Dual clumped (Δ₄₇-Δ₄₈) isotope data for amorphous carbonates and transformation products reveal a novel mechanism for disequilibrium clumped isotope 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. Currently, little is known about the isotopic composition of the anion in amorphous carbonates and how isotopic values evolve during transformation into a mineral. Here, we examined the evolution of isotopic values in amorphous carbonates and mineral transformation products to identify the potential origins of disequilibrium isotopic compositions in carbonate minerals that form from an amorphous precursor. We measured dual carbonate clumped isotopes (¹³C¹⁸O¹⁶O - Δ₄₇; ¹²C¹⁸O¹⁸O - Δ₄₈), bulk stable isotope ratios (δ¹³C, δ¹⁸O), and chemical and structural data throughout the transformation of amorphous calcium magnesium carbonate (ACMC) 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 significantly during transformation, indicating dissolution of ACMC and reprecipitation of HMC. After crystallization, the Δ₄₇ and Δ₄₈ values achieved a disequilibrium steady state, while δ¹⁸O values continued to evolve. For the fully crystallized HMC samples, the low temperature samples formed at 10 °C had the greatest extent of oxygen isotope disequilibrium (measured value – equilibrium value = −39‰); conversely; the greatest clumped isotope disequilibrium was observed in high temperature samples formed at 40 and 60 °C (0.068‰ for Δ₄₇, 0.072‰ for Δ₄₈). These results are consistent with a new potential mechanism of disequilibrium clumped isotope values in carbonate minerals. Specifically, the dissolution of ACMC during transformation causes disequilibrium oxygen and clumped isotope values in the dissolved inorganic carbon (DIC) pool. The extent of isotopic disequilibrium in DIC during transformation is temperature dependent, and is recorded by the forming mineral. Isotopic results may also reflect mixing effects, as scanning electron microscopy (SEM) showed ACMC and HMC existing simultaneously during transformation, indicating that ACMC likely progressively dissolves and reprecipitates as the DIC pool isotopically evolves. This may result in heterogeneous isotopic values in HMC. In total, these data suggest a highly dynamic localized environment could exist in biomineralizing organisms and abiotic systems that utilize amorphous precursors to form carbonate minerals, potentially resulting in isotopic values that are not representative of formation temperature.