶显示出相当复杂的化学分带和结构分带模式。了解晶体群的分区模式和多样性，可以为火成岩形成的过程和时间尺度提供线索。然而，除了晶体群的“真正”天然复杂性之外，斜长石类型的大量变化可以部分地是使用二维（2D）岩石薄片和三维（3D）斜长石晶体的随机切割的人工制品。因此，斜长石种群的真实数量的识别，并决定哪些是“代表性”的晶体截面用于详细的微量元素和同位素分析是不明显的，往往是主观的。在这里，我们接近这个问题的一系列数值模拟和统计分析的各种斜长石晶体分区在3D。我们分析了基于2D化学地图（例如，背散射电子图像，BSE）。我们首先分析了单晶的随机截面，然后研究了样品中不同晶体布居数的混合效应。通过量化约100个2D斜长石截面的组成直方图的相似性，可以识别代表真实的3D晶体群的所谓参考和理想截面。这些部分类型允许过滤掉随机切割的影响，并解释了给定样品的斜长石成分数据的90%以上。我们的方法可以识别的主要晶体人口和代表性的晶体，然后可以用于更强大的解释岩浆过程和时间尺度。
Many plagioclase phenocrysts from volcanic and plutonic rocks display quite complex chemical and textural zoning patterns. Understanding the zoning patterns and variety of crystal populations holds clues to the processes and timescales that lead to the formation of the igneous rocks. However, in addition to a "true" natural complexity of the crystal population, the large variety of plagioclase types can be partly artifacts of the use of two-dimensional (2D) petrographic thin sections and random cuts of three-dimensional (3D) plagioclase crystals. Thus, the identification of the true number of plagioclase populations, and the decision of which are "representative" crystal sections to be used for detailed trace element and isotope analysis is not obvious and tends to be subjective.Here we approach this problem with a series of numerical simulations and statistical analyses of a variety of plagioclase crystals zoned in 3D. We analyze the effect of increasing complexity of zoning based on 2D chemical maps (e.g., backscattered electron images, BSE). We first analyze the random sections of single crystals, and then study the effect of mixing of different crystal populations in the samples. By quantifying the similarity of the compositional histogram of about a hundred 2D plagioclase sections it is possible to identify the so-called reference and ideal sections that are representative of the real 3D crystal populations. These section types allow filtering out the random-cut effects and explain more than 90% of the plagioclase compositional data of a given sample. Our method allows the identification of the main crystal populations and representative crystals that can then be used for a more robust interpretation of magmatic processes and timescales.