Moderately Volatile Elements (MVEs) in chondritic meteorites vary as a smooth function of their condensation temperatures, calculated for a gas of nebular composition (Tc). However, their abundances in differentiated bodies (e.g., Earth) do not vary systematically with Tc, and are variably depleted relative to the solar (CI) composition. Unlike in chondrites, volatile depletion in planetary bodies may also arise from accretion of volatile-poor material, evaporation following giant impacts, core formation and collisional erosion. These processes occur at conditions different to those of the solar nebula, rendering Tc alone inappropriate in understanding the behaviour and origin of volatile elements in the terrestrial planets. Here, we adopt a two-pronged experimenal and isotopic approach:
Element volatilies are measured directly in controlled-atmosphere furnace experiments over a range of oxygen fugacities (fO2) and temperatures. Although most elemental vapour pressures increase at low fO2 and high T, their relative volatilities change with both parameters, such that they differ significantly from their Tc. As a result, the signature of volatile depletion occurring under more oxidising conditions (i.e., during late-stage planetary accretion) should be distinct from that arising due to nebular depletion.
However, this relationship is complicated by the dual siderophile/lithophile character of some MVEs, making it difficult to ascertain whether depletion arose due to core formation or volatile depletion. To that end, the stable isotopes of heavy elements in planetary basalts may record the fingeprints of such processes. The Fe isotope compositions of inner solar system bodies point to the influence of volatile depletion under nebular conditions, a view consisent with the Zn isotope composition of the Earth, which falls on the carbonaceous chondrite array. However, the step-wise depletion in volatile elements in the Moon, combined with its heavy Zn isotopes, suggest that it underwent additional volatile loss under more oxidising conditions, likely during the giant impact.