Distribution and isotope fractionation of traces during carbonate mineral formation from amorphous precursors

The formation of calcium carbonate from an amorphous precursor phase is a crystallization route selected by a large number of marine calcifiers. To date, however, only little is known about the fate of traces and their isotopes during the transformation process of the initial amorphous phase to the final crystalline product, although they are routinely used for environmental reconstruction. Here we report on our recent findings on the chemical and isotopic compositions of Mg and B in calcite and aragonite that formed via the crystallization of an amorphous phase precursor and we compare them with the results obtained during classical overgrowth experiments. During the formation of the amorphous precursor a weak Mg isotope fractionation occurs between the solid and aqueous phases, whereas the transformation of the precursor to the final crystalline product yields Mg isotope compositions that resemble near equilibrium values. This behaviour contradicts the strong kinetic effects that were reported earlier during calcite classical growth at low degrees of solution supersaturation with respect to this mineral. In the case of boron, which is commonly used as a pH proxy, the obtained results suggest that calcite formed at pH = 8.9 is in good agreement with the boron pH proxy and the presence of only borate ions in the solid. In contrast formation of calcite at pH=8.3 and aragonite deviate from the values measured earlier in overgrowth experiments but are in good agreement with the ab-initio calculations by Balan el al. (2018).

Balan et al. (2018) GCA 222,117-129.