Water Controls on Explosive Volcanism With Implications for Crustal Magma Chambers on the Moon

Abstract The explosive volcanic eruptions were common on the Moon, as revealed by the returned lunar glass beads along with the widely occurring pyroclastic deposits. It is typically considered that under the reduced magmatic conditions, CO and/or H 2 were responsible for explosive volcanism that generated the pyroclastic deposits on the Moon. However, the discovery of magmatic water in lunar pyroclasts caused a paradigm shift from a dry to a wet Moon, and yet the role of water in lunar explosive volcanism has remained poorly constrained. Furthermore, the role of bubbles, as indicated by the presence of vesicles in glass beads, in causing the eruption explosivity is yet to be determined under lunar magmatic conditions. Here, we use a coupled bubble growth and magma ascent model to show that H 2 O, rather than CO or H 2 , is the primary driver for explosive lunar volcanism. The modeled water contents in the magma at fragmentation are consistent with those inferred from measurements in lunar glasses as well as pyroclastic deposits. The model results show that at ≳500 ppm H 2 O and ≲20 ppm CO, degassing of H 2 O and diffusive bubble growth at shallow depths of a few hundred meters increase the gas volume sufficiently high to cause magma fragmentation. Our calculations suggest that the transient stalling of magma at ≲17 km crustal depths is likely a favorable condition for eruption explosivity on the Moon. The implications of our eruption model outcomes for the Moon's source and surface water contents are also evaluated.