Sculpting the Volatile Content of the Earth through Giant Impact-induced Atmospheric Loss and Magma Oceans

Using new noble gas data and giant impact calculations, we propose a sequence of events during the late stages of Earth's accretion that sets the terrestrial volatile budget and the composition of the early atmosphere. Noble gas data from the deep mantle record a signature of nebular ³He/²²Ne; however, the ³He/²²Ne ratio in the depleted mantle is at least a factor of 6.5 higher. The high ³He/²²Ne of the depleted mantle requires at least two episodes of atmospheric loss and magma ocean outgassing after the dispersal of the solar nebula. The last atmospheric loss and magma ocean outgassing event was likely associated with the Moon-forming giant impact. The large difference in ³He/²²Ne ratio between the deep and the shallow mantle indicates that these magma oceans did not completely homogenize the mantle. We find that the heterogeneous deposition of impact energy combined with perovskite's high liquidus temperature and low entropy gain during shock compression leads to incomplete melting of the mantle during giant impacts. We suggest that the viscosity contrast between a fully-melted upper mantle and partially-melted/solid lower mantle inhibited complete mixing after a giant impact and aided preservation of distinct ³He/²²Ne reservoirs in the mantle. At least two giant impact events are also required by a new formation model for the Moon. The hypothesis posits a giant impact onto a fast-spinning Earth to produce a lunar disk with bulk silicate Earth composition. The Earth's high spin rate would have been generated by one or more giant impacts prior to the Moon-forming event. We find that fast-spinning planets more easily lose their atmospheres via ground motion from the impact-induced shock wave. Thus, the new model for lunar origin facilitates the atmospheric loss required by the ³He/²²Ne data. Giant impact-induced atmospheric loss, particularly in the presence of a water ocean, provides an explanation for other features of the terrestrial volatile budget, such as the depletion of N compared to C. If so, the late veneer contribution to the volatile budget was not sufficiently large to overprint the volatile characteristics acquired and sculpted during the main stages of Earth's accretion. Finally, the timing of the end of volatile outgassing associated with giant impact-induced magma oceans is constrained by the I-Pu-Xe system. Our new Xe data from MORBs indicate that catastrophic outgassing of the mantle ceased between 31 and 52 Ma, significantly earlier than previous estimates of around 100 Ma. Since the Moon-forming giant impact generated the last magma ocean, the formation of the Moon occurred between 31 and 52 Ma.