Nucleation and growth of dolomite in an alkaline lake

Dolomite precipitation from supersaturated solutions at Earth’s surface temperature is considered kinetically inhibited. Alkaline lakes are among the few modern environments hydrochemically conducive to low-temperature dolomite formation. However, the nucleation and growth of dolomite under alkaline conditions, and how these processes are related to alkaline water chemistry, remain poorly understood. We report extensive dolomites recently observed in the lower Permian Fengcheng Formation of the Junggar Basin (northwest China); well-preserved ancient alkaline deposits. High-resolution, in situ secondary ion mass spectrometry (SIMS)−based carbon and oxygen isotope analyses reveal systematic zonation in dolomite crystals: δ18OVPDB (VPDB—Vienna Peedee belemnite) decreases from the core (3.30‰) to the intermediate (0.35‰) rim (−11.12‰), whereas δ13CVPDB increases from the core (0.89‰) to the intermediate (4.74‰) rim (6.81‰). The positive δ18O values of the core zone, together with its finely preserved nucleus and radial cathodoluminescence (CL) textures, suggest syndepositional nucleation under intensive evaporative conditions. The opposing core-to-rim trends in δ13C and δ18O of the studied dolomites capture the progressive evolution of the dolomites, with increasing formation temperatures and intensifying methanogenesis, from initial nucleation in the evaporative water column through growth near the sediment−water interface to final overgrowth in the burial methanogenesis zone. Our results suggest that alkaline lake conditions can promote the direct nucleation of dolomite via an inorganic pathway, providing important constraints on the role of alkaline environments in resolving the long-standing “dolomite problem” in abiotic systems.