Model for the Origin, Ascent, and Eruption of Lunar Picritic Magmas

Abstract

A model for the origin, ascent, and eruption of the lunar A17 orange glass magma has been constructed using petrological constraints from gas solubility experiments and from analyses of the lunar sample 74220 to better determine the nature and origin of this unique explosive eruption. Three stages of the eruption have been identified. Stage 1 of the eruption model extends from ~550 km, the A17 orange glass magma source region based on phase equilibria studies, to 50 km depth in the Moon. Stage 2 extends from ~50 km to 500 m, where a C-O-H-S gas phase formed and grew in volume based on melt inclusion analyses and measurements. The volume of the gas phase at 500 m depth below the surface is calculated to be 7 to 15 vol% of the magma (closed-system) using the minimum and maximum estimates of CO, H2O, and S loss from the melt. In Stage 3, depths shallower than ~450 m, the rising magma exsolved an additional 800–900 ppm H2O and 300 ppm S, increasing the moles in the gas by a factor of 3 to 4. The closed-system gas phase is calculated to reach ~70 vol% at ~130 m depth, enough to fragment the magma and form pyroclastic beads. However, fragmentation (bead formation) is interpreted to have occurred at depths ranging from 600 to 300 m below the lunar surface based on the pressure necessary to explain the C content of the orange glass beads. The gas volume (70%) required to fragment the ascending magma at this depth is a factor of ~5 greater than the volume determined for closed-system degassing of an orange glass magma at 500 m, strongly implying that the gas was produced by open-system degassing as the magma ascended from greater depths.