Deformation-induced graphitization and muscovite recrystallization in a ductile fault zone

IF 3.5 2区 地球科学 Q1 GEOLOGY Journal of Metamorphic Geology Pub Date : 2024-02-14 DOI:10.1111/jmg.12763
M. Rebecca Stokes, Aaron M. Jubb, Ryan J. McAleer, David L. Bish, Robert P. Wintsch
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Abstract

A suite of slate samples collected along a 2 km transect crossing the Lishan fault in central Taiwan were evaluated to assess the role of ductile deformation in natural graphitization at lower greenschist facies metamorphic conditions. The process of natural aromatization, or graphitization, of an organic precursor is well established as a thermally driven process; however, experimental studies have shown that the energy provided by deformation can substantially reduce the activation energy required for graphitization. This study provides a natural example of deformation-induced graphitization. A strain gradient approaching the Lishan fault was established by scanning electron microscope imaging and X-ray diffraction analysis of phyllosilicates and quartz that showed an increase in the strength of slaty cleavage development via dissolution-precipitation processes. Thermal conditions were constrained to be near isothermal using calcite-dolomite geothermometry. Raman spectroscopic results from carbonaceous material, including D1-full width-at-half-maximum (FWHM), G-FWHM, Raman band separation (RBS), and a lesser-known vibrational mode B2g-FWHM, showed robust linear trends across the same sampling transect. However, the G-FWHM parameter showed a trend opposite of that expected from thermally driven graphitization. The Raman results are interpreted to reflect a strain-driven reduction in graphite crystallite size (decrease in G-FWHM) but improvement in structural ordering in individual coherent domains. A multiple linear regression with an R2 value of 0.92 predicts the graphite D1-FWHM values from the XRD-derived ratio of muscovite populations and muscovite microstrain, demonstrating the concomitant recrystallization of silicates and carbonaceous material across the strain gradient, despite the disparate processes accommodating the deformation. This study demonstrates the role of deformation in natural graphitization and provides a new perspective on the use of graphite as a geothermometer in strongly deformed greenschist facies rocks.

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韧性断层带中形变诱导的石墨化和黝帘石重结晶
为了评估韧性变形在低绿岩面变质条件下天然石墨化过程中的作用,我们对沿台湾中部里山断层2公里横断面采集的一系列板岩样本进行了评估。有机前驱体的天然芳香化或石墨化过程是一个热驱动过程,这一点已得到公认;然而,实验研究表明,变形提供的能量可大大降低石墨化所需的活化能。本研究提供了一个变形诱导石墨化的自然实例。通过对植硅酸盐和石英进行扫描电子显微镜成像和 X 射线衍射分析,确定了接近里山断层的应变梯度,该梯度显示,通过溶解沉淀过程,板状裂隙发育的强度有所提高。通过方解石-白云石地温测定法,热条件被确定为接近等温。碳质材料的拉曼光谱结果,包括 D1-全宽-半最大值(FWHM)、G-FWHM、拉曼光谱带分离(RBS)和一种鲜为人知的振动模式 B2g-FWHM,在同一采样断面上显示出强劲的线性趋势。但是,G-FWHM 参数的变化趋势与热驱动石墨化的预期趋势相反。拉曼结果被解释为反映了应变驱动的石墨晶体尺寸减小(G-FWHM 下降),但单个相干畴的结构有序性得到改善。根据 XRD 导出的黝帘石种群比率和黝帘石微应变,以 R2 值为 0.92 的多元线性回归预测了石墨 D1-FWHM 值,表明尽管变形过程各不相同,但硅酸盐和碳质材料在整个应变梯度上同时发生了再结晶。这项研究证明了变形在天然石墨化中的作用,并为将石墨用作强烈变形绿岩面岩石的地质温度计提供了新的视角。
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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.
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