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Micro‐scale chemical and physical patterns in an interface of hydrothermal dolomitization reveals the governing transport mechanisms in nature: Case of the Layens anticline, Pyrenees, France
Sedimentology ( IF 3.5 ) Pub Date : 2020-10-15 , DOI: 10.1111/sed.12808
Stephen Centrella 1, 2 , Nicolas E. Beaudoin 1 , Hannelore Derluyn 1, 2 , Geoffrey Motte 1 , Guilhem Hoareau 1 , Pierre Lanari 3 , Francesca Piccoli 3 , Christophe Pecheyran 4 , Jean Paul Callot 1
Affiliation  

Hydrothermal dolomitization is an important diagenetic process that occurs in tectonic environments worldwide and forms conventional reservoirs associated with ore deposits and hydrocarbon accumulation, while forming efficient reservoirs for carbon sequestration. However, the current state of knowledge about the availability and reaction rate of Mg in dolomitizing fluids fails to explain the large volumes of hydrothermal dolomites geobodies observed in extended margins or in fold‐and‐thrust belts. To better understand this widespread phenomenon, it is essential to recognize the governing and limiting transport mode of the dolomitizing fluid. This contribution investigates the chemical and physical patterns developed between the original calcite and the newly formed dolomite. An extensive analytical study of well‐preserved dolomitization interfaces observed at outcrop scale in Callovian–Oxfordian limestones in the Layens anticline (north‐western Pyrenees, France) is presented. Through the use of scanning electron microscopy, electron backscattered diffraction, X‐ray microtomography, laser ablation inductively coupled plasma mass spectrometry (and mapping), the replacement related variations in elementary content, rock density, crystallographic properties and phase volumes and distribution were constrained. The results indicate a sequence of replacement, beginning with a fluid which starts to infiltrate the host rock by advection in the grain boundary network causing at the same time the replacement of calcite by diffusion‐limited dissolution and associated dolomite precipitation. The progressive replacement of calcite grains by dolomite is led by dissolution inside the grain enhanced by replacement related porosity creation, leading to a progressive decrease of local calcite grain size isolated as islands until the replacement is complete. The replacement of calcite by dolomite led to a mass loss without volume change, through generation of ca 11 vol.% porosity. Based on analytical observations of a natural sample, a conceptual model that accounts for the transport mode governing the different steps of hydrothermal dolomitization at crystal‐scale is proposed.

中文翻译:

热液白云石化界面中的微观化学和物理模式揭示了自然界中的主要运输机制:Layens背斜案例,法国比利牛斯

热液白云石化是一个重要的成岩过程,发生在世界范围的构造环境中,形成了与矿床和碳氢化合物积累相关的常规储层,同时形成了有效的固碳储层。但是,关于白云石化流体中镁的有效性和反应速率的当前知识状态不能解释在扩展边缘或褶皱冲断带中观察到的大量热液白云岩地质体。为了更好地理解这种普遍现象,必须认识到白云石化流体的控制和限制运输方式。该贡献调查了原始方解石与新形成的白云石之间形成的化学和物理模式。提出了在Layens背斜(法国西北比利牛斯)的卡洛夫-牛津石灰岩露头规模观察到的保存完好的白云石化界面的广泛分析研究。通过使用扫描电子显微镜,电子反向散射衍射,X射线显微断层照相术,激光烧蚀电感耦合等离子体质谱法(和作图),限制了元素含量,岩石密度,晶体学性质以及相体积和分布方面与置换有关的变化。结果表明了置换的顺序,首先是一种流体,该流体开始通过对流在晶界网络中渗透进入基质岩石,同时导致方解石被扩散受限的溶蚀和相关的白云石沉淀所替代。用白云石逐渐取代方解石晶粒是由于晶粒内部的溶解所引起的,而溶解是由于与置换相关的孔隙的产生而增强的,从而导致局部方解石晶粒尺寸的逐渐减小,这些局部方解石被孤立成小岛,直到置换完成。用白云石代替方解石会导致质量损失,而体积不会改变,孔隙率11%。基于对天然样品的分析观察,提出了一个概念模型,该模型解释了在晶体尺度上控制热液白云石化不同步骤的传输模式。
更新日期:2020-10-15
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