Spectral and chemical properties of rare lunar impact glasses: Implications for the regolith on Mercury

The Apollo 14 regolith breccia 14076 contains glass beads with extreme compositions that fall in two groups; (1) both high-alumina, silica-poor (HASP), interpreted as evaporation-residues, and (2) gas-associated spheroidal precipitates (GASP), impact vapor condensed to glassy spheroids [1,2]. Here, we present high-resolution quantitative EPMA data of the previously studied thin section 14076,5 and for the first-time data on thin section 14076,21. In addition, we present spectroscopic data of these unique glass samples, including in-situ mid-IR reflectance spectroscopy and Raman spectroscopy. The spectral characterization of the unique glasses in sample 14076 is in preparation for the ESA/JAXA mission BepiColombo to Mercury, which carries the Mercury Radiometer and Thermal Infrared Spectrometer (MERTIS) that will map the surface of the planet in the spectral range of 7 to 14 µm. Given the higher gravity and surface temperature on Mercury compared to the Moon, the regolith on Mercury likely contains exotic materials similar to HASP and GASP that formed in energetic impacts. Models suggest that these materials have more than an order of magnitude higher abundances on Mercury compared to the Moon [3], at levels detectable by the BepiColombo mission. A potential quantification of these materials in the Mercury regolith requires detailed spectral and chemical characterization of similar materials in the laboratory. Here, Apollo 14 sample 14076 provides a unique “analogue” material.

Quantitative elemental maps for SiO₂, TiO₂, Al₂O₃, CaO, MgO, FeO, Na₂O, K₂O, CrO, and S were measured with a JEOL JXA 8530F electron microprobe operated at 15 keV accelerating voltage and a probe current of 80 nA. Spatial resolution varied with the highest resolution scan steps of 0.1 µm and mapped areas of up to 200 µm in diameter. Average compositions of homogeneous areas were manually extracted from the quantitative maps using Fiji (ImageJ). Fourier Transform Infrared (FTIR) spectra were measured with a Bruker Hyperion 3000 microscope attached to a Vertex 80v FTIR-spectrometer and equipped with a focal plane array (FPA) mapping detector and a 15x Cassegrain objective. Spectra were recorded over the spectral range from 2.5 to 16.7 µm wavelength. Raman spectra were recorded with a high-resolution Horiba HR800 spectrometer and an Olympus microscope, focusing the laser beam to a 1-2 µm spot on the sample. 

The GASP spherules and agglutinates fall in two compositional categories. Si-GASP [2] have SiO₂ concentrations of 97±2 wt.% with ~1 wt.% FeO and 0.5 wt.% CaO. In contrast, Fe-GASP have lower SiO₂ contents of 61±6 wt.%, and high FeO (27±7 wt.%) and MgO (up to 20 wt.%) concentrations. These two GASP components are the product of silicate melt immiscibility in the SiO₂-FeO and SiO₂-MgO systems [4]. The observation of Fe-metal nuggets in some of the FeO-rich glasses constrain to fO₂ of the GASP beads at the iron-wüstite (IW) mineral buffer. The most abundant HASP particles are glassy and fall in a very narrow compositional field with 22±1 wt.% SiO₂, 52.4±0.8 wt.% Al₂O₃, and 27.9±0.6 wt.% CaO. Quench crystals in some HASP particles can be identified as the lunar mineral yoshiokaite [5], for which we provide the first FTIR-spectrum. Figure 1 shows a data compilation of mid-infrared spectroscopy “Christiansen Features” in glasses as a function of the SiO₂ concentration. The HASP and GASP glasses have extremely low-, and extremely high SiO₂ contents respectively, and accordingly extreme spectral properties not previously characterized by FTIR or Raman.

Finally, our observations of Fe metal nuggets in the FeO-bearing GASP glass provide a direct constraint on fO₂ in the condensing impact vapor plume, at the IW buffer. This is the same oxygen fugacity as determined for lunar mare basalts at IW-2 to IW+0.2 [11], which are a potential impact target and source of the material in the plume. This suggests that evaporation and condensation inside the plume did not affect the fO₂.

Figure 1: Relationship between the FTIR Christiansen Feature and the SiO₂ concentration in the respective glasses in wt.%. The data represent experimentally synthesized glasses with compositions representing compositions from Mercury, the Moon, Venus, Mars, and the Earth, laser impact experiments, and terrestrial impact glasses and tektites [6-10]. The HASP and GASP glasses fall in the shaded regions with very high- and very low SiO₂ contents, significantly extending the compositional range for which mid-IR spectral properties are determined.

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