  Silicate vapors play a key role in planetary evolution, especially dominating
early stages of rocky planet formation through outgassed magma ocean
atmospheres. Our open-source thermodynamic modeling software "VapoRock"
combines the MELTS liquid model (Ghiorso et al., 1995) with gas-species
properties from multiple thermochemistry tables (e.g., Chase et al., 1998).
VapoRock calculates the partial pressures of 34 gaseous species in equilibrium
with magmatic liquid in the system Si-Mg-Fe-Al-Ca-Na-K-Ti-Cr-O at desired
temperatures and oxygen fugacities (fO2, or partial pressure of O2). Comparison
with experiments shows that pressures and melt-oxide activities (which vary
over many orders of magnitude) are reproduced to within a factor of ~3,
consistent with measurement uncertainties. We also benchmark the model against
a wide selection of igneous rock compositions including bulk silicate Earth,
predicting elemental vapor abundances that are comparable (Na, Ca, & Al) or
more realistic (K, Si, Mg, Fe, & Ti) than those of the closed-source MAGMA code
(with maximum deviations by factors of 10-300 for K & Si). Vapor abundances
depend critically on the activities of liquid components. The MELTS model
underpinning VapoRock was calibrated and extensively tested on natural igneous
liquids. In contrast, MAGMA's liquid model assumes ideal mixtures of a limited
set of chemically simplified pseudo-species, which only roughly approximates
the non-ideal compositional interactions typical of many-component natural
silicate melts. Finally, we explore how relative abundances of SiO and SiO2
provide a spectroscopically measurable proxy for oxygen fugacity in
devolatilized exoplanetary atmospheres, potentially constraining fO2 in
outgassed exoplanetary mantles.
