Effects of temperature and transport conditions on calcite growth in the presence of Mg2+: Implications for paleothermometry

This study links direct measurement of Mg-calcite growth kinetics with high-spatial-resolution analysis of Mg contents in experimental crystals, with particular attention to the effects of temperature on growth rate and reactant transport conditions on Mg distribution. In contrast to previous experiments on Mg partitioning into calcite, here the layer-growth mechanism was observed in situ and step speeds precisely measured with fluid cell atomic force microscopy over a range of temperatures, degrees of supersaturation, and solution Mg concentrations. Data collected from 15° to 30°C yield an activation energy for calcite precipitation of 33 kJ/mol for solutions with [Mg] = 5 × 10−5 molal. Electron microprobe analyses of large hillocks grown at corresponding conditions demonstrate that Mg has a strong preference for incorporation at negative (acute) step edges, rather than at positive (obtuse) edges when growth rate is limited by surface reactions. This preference is reversed when growth is instead limited by diffusion of reactants through a boundary layer at the mineral-solution interface. These findings show that temperature is not the only strong control on the extent of Mg incorporation and distribution in calcite; transport conditions during mineral growth may also be a first-order factor governing the compositions of natural calcite samples.