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.

中文翻译：

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沉积白垩低镁方解石微晶中的镁分带和非均质性

成岩低镁方解石 (LMC) 微晶构成了石灰岩储层中大多数微孔的骨架，因此为碳氢化合物、水和人为 CO2 创造了储存能力。以 LMC 微晶为主的石灰岩也常用于古气候重建和化学地层对比。众所周知，LMC 微晶具有一系列纹理（例如，粒状、拟合、簇状），但关于这些纹理如何形成存在不确定性。一种假设是，在晶体生长过程中，镁被纳入成岩过度生长（水泥），其中化学分带和微观结构可能反映成岩过程。为了评估 LMC 微晶中的小尺度地球化学分带，本研究使用扫描电子显微镜 (SEM) 和能量色散光谱 (EDS) 来测量来自 Tor Fm 的晚白垩世海洋白垩的 LMC 颗粒微晶内部的 Mg/Ca 比率。（挪威北海）。在所有直径 > 5 μm 的 LMC 微晶中都发现了 Mg/Ca 分带。通常，大晶体的核心具有较低的 Mg/Ca (≈ 5.9 mmol/mol)，边缘具有升高的 Mg/Ca (≈ 13 mmol/mol)。较小的微晶 (< 5 μm) 没有显示可分辨的分区，但确实表现出 2.9 至 32.2 mmol/mol 的 Mg/Ca 含量范围。测量的 Mg/Ca 值被任意分为三类：低 Mg（平均 ≈ 5.9 mmol/mol）、中等 Mg（平均 ≈ 13.3 mmol/mol）和高 Mg（平均 ≈ 20 mmol/mol）。在 LMC 微晶内观察到的分带和 Mg 富集被解释为反映沉积以及多种成岩信号，例如通过再结晶和孔隙填充胶结形成的建设性沉淀。尽管来自 Tor Fm 的白垩。以粒状自形 LMC 微晶为主，使用 SEM-EDS 发现其他 LMC 微晶结构中的 Mg/Ca 异质性可以深入了解在微孔为主的石灰岩中产生结构变化的成岩过程。镁数据还更广泛地表明，在其他情况下被认为是均质的材料中存在有用的、可测量的成岩信息。区分可能的主要成分和次生胶结作用对依赖于细粒碳酸盐沉积物（例如泥晶）的主要化学性质的研究具有影响，