Constraints on early Earth's water budget from the evolution of the lunar hydrogen cycle

Abstract

During the Hadean, Earth recovered from the Moon-forming giant impact, became covered with liquid water oceans, and witnessed the onset of plate tectonics and life. Quantifying the abundances, distribution, and chemical states of water in the atmosphere, on the surface, and in the interior of the early Earth is essential to constrain the early evolution of System Earth. Assessing these parameters is hampered by the general dearth of early Earth samples, the difficulty of distinguishing primary signatures from later alteration processes in such samples, leading to large uncertainties on the influx and outflux of water to and from the early Earth. Given the close proximity of Earth and Moon, constraints on the early hydrogen cycle in the Moon may reflect coeval aspects of the water cycle on early Earth. Here, we assess constraints on the lunar water cycle from the time the Moon formed until the end of late accretion at ~3.8 Ga, and implications of these constraints for the early Earth water budget. Dynamic accretion models suggest the Moon initially contained ~455 ppm of water. Recent experimental studies of lunar magma ocean crystallization suggest similarly substantial initial lunar water contents. Hydrogen concentration measurements in lunar plagioclase crystals derived from the magma ocean illustrate that the Moon experienced significant degassing during the solidification of the lunar magma ocean (thought to have occurred between 4.5 and ~ 4.3 Ga). Hydrogen and chlorine systematics in lunar magmatic apatite grains formed between ~4.1 Ga and ~ 3 Ga indicate that lunar hydrogen reservoirs were replenished by volatile delivery during late accretion (~4.1–3.8 Ga), after which the water abundance of the Moon stabilized. Using this knowledge of the lunar water cycle to model Earth's early water budget leads to two scenarios that are consistent with the observed present-day terrestrial water content of 1000–3000 ppm: (1) Earth contained significantly more water than the Moon-forming material immediately after the giant impact, suggesting hydrogen heterogeneity in the initial Earth-Moon system; (2) Earth did not experience significant degassing in the aftermath of the giant impact, and the late accretion mass added to Earth was large and water-rich