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, H 2 O, and S loss from the melt. In Stage 3, depths shallower than ~450 m, the rising magma exsolved an additional 800-900 ppm H 2 O 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.Formation of the dike carrying the magma up from the ~550 km deep source is considered to occur by a crack propagation mechanism Head 2003, 2017). The rapid dike-propagation process facilitates gas collection by open-system degassing in the upper part of the dike. This is necessary to achieve the gas volumes required for magma fragmentation at 600 m depths, and the magma-ascent velocities to explain the wide areal distribution of the bead deposit. The explosive nature of the picritic orange glass eruption, and the homogeneity of the bead compositions, are consistent with this gasassisted eruption scenario, as is the evidence of a Fe-metal forming reduction event during Stage 2 followed by a Stage 3 oxidation event in the ascending magma.

The Cauchy 5 Small, Low‐Volume Lunar Shield Volcano: Evidence for Volatile Exsolution‐Eruption Patterns and Type 1/Type 2 Hybrid Irregular Mare Patch Formation

“…We first describe the setting and characteristics of the Cauchy 5 small shield volcano and its related deposits and features. Second, we explore the predictions of models for the intrusion and eruption of dikes producing small-volume eruptions and the nature of the predicted effusion and volatile release phases in such eruptions (Rutherford et al, 2017;. We then compare these predictions with the characteristics of the Cauchy 5 small shield volcano and the two types of IMPs and conclude with a discussion of the formation of Cauchy 5 and the origin of the unusual ages of its IMP populations.…”

“…As the magma rise speed decreases to less than~1 m/s, Phase 4 is initiated. Due to the very slow magma rise speed, ascending bubbles of CO released at great depth (Rutherford et al, 2017) have sufficient time to form, expand, rise, and coalesce into slugs. Strombolian activity (bursting of coalesced gas slugs at the top of the lava lake; Blackburn et al, 1976;Ripepe et al, 2008) will be the result.…”

“…Strombolian activity (bursting of coalesced gas slugs at the top of the lava lake; Blackburn et al, 1976;Ripepe et al, 2008) will be the result. However, beneath the undisturbed parts of the lava lake, relatively soluble and therefore shallow-nucleating water and sulfur compounds (e.g., Rutherford et al, 2017) will have had time to exsolve, leading to very high vesicularity.…”

“…These theoretical analyses of the nature of low-volume, low-effusion rate small shield volcano eruptions on the Moon (Figure 11b) (e.g., Rutherford et al, 2017;Wilson et al, 2019;Wilson & Head, 2017a) provide a framework of predictions for assessing and interpreting the nature, structure, morphology, and history of the Cauchy 5 small shield volcano.…”

A theoretical model for the formation of Ring Moat Dome Structures: Products of second boiling in lunar basaltic lava flows

“…Lunar magmas are erupted into a vacuum, and the presence of even very small amounts of volatiles can ensure that explosive activity occurs (Wilson and Head, 1981). Based on direct analysis of returned pyroclast samples and high-pressure laboratory experiments on samples with the same composition, lunar magma volatiles are found to be dominated by up to ~1000 ppm CO released mainly at depths between 500 and 50 km with an admixture of at least several hundred ppm H2O and sulfur species released at depth less than 500 m (Rutherford et al, 2017). The expansion of gas bubbles nucleating with initial diameters of ~10 m causes close packing to occur when the bubbles have grown to a few hundred microns, and the magma is then fragmented into sub-mm size pyroclastic droplets to emerge from the vent in a hawaiian-style fire fountain or curtain-of-fire eruption fed by a lava volume flux of ~10 6 m 3 s -1 (Wilson and Head, 2017a).…”