Rhodochrosite crystals were precipitated from Na-Mn-Cl-HCO3 parent solutions following passive, forced and combined passive-to-forced CO2 degassing methods. Forced and combined passive-to-forced CO2 degassing produced rhodochrosite crystals with a small non-equilibrium oxygen isotope effect whereas passive CO2 degassing protocols yielded rhodochrosite in apparent isotopic equilibrium with water. On the basis of the apparent equilibrium isotopic data, a new temperature-dependent relation is proposed for the oxygen isotope fractionation between rhodochrosite and water between 10 and 40 °C: 1000 ln αrhodochrosite s(-) water = 17.84 ± 0.18 (103 / T) - 30.24 ± 0.62 or 1000 ln αrhodochrosite s(-) water = 2.65 ± 0.03 (106 / T2) - 0.26 ± 0.35 where αrhodochrosite-water is the fractionation factor between rhodochrosite and water, and T is in kelvins. Over the temperature range investigated, rhodochrosite concentrates 18O relative to both calcite and aragonite, a result that is consistent with the relative ionic radii of Ca2+ and Mn2+ and recent theoretical calculations.
ASJC Scopus subject areas
- Geochemistry and Petrology