Influence of precipitation rate and temperature on the partitioning of magnesium and strontium in calcite overgrowths
To study the incorporation of magnesium (Mg) and strontium (Sr) in calcite precipitated over synthetic calcite seeds (overgrowths) as a function of the precipitation rate (R*, μmol/m2 h), we performed precipitation experiments wherein temperature and precipitation rates were decoupled at intervals of approximately 3.63–5.22 μmol/m2 h. In most sample reactions, high‑magnesium calcite (HMC) overgrowths co-precipitated with aragonite from the stirred solutions exposed to an atmosphere of NH3 and CO2 gases throughout the spontaneous decomposition of (NH4)2CO3. The percentage of aragonite in the solid CaCO3 increased with both temperature and dissolved inorganic carbon (DIC). The order of reaction with respect to the [DIC] is temperature dependent and is 1.9, 2.4, and 2.9 at temperatures of 12.5, 25.0, and 37.5 ◦C, respectively. The magnesium distribution coefficient (DMg) increases significantly with increasing R*, temperature, and Mg/Ca ratio in the fluid. The strontium distribution coefficient (DSr) increases with R* and with increasing MgCO3 concentrations in the calcite overgrowths. However, it is independent of temperature.