Thermodynamics of Element Volatility and its Application to Planetary Processes

Abstract

3) Evaporation on small planetary bodies (section 4.0.) 4) Evaporation during giant impacts (section 4.0.)To constrain the conditions under which these processes occur necessitates thermodynamic data of gaseous-and condensed species. Therefore, a review of our knowledge and lack thereof, is also presented (section 2.0.). Much of our information on thermodynamic quantities of gaseous species comes from spectrometric measurements, undertaken from the 1950s onwards, largely on simple (binary and ternary) systems (e.g., see Brewer 1953;Margrave 1967;Lamoreaux et al. 1987, and section 2.0.). This information is now readily available in compilations of thermodynamic data such as JANAF (Chase 1998) or IVTAN (Glushko et al. 1999). However, the application of these data to more complex, natural geological systems is less straightforward.Instead, for lack of any other comprehensive volatility scale, 50% nebular condensation temperatures, Tc, (Grossman and Larimer 1974;Lodders 2003), which describe the temperature at which half of the abundance of an element is condensed in the solar nebula, are frequently used to discuss elemental volatility in planetary environments. However, these Tc are strictly relevant only at the pressure and temperature conditions of a solar-composition protoplanetary accretion disk (e.g., the solar nebula). Namely, at low total pressures (PT ≲10 -2 bar), solar or close to solar metallicity 2 (i.e., PT ~ PH2 + PH + PHe), oxygen fugacities ~ 6 log bar units below the Iron -Wüstite (IW) buffer (due to the low H2O/H2 ratios, 5 × 10 -4 ; Rubin et al. 1988;Grossman et al., 2008). Boss (1998) and Woolum and Cassen (1999), to give two examples, used astronomical observations to model midplane temperatures in protoplanetary accretion disks in the planet-forming region and found temperatures <1600 K.Petrological observations of Calcium, Al-rich inclusions (CAI) with the highly fractionated group II rare-earth element (REE) pattern 3 (Boynton 1975) indicate they formed by fractional condensation (Boynton 1975;Davis and Grossman 1979) at temperatures between 1676 K, 1463 K, 1296 K at total pressures (PT) of 10 -3 bar, 10 -6 bar, and 10 -9 bar, respectively (Kornacki and Fegley 1986). At total pressures above ≈10 -2 bar, liquids condense in place of solids (Ebel, 2004) and departures from solar composition (e.g., by changes in metallicity or H-loss on a planetary body) lead to condensation temperatures different to those in a solar-composition system (Larimer and Bartholomay 1979; Lodders and Fegley 1999; Schaefer and Fegley 2010). Thus, the complexity of evaporation and condensation reactions are such that, at conditions outside this range, the relevant gas species, their equilibrium partial pressures and mechanisms driving their escape diverge greatly. Potentially powerful tracers of the conditions under which these gas-liquid/solid processes occur are the moderately volatile elements (MVEs), defined as those that condense from a solar composition nebular condensation and evaporation of silicates under post-n...

Isotopic evidence for the formation of the Moon in a canonical giant impact

“…However, even if volatilization induced V isotope effects, then isotopic fractionation during either (i) partial condensation of an originally BSE-like vapor phase or (ii) evaporation of a partially molten proto-Moon 22 would both produce heavy isotope enrichments relative to the BSE, which is opposite to what we observe here for lunar V. If the Moon represents a partial condensate of a protolunar disk 23 that resulted in a light V isotope composition of the Moon relative to Earth, then we would expect similarly refractory elements like Ti and Sr to show similar stable isotope offsets as V, which is not observed 24 , 25 . Furthermore, equilibrium isotope exchange reactions in the protolunar disk may be expected to produce limited isotope fractionation because V, like Si or Ti, is associated with at least one atom of oxygen (e.g., VO, VO 2 , V 4 O 10 ) in both the solid and gas phases 26 , which limits the potential for significant equilibrium isotope fractionation 27 . The partial vaporization behavior and thermodynamics of V under protolunar disk conditions are unknown, making quantitative assessments of such equilibrium effects very difficult.…”

Tidal pull of the Earth strips the proto-Moon of its volatiles

“…The composition of the gas at the surface of the magma is computed using a thermodynamic approach (Appendix I) taking advantage of recent laboratory measurements of chemical activities of melt oxide species (Sossi and Fegley Jr. (2018); Sossi et al (2019)). The equilibrium partial pressure may be calculated for any gas species containing a metal, M , according to the generalised congruent vaporisation reaction:…”

Vertically Resolved Magma Ocean–Protoatmosphere Evolution: H2, H2O, CO2, CH4, CO, O2, and N2as Primary Absorbers

“…This emphasizes that redox evolution of the planetary mantle and atmospheric speciation are tightly interconnected, as suggested by laboratory experiments (Deng et al., 2020; Grewal et al., 2020). Considering more realistic outgassing speciation as a result of the rock composition (Herbort et al., 2020; Schaefer & Fegley, 2017; Sossi & Fegley, 2018) is thus necessary. In addition, we here ignore the potential effects of dynamic trapping of volatiles due to inefficient melt drainage out of the freezing front during solidification (Hier‐Majumder & Hirschmann, 2017).…”

“…The chemical composition (as quantified by its redox state, f O 2 ) of the magma ocean controls the speciation of outgassed volatiles (e.g. Sossi & Fegley, 2018). These chosen relations hence serve to illustrate the solubility behaviour for a mantle composition that allows the stability of given volatile species.…”

“…Considering more realistic outgassing speciation as a result of the rock composition (Schaefer & Fegley, 2017;Sossi & Fegley, 2018;Herbort et al, 2020) is thus necessary. In addition, we here ignore the potential effects of dynamic trapping of volatiles due to inefficient melt drainage out of the freezing front during solidification (Hier-Majumder & Hirschmann, 2017).…”