Abstract The formation mechanism of dolomite (CaMg(CO3)2), especially primary dolomite that precipitates directly from solution, has been challenging to decipher. The kinetic barrier of dehydrating Mg2+-water complex at Earth’s surface temperatures has hindered laboratory synthesis in the past and compromised the understanding of dolomite formation. Recent advances in room temperature laboratory synthesis of dolomite have established that high concentration of dissolved silica could promote precipitation of disordered dolomite, a precursor phase for sedimentary dolomite. While silica appears to address many of the temporal and spatial distributions of dolomite through geologic time, the origin of micritic dolomite in deep-time sedimentary records and the effect of silica on sedimentary dolomite during diagenesis remains largely unclear. This work aims to understand the effect of dissolved silica on carbonate recrystallization process during early diagenesis through laboratory annealing experiments combined with synchrotron-based high resolution powder X-ray diffraction (XRD), as well as validating our hypothesis by examining the micritic Early Silurian dolomite from Byron Formation. Our annealing experiments of calcite, dolomite and high magnesium calcite (HMC) seeds demonstrate that the presence of dissolved silica alters the carbonate surface free energy, likely through adsorbed and incorporated silica, and substantially inhibits carbonate crystal growth during recrystallization. Our results also show that lower temperatures and higher silica concentrations lead to reduced crystal growth rate. This suggests the dissolved silica-rich paleo-seawater, prior to the Mesozoic, restricted the crystallite size increase during early diagenesis and the crystal size are determined by the dissolved silica concentration during deposition. The oscillations of submillimeter lamellae in the early Silurian dolomites results from variation of dolomite crystal sizes rather than changes in mineral assemblage. Substantial silica (200 ppm) is associated with the micritic Early Silurian dolomite, and dolomite crystal sizes show an inverse correlation with silica content. The oscillating crystal size in the dolomite point to local dissolved silica variation during dolomite precipitation. This inhibition effect of the adsorbed silica on carbonate growth provides a new explanation for the well-preserved Paleozoic and Precambrian micritic dolomite. These findings also imply that the retention of carbonate rocks on silica during formation and diagenesis has an overlooked effect on the silica cycle.
Fang et al. (Wed,) studied this question.