Effect of organic ligands on Mg partitioning and Mg isotope fractionation during low-temperature precipitation of calcite in the absence of growth rate effects

Calcite growth rate has been previously shown to be the dominating parameter controlling both Mg partitioning and Mg isotope fractionation during calcite growth. In natural calcite precipitation environments - characterized by abundant organic material - the presence of dissolved organic molecules may affect these two parameters. In order to assess the role of organic molecules, steady state calcite growth experiments have been performed at 25 degrees C, 1 bar pCO(2) and constant, within analytical uncertainty growth rate (r(p) = 10(-7.4) mol m(-2) s(-1)) in the presence of aqueous Mg and six organic ligands in the concentration range from 0.01 to 10 mM. The organic ligands used in this study are: (i) acetic acid, (ii) citric acid, (iii) glutamic acid, (iv) salycilic acid, (v) glycine, and (vi) ethylenediaminetetraacetic acid (EDTA). These contain one or more carboxyl- and amino-groups that are commonly present in natural organic substances found in lacustrine, fluvial, soil, cave, as well as in marine and earliest diagenetic porewater environments. Results shown here indicate that the presence of these carboxyland amino-groups promotes an increase in the partition coefficient of Mg in calcite (D-Mg = (Mg/Ca)(calcite)/ (Mg/Ca)(fluid)) that can be attributed to their adsorption onto the calcite surfaces and the subsequent reduction of the active sites of growth. This increase of DMg values as a function of the supersaturation degree of calcite in the fluid phase can be described by the linear equation: LogD(Mg) = 0.3694 (+/- 0.0329) x SIcalcite - 1.9066 (+/- 0.0147) R-2 = 0.92 In contrast, data shown here suggest that the presence of organic ligands, with exception of citric acid, does not significantly affect the Mg isotope fractionation factor between calcite and reactive fluid. Citric acid likely exhibits larger fractionation between the Mg-ligand complexes and free aqueous Mg, compared to the other organic ligands studied in this work, as evidenced from the smaller Delta(26) Mgcalcite-fluid values. The main result shown here is that in Earth's surface precipitation environments the presence of organic ligands is not a major factor affecting the Mg content or Mg-isotope composition of inorganic calcite. But in the presence of higher contents of organic macromolecules from byproducts of metabolic activities, humid, and fulvic acids and/or exopolymeric substances and matrices, an impact on Mg content or Mg isotope composition of calcite cannot be ruled out. (C) 2017 Elsevier Ltd. All rights reserved.