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Annealing experiments on CO₂-bearing jadeite glass: an insight into the true temperature dependence of CO₂ speciation in silicate melts

Department of Earth Sciences, Wills Memorial Building, University of Bristol, Queen’s Rd., Bristol BS8 1RJ, UK

^* E-mail: simon.kohn{at}bristol.ac.uk

The thermodynamics and kinetics of CO₂ speciation in silicate melts have been studied by measuring the concentration of CO₂^mol and carbonate in jadeite glass annealed at 575, 450 and 400°C. Assuming that the reaction is

(1)

where CO₂^mol..O_br represents a CO₂ molecule weakly bonded to a bridging oxygen in the network and CO₃ represents a bridging carbonate group with no net negative charge, H for the reaction is –17 (+4/–8) kJ mol^–1 and S is –24 (+6/–9) J K¹ mol^–1. The rate of equilibration of the species was measured at each temperature and the rate constants were deduced. The temperature dependence of the rate constants was used to determine the activation energy of the forward and reverse reactions which are 68 (+3/–31) kJ mol^–1 and 86 (+1/–69) kJ mol^–1 respectively. The data suggest that CO₂^mol may be much more abundant in silicate melts than previously assumed on the basis of studies of CO₂-bearing glasses. Models of solubility, diffusion, and isotope fractionation should take this into account.

KEYWORDS: jadeite glass, silicate melts, CO₂ speciation, annealing experiments

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