Mg zonation and heterogeneity in low-Mg calcite microcrystals of a depositional chalk

Abstract

ABSTRACT 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.