DECIPHERING MG PARTITIONING INTO CALCITE: A NEW STUDY OF MACROSCALE BEHAVIOR THROUGH DIRECT MEASUREMENT OF NANOSCALE CONTROLS

The Mg/Ca paleothermometer is based upon the relationship between temperature and Mg partitioning into biogenic calcite. However, interpretations of bulk fluid-calcite experiments in the literature disagree upon the most fundamental assumptions regarding the influences of temperature, growth rate, impurity concentration, and salinity on Mg uptake. To begin addressing this problem, we conducted a new type of calcite growth study: while temperature and solution chemistry were precisely controlled, we directly quantified near-equilibrium, layer-mechanism growth rates and Mg contents of calcite crystals.

We measured migration rates of monomolecular steps in situ on individual growth spirals on {104} calcite faces using atomic force microscopy. Solutions containing CaCl₂, NaHCO₃, MgCl₂, and 0.1 molal NaCl flowed continuously over samples. Growth rates were determined at 15-30°C in solutions containing 0-10^-4 molal [Mg] and with supersaturations (ln [(a_Ca++ x a_CO3--)/K_sp]) of 0.3-1.0. In complementary experiments, we grew seed crystals under identical conditions for several days to produce large growth hillocks for high-resolution electron microprobe analysis. We find that Mg exhibits a pronounced, non-uniform partitioning into growth hillocks with a distinct preference for geometrically obtuse steps (~100 to >3000 ppm) over acute steps (0 to ~800 ppm). This partitioning is opposite to results of Paquette and Reeder (1995; GCA 59:735) for calcite grown at high ionic strength (1-2 molal) with multiple impurities in solution. Our work suggests that the interaction of Mg with calcite surfaces depends upon nanoscale site geometry and also upon the presence or absence of other trace elements. Preliminary findings suggest that of the factors named above, supersaturation has the strongest influence on Mg/Ca in the solid crystals.

Our work on inorganic controls of impurity partitioning in calcite sets the stage for investigation of biological effects and for robust interpretation of trace element signatures in biogenic calcite.