Prolonged cooling and degassing of Apollo 17 volcanic glasses on the lunar surface.

Abstract

Studying lunar pyroclastic volcanism provides key insights into the thermo-chemical evolution of the lunar mantle and the Moon's volatile budget. Pyroclastic deposits, although volumetrically minor compared to mare basalts, are derived from deeper, more primitive, and volatile-rich mantle sources. Here we synthesize volatile degassing recorded by Apollo 17 sample 74220, one of most studied pyroclastic deposits. By modeling the volatile degassing of olivine-hosted melt inclusions, melt embayments, and glass beads, we show that volatile depletion of these samples cannot be explained solely by degassing during free-flight, as previously proposed. Instead, the data require extended thermal histories, with beads undergoing slow cooling and prolonged degassing on the lunar surface for years, a scenario further supported by thermal modeling. Our findings suggest that pyroclastic deposits remained active volatile sources well beyond eruption, which is potentially important for sustaining a local transient lunar atmosphere and the long-term volatile cycle of the Moon.