Ra. Brooker et al., Structural controls on the solubility of CO2 in silicate melts Part I: bulk solubility data, CHEM GEOL, 174(1-3), 2001, pp. 225-239
CO2 solubility data are presented for a wide range of melt compositions in
the following systems: SiO2-Na2O-Al2O3 + CO, (SNA) at 2.0 GPa and 1600 degr
eesC; SNA + CaO (SNAC) and SNA + MgO + CaO (MSNAC) at 1.5 Cpa and 1275-1400
degreesC; and-for several "natural" magma compositions (Mg- and Ca-rich me
lilitites, andesite and phonolite) at 1.2-2.7 GPa and 1300-1600 degreesC. A
t a given pressure and temperature, the solubility is found to be a strong
function of the "non-bridging oxygen" (NBO) content of the melt, expressed
as the NBO/T ratio, where 'T represents tetrahedral network-forming cations
. The NBO/T ratio, calculated from the melt composition, thus provides a us
eful parameter for expressing and predicting the CO2 solubility. In highly
polymerised melts, other dissolution mechanisms involving bridging oxygens
become important and NBO/T is no longer the exclusive control on solubility
. In Fe-bearing systems, the best correlation between CO2 solubility and NB
O/T is found when both Fe3+ and Fe2+ are assumed to be tetrahedral (T), ind
icating that these cations should be considered in a polymerising role in t
he melt, with respect to CO2 dissolution. There is also evidence that some
fraction of the Mg2+ in the melt should be assigned to a polymerising role.
(C) 2001 Elsevier Science B.V. All rights reserved.