Elemental relationships in synthetic calcite at variable fluid chemistry and crystallization rate.

Abstract:

Minor and trace elemental relationships in natural calcium carbonate minerals are generally explained by changes in temperature and/or composition of the growth media. However, partitioning of elements between mineral and fluid often occurs at conditions far from equilibrium. Incorporation of minor and trace elements into calcite has been explored over the last several decades, however kinetic and thermodynamic effects were not fully resolved.

Here, we present ion microprobe data on Mg, Sr, Ba, and U distribution within individual calcite crystals grown from fluids with continuous increases of element to calcium ratios and decreases of crystal growth rates. Analyses were performed using CAMECA ims-1270 (UCLA). The depletion of fluid in Ca and the above mentioned elements caused a continuous increase in element to calcium ratios during each experiment. However, we were not able to fully resolve the influence of fluid depletion on apparent partition coefficients during disequilibrium crystallization. Our results suggest that growth rate strongly affects incorporation of Mg, Sr, Ba, and U into the calcite lattice. Growth Entrapment Model (GEM) explains observed relationships between growth rate and partition coefficients. However, elemental profiles across individual crystals suggest that the factors controlling incorporation process may not be ubiquitous for each element.

Calcite growth velocities obtained from our work overlap with the growth rate range reported for speleothems and marine biogenic carbonates. We anticipate comparing elemental relationships in synthetic calcite with those obtained in natural samples which were reported in previous literature.