HR: 0830h
AN: T11C-0405    [PDF]
TI: A Thermodynamic Model for Dissolution of Volatiles in Nominally Volatile-Free Minerals
AU: * Landuyt, W
EM: william.landuyt@yale.edu
AF: Yale University, Department of Geology & Geophysics, New Haven, CT 06520 United States
AU: Skemer, P
EM: philip.skemer@yale.edu
AF: Yale University, Department of Geology & Geophysics, New Haven, CT 06520 United States
AU: Karato, S
EM: shun-ichiro.karato@yale.edu
AF: Yale University, Department of Geology & Geophysics, New Haven, CT 06520 United States
AB: Most nominally volatile-free minerals are known to dissolve certain amounts of volatiles such as hydrogen or carbon. Understanding how the solubility of volatiles depends on thermodynamic conditions is critical to our understanding of the dynamics and evolution of this planet. A simple thermodynamic analysis shows that the solubility of a volatile depends on pressure and temperature as $$C_x(P,T)={\rm A}f^{r}_{X_nO_m}(P,T)\mbox{exp}[-(\Delta{\rm E}+{\rm P}\Delta{\rm V})/RT]$$ where $f_{X_nO_m}$ is the fugacity of a fluid phase (such as $H_2O$ or $CO_2$) containing the volatile species X, r is a constant that depends on the mechanism of dissolution, and $\Delta$E and $\Delta$V are the change in internal energy and volume associated with the reaction. Therefore the important quantities controlling the P-T dependence of solubility are the fugacity of relevant fluid phase (water or carbon dioxide), the power, r, $\Delta$E and $\Delta$V. The fugacities of water and carbon dioxide are calculated from their respective equations of state, and the parameters r and $\Delta$V are determined from the experimental data on the dependence of solubility of volatiles on pressure. Experimental results on the solubility of hydrogen in olivine and wadsleyite, as well as solubility of carbon in olivine support a model in which the dominant volatile-bearing defect is a fully charge-compensated defect at Mg- and Si-site ($(2H)^{\times}_{Mg}$ and $C^{\times}_{Si}$) respectively. The present analysis shows that the volume term ($\Delta$V) corresponds approximately to the volume of MgO ($\sim$11 $cm^{3}$/mol) and $SiO_2$ ($\sim$22 $cm^3$/mol) in these minerals for hydrogen and carbon dissolution respectively. Due to a relatively small volume change, hydrogen solubility in minerals increases significantly with pressure, whereas the large volume change associated with carbon dissolution limits its solubility at high pressures. We note that although the dominant volatile-bearing defects are fully charge-compensated defects, defects that control the transport properties are in most cases charged defects. Thermodynamic analysis of experimental data on these properties must distinguish the contribution of dominant defects from that of minor defects.
DE: 3904 Defects
SC: Tectonophysics [T]
MN: 2003 Fall Meeting