1Xue Su,1Youxue Zhang,2Yang Liu,1Robert M.Holder
Earth and Planetary Science Letters 602, 117924 Link to Article [https://doi.org/10.1016/j.epsl.2022.117924]
1Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, MI 48109, USA
2Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
Formed in a fire-fountain eruption, lunar 74220 orange glass beads are excellent recorders of the volcanic plume associated with the gas-driven eruption on the Moon. The bead surface is often coated with a thin (about 20 nm thick) and volatile-rich layer, with scattered euhedral vapor-condensates (the largest dimension is of a few μm). Hence, bulk analyses of glass beads show high concentrations of volatiles and moderately volatile elements, without the characteristic depletion of these elements in lunar basalts. The bead interiors contain lower concentrations of volatiles than olivine-hosted melt inclusions in the same sample, indicating loss of volatiles. These observations are commonly explained as outgassing from the melt during eruption and subsequent re-condensation of some of the gas species onto the surface of the quenched beads. Here, we report the first discovery of pervasive “U-shaped” Na, K and Cu concentration profiles across lunar 74220 orange beads with Na, K and Cu enrichment near bead surfaces and Na, K and Cu loss in the bead interiors. We propose that such U-shaped Na, K and Cu profiles were formed by initial outgassing and subsequent in-gassing of Na, K and Cu when the beads were flying from the vent onto the surface through the cooling volcanic gas plume. Hence, in-gassing and the formation of surface coatings are two processes that are genetically linked during the pyroclastic eruption and evolution of the gas cloud. The condensation of grains on bead surfaces is due to oversaturation of solid phases in the cooling volcanic plume.
To quantify the processes that formed the U-shaped profiles, we developed a diffusion and surface-equilibrium model using available literature data on Na and Cu diffusivity in basaltic melts. The model assumed an asymptotic cooling history for spherical glass beads, a homogeneous initial composition, and surface equilibrium with the ambient atmosphere. The model reproduced U-shaped Na and Cu concentration profiles with outgassing at high temperature and subsequent in-gassing as beads cooled. By fitting the measured Na and Cu profiles, we found that the cooling time scales of individual orange glass beads range from 48 to 179 s. This is the first time that both outgassing and in-gassing were modeled and the cooling time scales of individual 74220 volcanic orange glass beads were estimated. The discovery of the U-shaped profiles of moderately volatile elements inside volcanic beads provides significant constraints on partial pressures of relevant volcanic gas species in the eruption plume.