In-situ observation of calcium carbonate phase heterogeneity during mineral precipitation: Implications for the interpretation of calcium carbonate based proxies

Open Access
Author:
Gonzales, Matthew Scott
Graduate Program:
Geosciences
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
May 14, 2014
Committee Members:
  • Matthew Scott Fantle, Thesis Advisor
  • Michael Allan Arthur, Thesis Advisor
  • Peter J Heaney, Thesis Advisor
  • James David Kubicki, Thesis Advisor
Keywords:
  • Calcite
  • vaterite
  • calcium isotopes
  • paleoproxy development
Abstract:
The isotopic and trace element composition of calcium carbonate (CaCO3) minerals are powerful proxies for global weathering rates, changes in ocean chemistry, and global and local scale climate. The use of CaCO3 based proxies is complicated by the isotopic and elemental fractionation during precipitation, the formation of multiple polymorphs under Earth surface conditions, and the observation the isotopic and elemental partitioning is a function of the mineralogy of the solid phase. While calcite is the most stable polymorph, and thus the most dominant mineralogy found in the rock record, the pathway by which calcite is formed remains a topic of interest research. In this study, the influence of solution chemistry on the formation of CaCO3 polymorphs was investigated. Unseeded precipitation was initiated in small volume (0.1 mL) flow-through reactors, and mineral growth observed in-situ using optical microscopy and Raman spectroscopy. Solutions containing CaCl2 and Na2CO3 were mixed in the reactor from separate reservoirs using syringe pumps to control flow rate (0.005-0.1 mL/min). Inital saturation indices (with respect to calcite) ranged from 1.1 to 2.3 while starting pH varied from 8.3 to 10.5. A subset of the experimental solutions included NaCl, NH4Cl, Na2HPO4, and L-arginine as low level additives. In nearly all experiments, both rhombic calcite and vaterite (in almond. shell, spherical, or flower morphologies) were observed. Calcite to vaterite ratios varied within the reactors between 0.1 and 187 and were dependent mainly on saturation state, though the location of growth within the reactor was also important. Both calcite and vaterite precipitated concurrently and phase transitions were limited to small amounts of vaterite spheres becoming calcite. The presence of additives such as Na2HPO4, L-arginine, and, to a lesser extent, NH4Cl inhibited the precipitation of vaterite, while NaCl slightly promoted to precipitation of calcite. The results suggest that the interpretation of Ca isotope data from precipitation experiments may be marked affected by sampling protocol, the effects of additives, and the method by which saturation is achieved to initiate precipitation.