The role of grain boundaries and transient porosity in rocks as fluid pathways for reaction front propagation
The role of grain boundaries and transient porosity in rocks as fluid pathways for reaction front propagation
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DOI：
10.1016/j.epsl.2013.10.050
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发表时间：
2013-04
影响因子：
5.3
通讯作者：
Laura Jonas;T. John;H. King;T. Geisler;A. Putnis
中科院分区：
文献类型：
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作者：
Laura Jonas;T. John;H. King;T. Geisler;A. Putnis
以磷酸钙对卡拉拉大理岩的假晶置换作用为模型体系，研究了不同流体通道对流体-岩石相互作用引起的反应前沿扩展的影响。在该模型中，岩石中存在的晶界以及在整个置换反应中发展的瞬态孔隙结构使得反应前沿能够进一步进展到岩石中以及每个单个颗粒的中心，直到转化完成。使用含磷酸盐溶液对大理石进行水热处理导致羟基磷灰石和β-TCP的形成; β-TCP相的形成可能由0.6wt.%的Na 2SO 4的存在促进。镁在母体碳酸盐相中。完全转化的单个颗粒在成分和结构上都表现出明显的分带性。而靠近晶界的区域由几乎纯的羟基磷灰石组成并显示出粗糙的孔隙度，靠近单个晶粒中心的区域显示出大量的β-TCP和非常细的多孔显微结构。如果氟作为另外的溶液组分加入，则最多3重量%的氟可以作为另外的溶液组分。F掺入到产物磷灰石中，避免了β-TCP的生成。使用同位素18 O作为置换反应的计时器，可以重建磷酸钙反应前沿的时间发展。拉曼分析表明，18 O掺入到羟基磷灰石的PO 4四面体中，导致垂直于大理石晶界的磷酸钙反应前沿出现明显的轮廓。通过使用18 O计时器，可以估计和比较该模型系统中不同流体路径的时间效率。结果表明，晶界是一个有效的途径，使流体渗透到岩石比新开发的通道状孔隙结构，作为单一矿物颗粒的中心的路径相比，一个以上的数量级更快。因此，在仅短的反应持续时间之后，流体可以沿着晶界前进相对大的距离沿着，而不会沿着路径沿着形成宽的反应前沿。
The pseudomorphic replacement of Carrara marble by calcium phosphates was used as a model system in order to study the influence of different fluid pathways for reaction front propagation induced by fluid–rock interaction. In this model, grain boundaries present in the rock as well as the transient porosity structures developing throughout the replacement reaction enable the reaction front to progress further into the rock as well as to the center of each single grain until transformation is complete. Hydrothermal treatment of the marble using phosphate bearing solutions led to the formation of hydroxylapatite and β-TCP; the formation of the latter phase was probably promoted by the presence of∼ 0.6 wt.% Mg in the parent carbonate phase. Completely transformed single grains show a distinctive zoning, both in composition and texture. Whereas areas next to the grain boundary consist of nearly pure hydroxylapatite and show a coarse porosity, areas close to the center of the single grains show a high amount of β-TCP and a very fine porous microstructure. If fluorine was added as an additional solution component, up to 3 wt.% of F were incorporated into the product apatite and the formation of β-TCP was avoided. The use of the isotope 18 O as a chronometer for the replacement reaction makes it possible to reconstruct the chronological development of the calcium phosphate reaction front. Raman analysis revealed that the incorporation of 18 O in the PO 4 tetrahedron of hydroxylapatite results in the development of distinct profiles in the calcium phosphate reaction front perpendicular to the grain boundaries of the marble. Through the use of the 18 O chronometer, it is possible to estimate and compare the time effectiveness of the different fluid pathways in this model system. The results demonstrate that the grain boundaries are an effective pathway enabling the fluid to penetrate the rock more than one order of magnitude faster compared to the newly developing channel-like porosity structures, which act as pathways towards the center of single mineral grains. Thus, after only short reaction durations, it may be possible for the fluid to progress relatively large distances along the grain boundaries without developing broad reaction fronts along the path.