Stable Isotope Fractionation during Experimental Formation of Norsethite (BaMg[CO3]2): A Mineral Analogue of Dolomite |
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Authors: | Michael E Bötcher |
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Institution: | (1) Department of Earth Sciences, Carleton University, 1125 Colonel By Drive, Ottawa, Ontario, K1S 5B6, Canada;(2) Present address: Alberta Geological Survey, Energy Resources Conservation Board, 4th Floor, Twin Atria Building, 4999 - 98 Avenue, Edmonton, Alberta, T6B 2X3, Canada |
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Abstract: | Stable oxygen and carbon isotopefractionation during the experimental formation ofordered norsethite (BaMgCO3]2) from thereaction of anhydrous BaCO3 (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 theBaCO3–H2O and MgCO3–H2O systems,13C/12C partitioning is more similar to thatfor BaCO3–CO2. Between 20° and90°C, the temperature dependences of the18O/16O and 13C/12C fractionationfactors are represented by the equations (T in °K):103 ln BaMgCO3]2-H2O = 2.83 106T--2.85, and 103ln BaMgCO3]2-CO2(gas) = 1.78 106T--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 BaMgCO3]2-CO2-H2O are: Oxygen isotopes: BaMg(CO3)2-H2O = 1.02941, BaMg(CO3)2-OH-(aq) = 1.07059, BaMg(CO3)2-CO2(gas) = 0.98868, and BaMg(CO3)2-H2CO3
* = 0.98843; carbon isotopes: BaMg(CO3)2-CO2(gas) = 1.00992, BaMg(CO3)2-H2CO3
* = 1.01099, BaMg(CO3)2-HCO3
- = 1.00194, BaMg(CO3)2-CO3
2- = 1.00491 or 1.00150.The spontaneous precipitation of aBaMgCO3]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. |
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Keywords: | carbonates norsethite dolomite stable isotope fractionation experiment low temperature |
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