Mg zonation and heterogeneity in low-Mg calcite microcrystals of a depositional chalk
Mg zonation and heterogeneity in low-Mg calcite microcrystals of a depositional chalk
复制标题
DOI：
10.2110/jsr.2020.173
复制
发表时间：
2021-08
影响因子：
2
通讯作者：
Chanse J. Rinderknecht;Franek Hasiuk;Stephan C. Oborny
中科院分区：
文献类型：
--
作者：
Chanse J. Rinderknecht;Franek Hasiuk;Stephan C. Oborny
成岩低镁方解石（LMC）微晶构成了石灰石储层中大多数微孔的框架，因此创造了碳氢化合物、水和人为二氧化碳的储存能力。以LMC微晶为主的灰岩也常用于古气候重建和化学地层对比。众所周知，LMC微晶体表现出一系列的结构（例如，颗粒状，贴合，簇状），但关于这些结构如何形成存在不确定性。一种假设是，在晶体生长过程中，镁被掺入成岩过度生长（胶结）中，其中的化学分带和显微结构可能反映了成岩过程。为了评价LMC微晶的小尺度地球化学分带性，本研究利用扫描电镜（SEM）和能谱仪（EDS）测量了来自Tor Fm晚白垩世海相白垩粉的LMC颗粒微晶内部的Mg/Ca比值。（挪威北海）。直径为bbb50 μm的LMC微晶体均存在Mg/Ca带化现象。一般来说，大晶体的核心Mg/Ca较低（≈5.9 mmol/mol），而边缘Mg/Ca较高（≈13 mmol/mol）。较小的微晶（< 5 μm）没有明显的分带性，但Mg/Ca含量在2.9 ~ 32.2 mmol/mol范围内。测量到的Mg/Ca值被任意分为低Mg（平均≈5.9 mmol/mol）、中Mg（平均≈13.3 mmol/mol）和高Mg（平均≈20 mmol/mol）三个种群。LMC微晶中的分带和Mg富集被解释为反映了沉积和多种成岩信号，如通过再结晶和孔隙填充胶结作用产生的构造沉淀。尽管粉笔来自Tor Fm。以粒状自面体LMC微晶为主，利用SEM-EDS发现其他LMC微晶结构中Mg/Ca的非均质性，可能有助于深入了解微孔为主的灰岩中产生结构变化的成岩过程。Mg数据还更广泛地表明，在被认为是均匀的材料中存在有用的、可测量的成岩信息。区分可能的原生组成和次生胶结作用，对依赖于细粒碳酸盐矿床（如泥晶）原生化学的研究具有重要意义，如古气候学、镁古温度学和化学地层学。
Diagenetic low-magnesium calcite (LMC) microcrystals constitute the framework that hosts most micropores in limestone reservoirs and therefore create the storage capacity for hydrocarbons, water, and anthropogenic CO2. Limestones dominated by LMC microcrystals are also commonly used for paleoclimate reconstructions and chemostratigraphic correlations. LMC microcrystals are well known to exhibit a range of textures (e.g., granular, fitted, clustered), but there exists uncertainty with regard to how these textures form. One hypothesis is that during crystal growth, Mg is incorporated into diagenetic overgrowths (cement), where the chemical zonation and microtexture may reflect diagenetic processes. To evaluate small-scale geochemical zonation in LMC microcrystals, this study uses scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) to measure the Mg/Ca ratio across the interiors of LMC granular microcrystals from a Late Cretaceous marine chalk from the Tor Fm. (Norwegian North Sea). Mg/Ca zonation was identified in all LMC microcrystals with a diameter > 5 μm. Generally, the cores of large crystals have lower Mg/Ca (≈ 5.9 mmol/mol) and the rims have elevated Mg/Ca (≈ 13 mmol/mol). Smaller microcrystals (< 5 μm) show no resolvable zonation, but do exhibit a wide range in Mg/Ca content from 2.9 to 32.2 mmol/mol. Measured Mg/Ca values are arbitrarily divided into three populations: low Mg (average ≈ 5.9 mmol/mol), intermediate Mg (average ≈ 13.3 mmol/mol), and high Mg (average ≈ 20 mmol/mol). The observed zonation and Mg enrichment within LMC microcrystals is interpreted to reflect depositional as well as multiple diagenetic signals, such as constructive precipitation through recrystallization and pore-filling cementation. Although chalk from the Tor Fm. is dominated by granular euhedral LMC microcrystals, using SEM-EDS to find Mg/Ca heterogeneity in other LMC microcrystal textures may provide insight into the diagenetic processes that create textural variations in micropore-dominated limestones. The Mg data also more broadly suggest that there is useful, measurable diagenetic information in material that is otherwise considered homogeneous. Distinguishing between possible primary compositions and secondary cementation has implications for studies that rely on the primary chemistry of fine-grained carbonate deposits (e.g., micrite), such as paleoclimatology, Mg paleothermometry, and chemostratigraphy.