Strontium partitioning in calcite and its controling factors

Strontium (Sr) is a common trace element in calcite which is incorporated during calcite precipitation through either inorganic or organic pathways. The ratio of Sr to calcium (Ca) in carbonate rocks and minerals has been widely used in studies of paleoceanography, marine sediment diagenesis, and hydrothermal alteration of oceanic crust. The partitioning coefficient of Sr (K_Sr) describes fundamentally the partitioning of Sr between calcite and aqueous solutions, but is a complicated function of environmental conditions and water composition. It has long been recognized that K_Sr is strongly dependent on the precipitation rate of calcite (R_p), which has been formulated in a surface kinetic model [1] and a subsequent ion-by-ion model [2]. We re-evaluate available experimental data of Sr partitioning in calcite and find apparent dependence of K_Sr on calcite oversaturation and solution pH [3]. An ion-by-ion model is developed that successfully reproduces the observed K_Sr values at given solution chemistry [4]. Our model also reproduces observed K_Sr-R_p relationships at various temperatures of 5, 25 and 40 ^oC. This model provides an opportunity to evaluate effects of past seawater composition on Sr partitioning and their possible roles in reconstructing seawater Sr/Ca ratio in the geological history [5], in using pore fluid Sr concentration to extract sediment-fluid exchange rates in deep sea carbonate sediments [5], and in understanding Sr partitioning in biogenic calcite such as foraminifera.

Reference

[1] DePaolo, D.J., Surface kinetic model for isotopic and trace element fractionation during precipitation of calcite from aqueous solutions. Geochimica et Cosmochimica Acta, 2011. 75(4): p. 1039-1056.

[2] Nielsen, L.C., J.J. De Yoreo, and D.J. DePaolo, General model for calcite growth kinetics in the presence of impurity ions. Geochimica et Cosmochimica Acta, 2013. 115: p. 100-114.

[3] Zhang, S. and D.J. DePaolo, Equilibrium calcite-fluid Sr/Ca partition coefficient from marine sediment and pore fluids. Geochimica Et Cosmochimica Acta, 2020. 289: p. 33-46.

[4] Jia, Q., et al., A model for pH dependent strontium partitioning during calcite precipitation from aqueous solutions. Chemical Geology, 2022. 608: p. 121042.

[5] Zhang, S., R.J. Zhou, and D.J. DePaolo, The seawater Sr/Ca ratio in the past 50 Myr from bulk carbonate sediments corrected for diagenesis. Earth and Planetary Science Letters, 2020. 530: p. 115949.