Experimental studies of oxygen isotope fractionation between rhodochrosite (MnCO3) and water at low temperatures

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: 1000lnαrhodochrosite–water=17.84±0.18(103/T)-30.24±0.62 or 1000lnαrhodochrosite–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.