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

다국어 초록 (Multilingual Abstract)

Rhodochrosite crystals were precipitated from Na-Mn-Cl-HCO<SUB>3</SUB> parent solutions following passive, forced and combined passive-to-forced CO<SUB>2</SUB> degassing methods. Forced and combined passive-to-forced CO<SUB>2</SUB> degassing produced rhodochrosite crystals with a small non-equilibrium oxygen isotope effect whereas passive CO<SUB>2</SUB> 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<SUP>o</SUP>C: 1000lnα<SUB>rhodochrosite-water</SUB>=17.84+/-0.18(10<SUP>3</SUP>/T)-30.24+/-0.62 or 1000lnα<SUB>rhodochrosite-water</SUB>=2.65+/-0.03(10<SUP>6</SUP>/T<SUP>2</SUP>)-0.26+/-0.35 where α<SUB>rhodochrosite-water</SUB> is the fractionation factor between rhodochrosite and water, and T is in kelvins. Over the temperature range investigated, rhodochrosite concentrates <SUP>18</SUP>O relative to both calcite and aragonite, a result that is consistent with the relative ionic radii of Ca<SUP>2+</SUP> and Mn<SUP>2+</SUP> and recent theoretical calculations.