Redox conditions in magma oceans (MOs) have a key influence on the mass and composition of Earth's early atmosphere. If the shallow part of the MO is oxidized, it may be overlain by an H 2O-CO 2 atmosphere, but if the near-surface magma is close to equilibrium with Fe-rich alloy, then the atmosphere will consist chiefly of H 2, H 2O, and CO, and on cooling will be rich in CH 4. Although MOs are intimately associated with core-forming metal, the redox conditions in their shallow parts are not necessarily reducing. The magmatic Fe 3+/Fe T ratio is set by equilibrium with metal at depth and homogenized through the magma column by convection. Indirect evidence suggests that the Fe 3+/Fe T ratio of magmas in equilibrium with alloy at high pressure is greater than at low pressure, such that the shallow part of the MO may be comparatively oxidized and coexist with an atmosphere consisting chiefly of H 2O and CO 2. The mass of the atmosphere is dictated by the concentrations of volatile-species dissolved in the magma, which in turn are determined by partitioning between magma and alloy. Very strong partitioning of C into alloy may capture most of the carbon delivered to the growing planet, leaving behind a C-poor bulk silicate Earth (BSE) and a C-poor atmosphere. However, modest solubility of CH 4 in the magma may allow the BSE to retain significant C. Alternatively, if partitioning of C into alloy is extreme but the fraction of metal equilibrated with the MO is small, the alloy may become saturated with diamond. Floatation of diamond in the MO may retain a substantial inventory of C in the early mantle. BSE C may also have been replenished in a late veneer. Following segregation of metal to the core, crystallization of the MO may have prompted precipitation of C-rich phases (graphite, diamond, carbide), limiting the C in the early atmosphere and creating a substantial interior C inventory that may account for the large fraction of BSE carbon in the mantle today. Such precipitation could have occurred owing to a combination of the redox evolution of the crystallizing MO and cooling.
Bibliographical noteFunding Information:
I am grateful for the hospitality of colleagues at the Research School of Earth Sciences, ANU, where this manuscript was prepared during sabbatical leave. The paper was improved by the comments of Bernard Marty and two anonymous referees. This work was supported by Grants from NASA ( NNX11AG64G ) and NSF ( EAR1119295 ).
- Deep carbon cycle
- Earth's early atmosphere
- Magma ocean
- Oxygen fugacity