A hydrogen-based oxidation mechanism relevant to planetary formation

• Under reducing conditions H2 is an important phase in the O–H system.
• H2 is easily lost from a cooling magma, explaining the generally dry Moon.
• Fire fountain eruptions on the Moon were driven by H2 gas.
• The high oxidation state of Earth is explained by hydrodynamic escape of H2.
• Loss of 1/3 of an ‘ocean equivalent’ of H2 drives upper mantle f(O2) to QFM.

It is now generally accepted that the Moon formed by collision of a Mars-sized impactor with the Earth (Hartmann and Davis, 1975), a process termed “the Giant Impact”. The oxygen isotope compositions of both bodies are indistinguishable (Wiechert et al., 2001), a result that would require extensive chemical homogenization. Recent simulations have been developed that predict intimate mixing immediately following the impact (Canup, 2012, Ćuk and Stewart, 2012 and Pahlevan and Stevenson, 2007), minimizing the oxygen isotope problem. Nevertheless, striking chemical differences remain. We propose that the process of hydrogen degassing and loss to space during magma cooling can explain differences in the water content, oxygen fugacity (f(O2)), and isotopic composition of Cl and H in the Earth–Moon system. At low f(O2), H2 gas is the stable O–H phase in basalts. The low solubility and rapid diffusivity of H2 explain the presently dry character of most lunar samples. Many of the apparently discrepant observations regarding the hydrous character of the Moon are reconciled by H2 degassing. Early H2 degassing also explains the high f(O2) of Earthʼs mantle. A loss of only 1/3 ‘ocean equivalent’ water from Earth by hydrodynamic escape of H2 would shift the f(O2) of the upper mantle from the very low oxidation state equivalent to the Moon and other primitive differentiated bodies to its present oxidized state (near the fayalite–magnetite–quartz buffer). No other processes, such as late addition of material to Earth or injection of Fe3+-rich deep mantle materials to the upper mantle are required to explain the early elevated oxidation state of Earthʼs upper mantle.

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