Evaporation of Neoproterozoic seawater drives amorphous calcium-magnesium carbonate production

• PO₄³⁻ in seawater inhibits calcite and aragonite precipitation during evaporation. • Amorphous calcium-magnesium carbonate (ACMC) saturation reached at ∼60 % evaporation. • ACMC detected in precipitates; composition depended on initial Ca/Mg in seawater. • ACMC transformed into monohydrocalcite and hydrous Mg carbonates. • Evaporation could produce widespread precursors for enigmatic carbonate fabrics. Non-classical crystallisation pathways involving metastable intermediates, such as amorphous calcium magnesium carbonate (ACMC), have been proposed as an explanation for enigmatic fabrics and minerals in the Precambrian carbonate rock record. However, it is uncertain how these metastable phases could have formed in natural environments. This study hypothesises that, if the precipitation of calcite and aragonite is inhibited, evaporating seawater can reach sufficient carbonate saturation to precipitate ACMC. We conducted evaporation experiments using synthetic Neoproterozoic seawater with and without dissolved PO 4 , an effective kinetic inhibitor of aragonite and calcite precipitation. Solution chemistry measurements show that phosphate-bearing seawater undergoing evaporative concentration reaches increasingly high alkalinity, pH, and carbonate saturation until the ACMC forms as the first solid phase, whereas aragonite dominated in phosphate-free systems. Slower evaporation permitted ACMC transformation into other metastable phases, including spherulitic monohydrocalcite and hydrous magnesium carbonates. Experimental data also show that the initial Mg/Ca ratio of the fluid influences its subsequent chemical evolution during evaporation, in turn controlling alkalinity, pH, the composition of precipitated ACMC, and the materials formed through its transformation. These results suggest that the widespread production of metastable precursors, such as amorphous or hydrated Ca-Mg carbonates, may have been common in ancient shallow marine carbonate systems, and they establish a link between aqueous chemistry and some of the unique fabrics and mineralogy preserved within Precambrian carbonates.