Experimental oxygen isotope fractionation between siderite-water and phosphoric acid liberated CO2-siderite

The equilibrium fractionation of O isotopes between synthetic siderite and water has been measured at temperatures ranging from 33° to 197°C. The fractionation between siderite and water over this temperature range can be represented by the equation: 10₃ ln α = 3.13 × 10⁶T⁻² − 3.50. Comparison between the experimental and theoretical fractionations is favorable only at approximately 200°C; at lower temperatures, they generally differ by up to 2 permil.

Siderite was prepared by the slow addition of ferrous chloride solutions to sodium bicarbonate solutions at the experimental temperatures. It was also used to determine the O isotope fractionation factors between phosphoric acid liberated CO₂ and siderite. The fractionation factors for this pair at 25° and 50°C are 1.01175 and 1.01075, respectively.

Preliminary results of the measured C isotope fractionation between siderite and Co₂ also indicate C isotopic equilibrium during precipitation of siderite. The measured distribution of ¹³C between siderite and CO₂ coincides with the theoretical values only at about 120°C. Experimental and theoretical C fractionations differ up to 3 permil at higher and lower temperatures.