Controls of temperature and mineral growth rate on Mg incorporation in aragonite

Abstract

The incorporation of Mg in aragonite was experimentally investigated as a function of mineral growth rate at 5, 15 and 25 °C using the constant addition technique. Aragonite growth rate was found to be the major parameter affecting the Mg partitioning coefficient (i.e. DMg=(MgCa)solid(MgCa)fluid) between aragonite and fluid, whereas the effect of temperature is smaller but measurable. At similar surface normalized growth rates, DMg values decrease as temperature increases from 5 to 25 °C. The magnitude of decrease as a function of temperature is similar for all the experiments of this study where growth rate varied in the range 10-8.6 ≤ rp ≤ 10-7.1 (mol/m2/s). The combined effect of aragonite growth rate and temperature on DMg can be described by the linear equation:Log DMg = 0.583(±0.020) Log rp − 0.026(±0.001) T + 0.863(±0.153); R2 = 0.97.where T is the temperature in degrees Celsius.The increase of DMg values at decreasing temperatures in experiments conducted at similar growth rates is consistent with the increase of fluid supersaturation with respect to aragonite. Thus, it can be inferred that increased Mg incorporation at higher supersaturation is associated with the greater presence of defect sites on the growing mineral surface, similar to the incorporation of other incompatible ions in carbonate minerals. Overall, the relationship between Mg content of aragonite with the degree of saturation of the fluid with respect to this mineral phase suggests that DMg values or Mg/Ca ratio in natural aragonites can be used as a proxy for saturation degree of the formation fluid with respect to CaCO3 minerals.