Stable oxygen and carbon isotopefractionation during the experimental formation ofordered norsethite (BaMg[CO
3]
2) from thereaction of anhydrous BaCO
3 (witherite) withrelatively low concentrated sodium-magnesiumbicarbonate solutions has been studied between20° and 135 °C. In the investigatedtemperature range,
18O and
13C are enrichedin norsethite with respect to water and gaseous carbondioxide, respectively. Whereas
18O/
16Opartitioning is intermediate between those of theBaCO
3–H
2O and MgCO
3–H
2O systems,
13C/
12C partitioning is more similar to thatfor BaCO
3–CO
2. Between 20° and90°C, the temperature dependences of the
18O/
16O and
13C/
12C fractionationfactors are represented by the equations (T in °K):10
3 ln
BaMg[CO
3]
2-H
2O = 2.83 10
6T
--2.85, and 10
3ln
BaMg[CO
3]
2-CO
2(gas) = 1.78 10
6T
--10.16. The later equation considers carbon isotope fractionationbetween the dissolved carbonate ion and carbon dioxide measured by Halaset al. (1997). Under standard state conditions (25 °C) the fractionation factors in the system BaMg[CO
3]
2-CO
2-H
2O are: Oxygen isotopes:
BaMg(CO
3)
2-H
2O = 1.02941,
BaMg(CO
3)
2-OH-(aq) = 1.07059,
BaMg(CO
3)
2-CO
2(gas) = 0.98868, and
BaMg(CO
3)
2-H
2CO
3
* = 0.98843; carbon isotopes:
BaMg(CO
3)
2-CO
2(gas) = 1.00992,
BaMg(CO
3)
2-H
2CO
3
* = 1.01099,
BaMg(CO
3)
2-HCO
3
- = 1.00194,
BaMg(CO
3)
2-CO
3
2- = 1.00491 or 1.00150.The spontaneous precipitation of aBaMg[CO
3]
2 gel at 20 °C,followed by the alteration of the products at20° or 60°C for 31 days,demonstrated isotope exchange reactions betweensolids and mother solutions dueto recrystallization. Isotope equilibrium, wasnot reached within run time.
carbonates - norsethite - dolomite - stable isotope fractionation - experiment - low temperature