Work Description

Date: 3 February, 2023

Dataset Title: Data of the paper "Outgassing and In-gassing of Na and Cu in Lunar 74220 Orange Glass Beads

Dataset Creators: Xue Su, Youxue Zhang, Yang Liu, and Robert M. Holder

Dataset Contact: xuesu@umich.edu

Funding: 80NSSC19K0782 (NASA), JPL’s Strategic University Research Partnerships (SURP) program

Key Points:
- Discovery of unexpected in-gassing of Na, Cu, K into degassed orange beads
- Constraints of cooling rates of individual beads through numerical modeling
- Provide key information for constraining the volcanic plume (cooling and expansion)

Research Overview:
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.

Methodology:
Spherical orange glass beads mostly of 200-300 µm in diameter were handpicked from pristine lunar sample 74220, 892. These glass beads were embedded in epoxy and polished to expose a section that cuts roughly through the geometric center of the beads. We examined the polished sections of orange glass beads using a JEOL JSM-7800FLV field-emission Scanning Electron Microscope (FE-SEM) at the University of Michigan and a Zeiss 1550 VP FE-SEM at the California Institute of Technology. Ten homogenous glass beads with no obvious crystals were selected for subsequent compositional analyses. Major and minor element compositions were determined using a CAMECA SX-100 Electron Microprobe (EMP) at the University of Michigan. Elemental profiles from rim to rim across the center of the orange glass beads were obtained. The procedure was optimized for an improved 1σ uncertainty of Na2O (±0.005wt%) and discernible Na2O variation trends across the orange glass beads. To obtain higher precision data, we analyzed selected orange glass beads using a CAMECA IMS 7-f GEO Secondary Ion Mass Spectrometry (SIMS) at Caltech. The traverses during SIMS measurements were placed near EMP traverses. To examine the distribution of Cu and K and further confirm the distribution of Na in the 74220 orange glass beads, we also conducted elemental mapping using Laser-Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) at the University of Michigan. A 7 × 7 μm square laser beam was rastered across the samples and produced 23Na, 39K and 63 Cu distribution maps of the samples. We converted LA-ICP-MS elemental maps to radial concentration profiles by averaging points of equal distance from the bead surfaces to the bead centers. The outermost points were excluded to avoid complications associated with partial ablation of epoxy or volatile-enriched surface condensates. Each radial elemental profile was then mirrored to provide a complete profile across the glass beads for illustration and model fitting.

Instrument specifications: SEM, EMPA, SIMS, LA-ICP-MS

The span of time that the data were collected and processed: 2 years

Files contained here:
The folder has 4 subfolders based on the data sources (instruments that were used in the work). The data contained in the folders are raw data. For processed data please see the supplementary materials of Su et al. (2023) EPSL paper. Ten orange glass beads were studied in this work (names: GB-A, GB-B, GB-C, GB-B1, GB-B2, GB-B5, GB-B6, GB-B7, GB3, GB-S6). The file names in the folders indicate which sample that was analyzed.
In the LA-ICP-MS folder: Each .csv file is an exported raw data file from LA-ICP-MS mapping and records the counts (cps) in each pixel. The cps matrix in size M*N (M rows and N columns) is corresponding to the mapping area: (M*beam size) in the vertical dimension and (N*beam size) in the horizontal dimension. The produced elemental map has M*N pixels. Epoxy may have negative values in the cps matrix.
In the SIMS folder: Raw data files were directly produced by the SIMS software which contain lots of information besides the data itself. For raw SIMS data, please see sheet named "Results", which shows the isotopic ratios of the targeted elements to internal standard with uncertainties. For processed SIMS data please see the summary excel file in the same folder.

Related publication(s):
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

Use and Access:
This data set is made available under an Attribution-NonCommercial 4.0 International license (CC BY-NC 4.0).

To Cite Data:
Su, X., Zhang, Y., Liu, Y., Holder, R. M. Data of the paper "Outgassing and In-gassing of Na and Cu in Lunar 74220 Orange Glass Beads" [Data set], University of Michigan - Deep Blue Data.