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

Abstract

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 degrees C: 10001n alpha(rhodochrosite-water) = 17.84 +/- 0.18(10(3)/T) - 30.24 +/- 0.62 or 10001n alpha(rhodochrosite-water) = 2.65 +/- 0.03(10(6)/T-2) - 0.26 +/- 0.35 where alpha(rhodochrosite-water) is the fractionation factor between rhodochrosite and water, and T is in kelvins. Over the temperature range investigated, rhodochrosite concentrates O-18 relative to both calcite and aragonite, a result that is consistent with the relative ionic radii of Ca2+ and Mn2+ and recent theoretical calculations. (C) 2009 Elsevier Ltd. All rights reserved.