Hydrothermal replacement of biogenic and abiogenic aragonite by Mg-carbonates - Relation between textural control on effective element fluxes and resulting carbonate phase
Hydrothermal replacement of biogenic and abiogenic aragonite by Mg-carbonates - Relation between textural control on effective element fluxes and resulting carbonate phase
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DOI：
10.1016/j.gca.2016.09.034
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发表时间：
2017-01-01
影响因子：
5
通讯作者：
Putlitz, Benita
中科院分区：
文献类型：
--
作者：
Jonas, Laura;Mueller, Thomas;Putlitz, Benita
白云化作用是白云岩（CaMg[CO3](2)）对方解石或文石（CaCO3）的二次替代作用，是最具体积意义的碳酸盐岩成岩作用之一。它发生在近地表和浅埋条件下，可以通过改变孔隙度和渗透率来显著改变岩石的性质。白云化前缘与流体通道直接耦合，这可能与前驱灰岩的初始孔隙度/渗透率、现有断层网络或替代反应产生的次生孔隙度/渗透率有关。本研究在热液条件下，实验研究了白云化过程中典型的前驱相镁碳酸盐对生物源文石和非生物源文石替代的结构控制。文石样品具有不同的纹理和微观结构特性，表现为致密（无机文石单晶）；中间产物（北极岛的双壳）和开放多孔结构（珊瑚Porites sp.的骨架）在200℃下与0.9 M MgCl2和0.015 M SrCl2溶液反应，文石被含钙菱镁矿和非化学量白云岩组成的Mg-Ca碳酸盐取代，通过溶解沉淀过程形成多孔反应前沿，逐渐取代文石前体。该反应导致反应前沿出现孔隙，并在反应界面处形成间隙和空洞等独特的微观结构。新形成的反应环由化学上不同的相组成，由清晰的边界隔开。结果表明，随着初始孔隙率和反应表面积的增加，相的数量及其化学变化量逐渐减少。这种观察结果可以用有效元素通量的变化来解释，这种变化导致反应边缘孔隙空间内流体的化学梯度不同。所观察到的反应速率在替换最初高度多孔的珊瑚时最高，而在替换单个文石晶体的致密结构时最低。因此，反应过程同样取决于反应界面处的流体与测试材料周围的体溶液之间的有效元素通量以及反应表面积。本研究表明，母材的织构和微观结构性能对产物相的化学成分有显著影响。此外，我们的数据强调了反应界面流体与局部微观结构控制的体溶液之间有效的流体介导元素交换的重要性。(C) 2016 Elsevier Ltd.版权所有。
Dolomitization, i.e., the secondary replacement of calcite or aragonite (CaCO3) by dolomite (CaMg[CO3](2)), is one of the most volumetrically important carbonate diagenetic processes. It occurs under near surface and shallow burial conditions and can significantly modify rock properties through changes in porosity and permeability. Dolomitization fronts are directly coupled to fluid pathways, which may be related to the initial porosity/permeability of the precursor limestone, an existing fault network or secondary porosity/permeability created through the replacement reaction. In this study, the textural control on the replacement of biogenic and abiogenic aragonite by Mg-carbonates, that are typical precursor phases in the dolomitization process, was experimentally studied under hydrothermal conditions. Aragonite samples with different textural and microstructural properties exhibiting a compact (inorganic aragonite single crystal), an intermediate (bivalve shell of Arctica islandica) and open porous structure (skeleton of coral Porites sp.) were reacted with a solution of 0.9 M MgCl2 and 0.015 M SrCl2 at 200 degrees C. The replacement of aragonite by a Ca-bearing magnesite and a Mg-Ca carbonate of non-stoichiometric dolomitic composition takes place via a dissolution-precipitation process and leads to the formation of a porous reaction front that progressively replaces the aragonite precursor. The reaction leads to the development of porosity within the reaction front and distinctive microstructures such as gaps and cavities at the reaction interface. The newly formed reaction rim consists of chemically distinct phases separated by sharp boundaries. It was found that the number of phases and their chemical variation decreases with increasing initial porosity and reactive surface area. This observation is explained by variations in effective element fluxes that result in differential chemical gradients in the fluid within the pore space of the reaction rim. Observed reaction rates are highest for the replacement of the initially highly porous coral and lowest for the compact structure of a single aragonite crystal. Therefore, the reaction progress equally depends on effective element fluxes between the fluid at the reaction interface and the bulk solution surrounding the test material as well as the reactive surface area. This study demonstrates that the textural and microstructural properties of the parent material have a significant influence on the chemical composition of the product phase. Moreover, our data highlight the importance of effective fluid-mediated element exchange between the fluid at the reaction interface and the bulk solution controlled by the local microstructure. (C) 2016 Elsevier Ltd. All rights reserved.