The existence of natural variations in isotopic abundance of carbon and oxygen in mantle derived materials requires careful consideration of isotopic fractionation, which may be large even at high temperatures, for carbon. The variations of isotopic fractionation in CO2-CaCO3 system have been the subject of several experimental and theoretical studies. Since the work of Bottinga (1968), a polynomial variation of
D 13C CO2-CaCO3 as a function of 1/T with a cross over around 200°C and a maximum in the 500-700°C range is commonly accepted. Nevertheless disagreement appears between the values and the temperature of the maximum of carbon isotopic fractionation (2,7” according to Bottinga (1968), 3,5” for Chacko et al., (1991) and more generally about the isotopic fractionation's chemical dependence.
In an effort to obtain faster exchange rates over a wider temperature range and to investigate the possible effects of chemical environment, we have undertaken experiments to measure the isotopic fractionation of carbon and oxygen between CO2 and two different carbonated liquids, by direct equilibration of the two phases between 500 and 900°C.
We used a natural natrocarbonatite from Tanzania (Oldoinyo Lengai, 1988) and a synthetic Lithium-Sodium-Potassium carbonate. Three sources of carbon dioxide with different isotopic characteristics were used. The experiments were run at low pressure (1-30 bars) in platinum capsules and the fractionation were determined by recovering both the CO2 and the quenched liquid carbonate. Isotopic equilibrium was checked by the identity of fractionation obtained upon approach from both isotopically heavier and lighter CO2.
The results show evidence of a large carbon isotopic fractionation between 4.0 and 4.5” for the temperature range 500-900°C. They are in agreement with the in situ measured fractionation for Oldoynio Lengai volcano eruption (1988) (Javoy et al., 1988). These high values are larger than the latest available data for the CO2-CaCO3 system (Chacko, 1991; Scheele and Hoefs, 1992; Rosenbaum, 1994), whereas they are close to the data of Javoy et al ., (1978) for tholeiites.
These new results show that equilibrium is faster established in carbonate liquids. The chemistry of the carbonates has significantly influence on the oxygen isotope fractionation. Whereas the carbon isotopic fractionation seems to be less dependent of the chemical and physical environment of carbon within the melt.
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