Progressive evolution of whole-rock composition during metamorphism revealed by multivariate statistical analyses
Progressive evolution of whole-rock composition during metamorphism revealed by multivariate statistical analyses
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多元统计分析揭示变质作用过程中全岩成分的渐进演化
DOI：
10.1111/jmg.12282
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发表时间：
2017
期刊：
影响因子：
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通讯作者：
Akaho Shotaro
中科院分区：
文献类型：
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作者：
Yoshida Kenta;Kuwatani Tatsu;Hirajima Takao;Iwamori Hikaru;Akaho Shotaro
基于多元统计分析描述了俯冲相关变质作用过程中变质岩的地球化学演化。研究的数据集包括一系列从日本四国中部三波川变质带收集的变质岩地图，该变质岩条件范围从泵辉石-阳起石相到绿帘石-角闪岩相。计算和信息科学的最新进展提供了许多能够揭示大型数据集中结构的算法。本研究将k均值聚类分析（KCA）和非负矩阵分解（NMF）应用于一系列变泥岩，这是三八川变质带的主要岩型。 KCA 描述高维数据的结构，而 NMF 提供端元分解，可用于评估连续成分趋势的空间分布。分析的数据集源自之前发表的工作，包含 296 个样品，其中分析了 14 种元素（Si、Ti、Al、Fe、Mn、Mg、Ca、Na、K、P、Rb、Sr、Zr 和 Ba）。 KCA 和 NMF 分析分别表明五个簇和四个末端成员，成功解释了数据集中的成分变化。 KCA 表明，采样区域西部（别子）的变泥岩样品的化学成分与东部（阿塞米川）的显着不同。在西部，团簇与变质程度表现出良好的相关性。随着变质程度的增加，SiO2和Na2O含量减少，其他成分增加。但东部地区成分随变质程度的变化不太明显。使用 NMF 进行端元分解表明，与变质程度相关的全岩石成分的演化变化近似于全岩石成分中石榴石类成分的化学计量增加，这可能是由于石榴石的沉淀和渐进脱水过程中其他成分的渗出所致。全岩成分演化的热力学模型得到了以下结果：（1）在石榴石带条件下，较低变质等级的全岩成分有利于石榴石的优先结晶，在相同的P-T条件下，黑云母与高品位岩石成分中的石榴石一起变得稳定； (2) 随着全岩成分的提高，保留了更多的 H2O。这些结果提供了对高变质条件下抑制脱水机制的深入了解。尽管尚未开发出这样的定量模型，但在俯冲带流体循环的正演模型中应考虑这种机制。
The geochemical evolution of metamorphic rocks during subduction‐related metamorphism is described on the basis of multivariate statistical analyses. The studied data set comprises a series of mapped metamorphic rocks collected from the Sanbagawa metamorphic belt in central Shikoku, Japan, where metamorphic conditions range from the pumpellyite–actinolite to epidote–amphibolite facies. Recent progress in computational and information science provides a number of algorithms capable of revealing structures in large data sets. This study appliesk‐means cluster analysis (KCA) and non‐negative matrix factorization (NMF) to a series of metapelites, which is the main lithotype of the Sanbagawa metamorphic belt. KCA describes the structures of the high‐dimensional data, while NMF provides end‐member decomposition which can be useful for evaluating the spatial distribution of continuous compositional trends. The analysed data set, derived from previously published work, contains 296 samples for which 14 elements (Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K, P, Rb, Sr, Zr and Ba) have been analysed. The KCA and NMF analyses indicate five clusters and four end‐members, respectively, successfully explaining compositional variations within the data set. KCA indicates that the chemical compositions of metapelite samples from the western (Besshi) part of the sampled area differ significantly from those in the east (Asemigawa). In the west, clusters show a good correlation with the metamorphic grade. With increasing metamorphic grade, there are decreases in SiO2and Na2O and increases in other components. However, the compositional change with metamorphic grade is less obvious in the eastern area. End‐member decomposition using NMF revealed that the evolutional change of whole‐rock composition, as correlated with metamorphic grade, approximates a stoichiometric increase of a garnet‐like component in the whole‐rock composition, possibly due to the precipitation of garnet and effusion of other components during progressive dehydration. Thermodynamic modelling of the evolution of the whole‐rock composition yielded the following results: (1) the whole‐rock composition at lower metamorphic grade favours the preferential crystallization of garnet under the conditions of the garnet zone, with biotite becoming stable together with garnet in higher‐grade rock compositions under the sameP–Tconditions; (2) with higher‐grade whole‐rock compositions, more H2O is retained. These results provide insight into the mechanism suppressing dehydration under high‐Pmetamorphic conditions. This mechanism should be considered in forward modelling of the fluid cycle in subduction zones, although such a quantitative model has yet to be developed.