A Revised Model for the Ascent and Eruption of Gas-saturated Lunar Picritic

Introduction: Prior to the discovery of H2O dissolved in the picritic lunar glass beads [1], models for the eruption of lunar picritic magmas involved both a column of melt extending deep into the lunar mantle, and a gas-assisted lift of the magma to achieve explosive eruption [2,3,4,5]. In a H-free moon, first gas phase formation would have involved the oxidation of reduced C (graphite) to produce a CO-rich gas (6). As little as 50 ppm C in a picritic magma was calculated as sufficient to cause fragmentation of the picritic orange glass magma as it erupted [5]. Additionally, as much as 400 ppm S and possibly other volatile species were partitioned into the gas phase associated with the orange glass magma during ascent [7]. However, recent experimental studies [8] indicate that gas formation is more complex than indicated in the above referenced studies, and needs to be significantly modified to incorporate CO solubility as Fe-carbonyl at lunar interior oxidation states. Further, the models of conduit flow do not adequately evaluate the dike formation stage of a fire–fountain eruption as is done in the model of [9]. The purpose of this paper is to integrate new data on C-O-H-S volatile speciation and solubility in picritic magmas into a revised eruption model based primarily on the dike formation process proposed by [3,4]. The Data: The melt inclusions trapped in olivine microphenocrysts of the A17 orange glass (74220) contain dissolved C and H ranging from 4 ppm C and 1400 ppm H2O down to 0.4 ppm C and 800 ppm H2O [10,11]. In the same inclusions, S decreases from 860 to 700 ppm as C and H decrease. The orange glass beads formed during the surface fire fountain eruption contain an average of 0.45 ppm C, 10-30 ppm H2O, and 250 ppm S. Diffusion-loss profiles and diffusion data suggest post-bead formation loss was relatively minor for C and S but was significant for H [1]. Experimental data for C and H solubility in a graphite-saturated, orange glass melt at lunar oxidation states (Fig. 1) show how the C content varies with pressure in vapor-saturated magma with~1000 ppm dissolved H2O. Using this