USING LABORATORY EXPERIMENTS TO INVESTIGATE THE CONTROLS ON LMC MICROCRYSTAL TEXTURES

Phanerozoic marine limestones are frequently comprised of diagenetic low-magnesium calcite (LMC) microcrystals. It has been shown that these microcrystals generally fit into one of six textural classes, which strongly correlate with porosity and permeability. Diagenetic LMC microcrystals also exhibit different elemental and isotopic signatures than their carbonate sediment counterparts. Numerous case studies have reported on the occurrence of natural LMC microcrystals, but little attention has been given to their genesis. A few experimental studies also exist, but they generally lack a systematic approach. As a result, the fundamental controls on LMC microcrystal texture are largely unknown. Here, findings are presented from a series of aragonite stabilization experiments designed to systematically investigate the controls on LMC microcrystal texture. In an experiment to evaluate reaction temperature, 100 mg of pulverized aragonite was reacted with 15 ml of distilled water at 70, 100, 150, or 215 °C. Despite the large temperature range, all products exhibit the granular-euhedral texture, comparable to what is observed in ancient limestones. Reaction rates increase with reaction temperature whereas average crystal sizes generally decrease. To evaluate reaction fluid chemistry, aragonite was transformed to LMC at 100 °C in synthetic solutions including CaCl₂, synthetic seawater, and synthetic seawater with reduced Mg/Ca ratios. In contrast to the distilled water, experiments in the CaCl₂ solution produced polyhedral crystals similar to the “rounded” crystal forms observed in nature and previously cited as evidence of dissolution. No LMC was observed after 7 months in seawater and modified seawater solutions, which supports the previous suggestion that magnesium inhibits the conversion of aragonite to LMC. To evaluate the effects of reactant texture, experiments conducted on crushed aragonite from single crystals, modern ooids, and Scleractinian corals. All products exhibited bimodality in crystal size. Ooids and corals transformed to polyhedral LMC crystals, whereas the single crystal aragonite yielded granular-euhedral LMC crystals. In the ooid and coral experiments, smaller crystals are more rounded compared to larger, euhedral crystals.