HR: 1330h
AN: B12C-0791    [PDF]
TI: Partitioning of Mg Into Calcite: Direct Measurement of the Relative Influences of Temperature, Growth Rate, and Solution Chemistry
AU: * Wasylenki, L E
EM: lew@vt.edu
AF: Dept. of Geosciences, Virginia Tech, Blacksburg, VA 24061 United States
AU: Dove, P M
EM: dove@vt.edu
AF: Dept. of Geosciences, Virginia Tech, Blacksburg, VA 24061 United States
AU: De Yoreo, J J
EM: deyoreo1@llnl.gov
AF: Dept. of Chem. and Mat. Sci., LLNL, Livermore, CA 94551 United States
AB: Magnesium contents of biogenic carbonates are recognized as potential paleotemperature proxies. However, the extents to which temperature, salinity, growth rates, fluid [Mg], and biological factors govern Mg contents in calcite are as yet unresolved. An ongoing source of uncertainty is that the crystal growth mechanism(s) in most past studies was inferred or unknown. Here we present two sets of complementary experiments on abiotic calcite growth in which nanoscale observation and measurement of layer-mechanism growth rates are linked with solid crystal Mg contents. Systematic variations in solution chemistry and temperature enable us to evaluate the relative influences of the inorganic controls on Mg incorporation. We directly measured monomolecular step velocities on calcite using fluid cell atomic force microscopy. Temperature varied from 15 to 30$\deg$C, supersaturation (ln [aCa$^{2+}$ x aCO$_{3}$$^{2-}$/K$_{sp}$]) from 0.3 to 1.0, and fluid [Mg] from 0 to 10$^{-4}$m. The resulting data relate temperature, growth rates, and fluid [Mg], yielding thermodynamic and kinetic information, including activation energies for calcite precipitation. Long-term experiments at corresponding conditions produced growth hillocks large enough for electron microprobe analysis of Mg concentrations and distributions. These results illustrate relationships between the parameters above and Mg contents of calcite. Our work shows unequivocally that degree of supersaturation, and thus growth rate, exerts a key influence on Mg partitioning. For example, at 23.5$\deg$C, despite decreasing Mg/Ca in solutions with increasing supersaturation, Mg contents in calcite increase several fold. Mg also displays a distinct preference for the obtuse (positive) steps on growth hillocks over acute (negative) steps, despite more pronounced morphological effects on acute steps as observed at the nanoscale. This finding contrasts with that of Paquette and Reeder (1995; GCA 59:735), who grew calcite in high ionic strength solutions with multiple impurities. This study advances our understanding of how inorganic factors control Mg signatures in calcite. Quantifying these effects is a critical step toward accurately interpreting the information recorded as trace element signatures in biogenic calcite.
DE: 1045 Low-temperature geochemistry
DE: 1615 Biogeochemical processes (4805)
DE: 3620 Crystal chemistry
DE: 4267 Paleoceanography
DE: 4875 Trace elements
SC: Biogeosciences [B]
MN: 2003 Fall Meeting