变质石榴石中重稀土元素模式的热力学和动力学贡献辨析

IF 3.5 2区 地球科学 Q1 GEOLOGY Journal of Metamorphic Geology Pub Date : 2022-12-08 DOI:10.1111/jmg.12703
Matthias Konrad-Schmolke, Ralf Halama, David Chew, Céline Heuzé, Jan De Hoog, Hana Ditterova
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引用次数: 5

摘要

变质石榴石中稀土元素(REE)浓度的变化是地球深部地球动力学和地球化学过程的重要信息来源。为了提取这些信息,必须区分和量化石榴石中REE吸收的热力学平衡和动力学贡献。利用高分辨率微量元素和μ拉曼图谱,结合热力学-地球化学-扩散模型,我们证明了变质石榴石中REE吸收的平衡和动力学方面可以通过解释单个样品中的2D微量元素图谱来区分。所研究的高压蓝片岩石榴石中的重(H)REE(Tb至Lu)分带包括一个内部,从核心到内缘总体减少,然后是一个HREE富集的同心区,并向最外缘急剧减少。石榴石核心的中心峰强度随REE原子序数的减小而减小。这种模式的总体形状类似于在不同岩石类型和构造环境的变质石榴石中经常观察到的形状。叠加在这一趋势之上的是HREE浓度的微小重复波动的同心模式,沿着整个石榴石半径至少有六组规则间隔的波峰和波谷。对观察到的包裹体套件、微量元素图谱和热力学-地球化学模型的比较表明,HREE浓度降低的内部是由未改变的矿物组合中的部分石榴石生长引起的,而REE富集区是由钛酸盐的分解引起的。我们认为,中心峰的宽度由相互连接的传输基质的整体渗透率和石榴石平衡的基质矿物的分数控制。叠加的REE波动是寄主岩石元素迁移特性变化的结果,标志着石榴石中从平衡REE吸收到迁移受限REE吸收的反复变化。这种波动的元素输运特性可以用脉冲状流体通量来最好地解释,这种流体通量有节奏地改变晶间输运矩阵的互连性。越来越多已发表的空间高分辨率REE分析表明,这种微量元素波动在变质石榴石中很常见,这表明脉冲流体通量导致的岩石渗透率反复变化是变质过程中的常见现象。
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Discrimination of thermodynamic and kinetic contributions to the heavy rare earth element patterns in metamorphic garnet

Variations of rare earth element (REE) concentrations in metamorphic garnet are an important source of information of geodynamic and geochemical processes in the deeper Earth. In order to extract this information, the thermodynamic equilibrium and kinetic contributions of the REE uptake in garnet must be distinguished and quantified. Utilizing high-resolution trace element and μ-Raman mapping together with combined thermodynamic–geochemical–diffusion models, we demonstrate that the equilibrium and kinetic aspects of the REE uptake in metamorphic garnet can be discriminated by interpreting 2D trace element mapping in a single sample. The heavy (H) REE (Tb to Lu) zoning in the investigated garnet from a high-pressure blueschist comprises an inner part with an overall decrease from core to inner rim, followed by a concentric zone of HREE enrichment and a drastic HREE decrease towards the outermost rim. The central peak in the garnet core decreases in intensity with decreasing atomic number of the REE. The broad overall shape of this pattern resembles those often observed in metamorphic garnet from different rock types and tectonic settings. Superimposed on this trend is a concentric pattern of minor recurring fluctuations in the HREE concentrations with at least six regularly spaced sets of peaks and troughs along the entire garnet radius. Comparison of the observed inclusion suite, the trace element maps and thermodynamic–geochemical models show that the inner part with decreasing HREE concentrations results from fractional garnet growth in an unchanged mineral assemblage, whereas the REE enrichment zone is caused by the breakdown of titanite. We suggest that the width of the central peak is controlled by the bulk permeability of the interconnected transport matrix and the fraction of matrix minerals that the garnet equilibrates with. The superimposed REE fluctuations result from changing element transport properties of the host rock and mark recurring changes from equilibrium REE uptake to transport-limited REE uptake in garnet. Such fluctuating element transport properties can be best explained by pulse-like fluid fluxes that rhythmically change the interconnectivity of the intercrystalline transport matrix. Increasing numbers of published spatially highly resolved REE analyses show that such trace element fluctuations are common in metamorphic garnet indicating that recurring changes in rock permeabilities due to pulsed fluid fluxes are a common phenomenon during metamorphism.

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来源期刊
CiteScore
6.60
自引率
11.80%
发文量
57
审稿时长
6-12 weeks
期刊介绍: The journal, which is published nine times a year, encompasses the entire range of metamorphic studies, from the scale of the individual crystal to that of lithospheric plates, including regional studies of metamorphic terranes, modelling of metamorphic processes, microstructural and deformation studies in relation to metamorphism, geochronology and geochemistry in metamorphic systems, the experimental study of metamorphic reactions, properties of metamorphic minerals and rocks and the economic aspects of metamorphic terranes.
期刊最新文献
Issue Information Zircon Coupled Dissolution–Precipitation Replacement During Melt–Rock Interaction Modifies Chemical Signatures Resulting in Misleading Ages Pressure–Temperature–Time Evolution of a Polymetamorphic Paragneiss With Pseudomorphs After Jadeite From the HP–UHP Gneiss-Eclogite Unit of the Variscan Erzgebirge Crystalline Complex, Germany Issue Information Experimental Replacement of Zircon by Melt-Mediated Coupled Dissolution-Precipitation Causes Dispersion in U–Pb Ages
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