RHOMBIC CALCITE MICROCRYSTALS AS A PROXY FOR METEORIC DIAGENESIS

Numerous Phanerozoic limestones are commonly comprised of calcite microcrystals that measure < 10 µm and exhibit various morphologies. These microcrystals have been interpreted to form during mineralogical stabilization of metastable carbonate sediments during diagenesis. Previous laboratory experiments show that calcite microcrystals precipitating under conditions similar to those that characterize meteoric diagenetic settings (impurity-free, low degree of supersaturation with respect to calcite, high fluid:solid ratio) exhibit the rhombic morphology, whereas calcite microcrystals precipitating under conditions similar to those that prevail in marine and marine burial diagenetic settings (impurity-rich, high degree of supersaturation, low fluid:solid ratio) exhibit non-rhombic morphologies. Based on these experimental observations, we propose that rhombic calcite microcrystals form exclusively in meteoric environments. This hypothesis is tested using new and previously published textural and geochemical data from the Phanerozoic rock record. The data show that the vast majority of Phanerozoic limestones characterized by rhombic microcrystals also exhibit petrographic and/or geochemical evidence indicative of meteoric diagenesis whereas non-rhombic morphologies are associated with marine burial conditions. By linking calcite microcrystal textures to specific diagenetic environments, our observations bring clarity to the conditions under which the various microcrystal textures form and imply that the rhombic microcrystal morphology may be a useful textural proxy for meteoric diagenesis.