HR: 0800h
AN: B21B-0869    [Abstracts]
TI: Distribution of Mg in Calcite is Strongly Influence by Transport Conditions at the Mineral-Solution Interface
AU: * Wasylenki, L E
EM: lew@vt.edu
AF: Department of Geosciences Virginia Tech, 4044 Derring Hall, Blacksburg, VA 24061 United States
AU: Dove, P M
EM: dove@vt.edu
AF: Department of Geosciences Virginia Tech, 4044 Derring Hall, Blacksburg, VA 24061 United States
AU: De Yoreo, J J
EM: deyoreo1@llnl.gov
AF: Department of Chemistry and Material Science, Lawrence Livermore National Lab, Livermore, CA 94551 United States
AB: Mg is widely recognized as a paleotemperature indicator in biogenic calcite samples. However, uncertainly continues about whether factors other than temperature act as primary controls on calcite Mg contents. To investigate the roles of surface structure and transport conditions in Mg uptake, we grew Mg-bearing calcite overgrowths on seed crystals using solutions with carefully prescribed levels of supersaturation, [Mg], pH, ionic strength, and constant temperature. In some growth experiments solutions were gently agitated; growth rates were limited by diffusive transport of ions at the crystal-solution interface. In other experiments fluid flowed vigorously, and growth rates were limited by the surface processes of adsorption, dehydration, and lattice bond formation. Control of growth rate by diffusion or surface reaction was determined by previous atomic force microscopy studies at the same conditions. Electron probe measurements yielded detailed information about Mg contents and distributions between the two geometrically distinct flanks of individual hillocks. Under diffusion-limited conditions, Mg displays a strong preference for incorporation into step edges with an obtuse (+) geometry. By contrast, Mg partitions preferentially into acute (-) step-edges when growth rate is controlled by surface processes. Thus the distribution of Mg in calcite is strongly influenced by transport conditions at the growing mineral surface. Although we do not know in detail the cause of differential incorporation nor how transport conditions change the distribution of Mg so dramatically, but the answers likely involve differences in the adsorption lifetime of Mg cations on step terraces relative to step migration rates. This study demonstrates that factors other than temperature strongly influence impurity uptake in calcite. The rate-controlling mechanisms for calcite growth in different organisms are as yet unknown, but transport conditions may have a first-order control on biomineral compositions.
DE: 4805 Biogeochemical cycles (1615)
DE: 4825 Geochemistry
DE: 4267 Paleoceanography
DE: 1045 Low-temperature geochemistry
DE: 1615 Biogeochemical processes (4805)
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting