THE EFFECTS OF PCO2 AND TEMPERATURE ON MAGNESIUM INCORPORATION IN CALCITE IN SEAWATER AND MGCL2-CACL2 SOLUTIONS

P(CO2) is a significant influence on the incorporation of MgCO3 in calcite precipitated from seawater and from MgCl2-CaCl2 (alpha-Mg2+/alpha-Ca2+ = 5.7) solutions at temperatures from 5 to 45-degrees-C. In seawater at 25-degrees-C, the thermodynamic distribution coefficient, D(Mg), increased from 0.013 at 10(-1) atm to 0.024 at 10(-4.5) atm, P(CO2).D(Mg) showed a similar dependence on P(CO2) at other temperatures, but trends are offset by temperature effects on D(Mg). In normal seawater, variations in temperature from 5 to 45-degrees-C, coupled with changes in P(CO2) from 10(-1) to 10(-4.5) atm, produce Mg calcite compositions ranging from 6 to 18 mol% MgCO3. D(Mg) depends linearly on log P(CO2), log m(HCO3-), and pH at any given temperature. There is no linear dependence of D(Mg) on log mCO3(2-), growth rate, or calcite saturation state. Overgrowths precipitated from MgCl2-CaCl2 solutions at equilibrium with different P(CO2) values showed compositional trends similar to those for seawater except that calcites were always enriched by about 2 mol% MgCO3 relative to compositions in seawater under the same P(CO2) and temperature conditions. D(Mg) values also are linearly dependent on pH and log alpha-HCO3- in these solutions. The compositional trends in the MgCl2-CaCl2 solutions correlate well with compositions of Mg-calcites precipitated by MUCCI et al. (1989) from sulfate-free seawater at 25-degrees-C and 10(-2.5) atm P(CO2). Modern marine Mg calcite cement compositions probably are influenced by local variations in porewater P(CO2) and sulfate concentration, as well as by changes in temperature. Global changes in atmospheric P(CO2) during the Phanerozoic may influence trends in compositions of ancient shallow marine Mg calcite.