Dual clumped isotope data for amorphous carbonates and transformation products reveal novel mechanisms for nonequilibrium effects

Amorphous precursors to minerals have been observed in laboratory materials and in nature, including across diverse phyla. These metastable phases allow for the incorporation of cations at higher concentrations than classical crystallization pathways. Thus their chemistry and behavior have implications in an array of disciplines. There are major knowledge gaps in characterization of the isotopic composition of the anion in amorphous carbonates and behavior during transformation. Here, we examined the evolution of the isotopic composition of amorphous carbonates and transformation products to constrain reaction mechanisms and potential origins of non-equilibrium isotopic compositions in calcite. We measured dual clumped isotopes (13C18O16O - Δ47; 12C18O18O - Δ48), bulk stable isotope ratios (δ13C, δ18O), and chemical and structural data throughout the transformation of an amorphous calcium magnesium carbonate (ACMC) precursor into high Mg-calcite (HMC) over 1 year, with crystallization occurring in solutions from 10 to 60 °C. The Δ47, Δ48, and δ18O values evolved considerably during transformation, with Δ47 and Δ48 values achieving a non-equilibrium steady-state, while δ18O continued to evolve after crystallization. Observations and modeling simulations were consistent with the dissolution of ACMC causing disequilibrium in the DIC pool, which was then recorded during subsequent crystallization of HMC. Isotopic results may also reflect mixing effects due to ACMC progressively dissolving and reprecipitating while the DIC pool isotopically evolved. These results have implications for applied studies as they demonstrate new mechanisms where nonequilibrium isotope effects could be produced in minerals formed from amorphous precursors, but with expression of the effects dependent on the conditions during transformation.