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- Creator:
- Su, Xue and Zhang, Youxue
- Description:
- The H2O concentration and H2O/Ce ratio in olivine-hosted melt inclusions are high (H2O up to 1410 ppm; H2O/Ce up to 77) in lunar sample 74220 but lower (H2O up to 430 ppm; H2O/Ce up to 9.4) in all other lunar samples studied before this work. This difference is absent for other volatiles (F, S, and Cl) in melt inclusions in 74220 and other lunar samples. Because H2O (or H) is a critical volatile component with significant ramifications on the origin and evolution of the Moon, it is important to understand what causes such a large gap in H2O/Ce ratio between 74220 and other lunar samples. Two explanations have been advanced. One is that volcanic product in sample 74220 has the highest cooling rate and thus best preserved H2O in melt inclusions compared to melt inclusions in other samples. The other explanation is that sample 74220 is a localized heterogeneity enriched in some volatiles. To distinguish the two possibilities, here we present new data from three rapidly cooled lunar samples: olivine-hosted glassy melt inclusions (OHMIs) in 74220 regolith and 79135 regolith breccia, and pyroxene-hosted glassy melt inclusions (PHMIs) in 15597 pigeonite basalts. If the gap is due to the difference in cooling rates, samples with cooling rates between 74220 and other studied lunar samples should have preserved intermediate H2O concentrations and H2O/Ce ratios. Our results show that melt inclusions in 79135 and 15597 contain high H2O concentrations (up to 969 ppm in 79135 and up to 793 ppm in 15597) and high H2O/Ce ratios (up to 21 in 79135 and up to 13 in 15997). Combined with literature data, we confirm that H2O/Ce ratios of different lunar samples are positively correlated to the cooling rates and independent of the type of mare basalts. Our work bridges the big gap in H2O/Ce ratio among 74220 and other lunar samples. We hence reinforce the interpretation that the lunar sample with the highest cooling rate best represents pre-eruptive volatiles in lunar basalts due to the least degassing. H2O, F, P, S and Cl concentrations in the lunar primitive mantle are also estimated in this work. and *** 2024-03-19: In addition to the files in the previous version, this updated deposit contains more data files as the supplementary files of the paper. For example, we added a summary excel file containing data that are used for figures in the paper, and an excel file contains data in the tables of the paper for easy use by readers. See ReadMe.txt for changes.
- Keyword:
- Water in the Moon, volatiles in the Moon, olivine-hosted melt inclusions, pyroxene-hosted melt inclusions, F/P ratio, S/Dy ratio, Cl/Ba ratio
- Discipline:
- Science
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- Creator:
- Su, Xue, Zhang, Youxue, Liu, Yang, and Holder, Robert M.
- Description:
- It is commonly thought that volcanic glass only records volatile loss during the eruptions in the Moon. However, our recent work shows that Na, K and Cu (moderately volatile elements) in lunar 74220 orange glass beads are enriched near the bead surfaces and depleted in the bead interiors, forming an overall “U-shaped” profile. The “U-shaped” profile means that rather than being “lost” into space, Na, K and Cu were “gained” into the volcanic glass during the eruption, which is contrary to the “volatile loss” story. Three different instruments (EMP, SIMS and LA-ICP-MS) were used to verify the discovery. 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. 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 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.
- Keyword:
- Moon, Volcanic orange glass beads, Moderately volatile elements, Outgassing and in-gassing, and Cooling time scales
- Citation to related publication:
- Su, X., Zhang, Y., Liu, Y. and Holder, R.M. (2023) Outgassing and in-gassing of Na, K and Cu in lunar 74220 orange glass beads. Earth and Planetary Science Letters 602. https://doi.org/10.1016/j.epsl.2022.117924
- Discipline:
- Science