An Investigation of the Mechanisms of Calcium Isotopic Fractionation in Gypsum

Open Access
Author:
Harouaka, Khadouja
Graduate Program:
Geosciences
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
November 11, 2011
Committee Members:
  • Matthew Scott Fantle, Thesis Advisor
Keywords:
  • isotopic fractionation
  • calcium
  • gypsum
Abstract:
This study reports the first ever systematic investigation of Ca isotopic fractionation in gypsum. A series of gypsum precipitation experiments were carried out to determine the relationship between Δ44Cax-s and precipitation rate as a function of saturation state and Ca2+/SO42- ionic ratio. The ionic strengths of all the experiments were fixed at 0.6 M, and the ionic ratios were fixed between 3 and 0.33. The initial saturation states of the precipitation solutions ranged between 4.7 and 1.6. The precipitated gypsum had a Δ44Cax-s range of 1.43‰ where the fastest and slowest precipitation rates produced crystals with a Δ44Cax-s of -2.25 and -0.82‰ respectively. This is the same range of Ca fractionation observed in naturally occurring evaporites from the ODP 654 site at the Tyrrhenian Basin (Hensley, 2006). The weight averaged surface area normalized rates ranges from 2.29x105 to 1.00x103 μmol/m2/h. In general, stirred solutions demonstrated a negative dependence of Δ44Cax-s on precipitation rate, in accordance with the findings of Tang et al. (2008b). Unstirred solutions showed a slight positive relationship between Δ44Cax-s and precipitation rate, which is the trend observed by Lemarchand et al. (2004). The Δ44Cax-s had strong dependence on precipitation rate as a function of saturation state and stirring. Rate as a function of ionic ratio did not affect the Ca fractionation of the precipitated crystals. Crystal morphology was found to correlate with precipitation rates, such that fast rates produced small (10-20 μm in diameter) platelet like crystals and slow precipitation rates produced large needle like crystals (>1000 μm in diameter). The small crystals were also found to isotopically equilibrate with the precipitation solution over time once the system had reached chemical equilibrium (Ω=1). The trends in Δ44Cax-s observed in stirred and unstirred solutions may suggest a Ca fractionation mechanism that is controlled by surface reactions in stirred solutions, and by diffusion in unstirred solutions. Surface reactions describe the incorporation of outer sphere complexes into the crystal lattice, a process which may fractionate Ca in favor heavier isotopes. The results of this study are significant as they provide valuable insight to the ongoing debate about what controls Ca fractionation during mineral precipitation. The observed Ca fractionation in gypsum also calls into question the interpretations of δ44Ca measurements in nature with respect to diagenesis in soils, and in reconstructing ancient sweater chemistry from calcium carbonates.