OXIDATION STATE OF LUNAR IMPACT MELTS: APPLICATION OF A NEW CR MELT OXYBAROMETER

The 842 pounds of lunar samples returned by the six Apollo missions were all sourced within the lunar regolith. The petrologic history of the Moon is preserved in the regolith which contains rock and mineral fragments as well as both volcanic and impact-derived glasses. Impact-melt glass has been identified in lunar meteorites and agglutinates, which are the unique products of micrometeorite impacts into the lunar regolith. Volcanic glass beads are the result of explosive, fire-fountaining eruptions fueled by CO or H₂O degassing. These glass beads are especially valuable when investigating the interior of the Moon as these glasses represent direct melts from the mantle and record information about their formation conditions and ascent through the crust.

Both glass types preserve information on the oxygen fugacity (fO₂) conditions prevalent during their formation. Determining the fO₂ of impact glasses allows for assessment of volatile conditions during the impact event; the fO₂ of volcanic glasses records information on the formation and evolution of the lunar interior. Melt fO₂ is quantified via multivalent element ratios, generally Fe^2+/3+. Iron, however, is not ideal for the reduced conditions of the lunar interior and surface. The transition of Cr³⁺ to Cr²⁺ occurs at lower fO₂ making it a more suitable oxybarometer for lunar melts.

We have developed a calibration set by which to measure Cr oxidation states in glasses with X-ray Absorption Spectroscopy (XAS). Synthetic glasses representing basalts to basaltic andesites from Mars, the Moon, and earth (SiO2 wt.% 38.5-57.9, FeO wt.% 22.9-5.82) were synthesized and equilibrated under oxygen fugacities of IW-2, IW, QFM, and air. XAS analyses were completed at the GSECARS beamline at Argonne National Lab. fO₂ predictions have an RMS error of 1.28.

To demonstrate the use of this method, we have measured Cr in situ within volcanic, agglutinic, and impact melt glasses in lunar samples. Variation in fO₂ is seen within the samples with the volcanic beads ranging from IW to IW-1 while the impact melt and agglutinic glass reaches a lower fO₂ of IW-2. Measuring fO₂ signatures in glass beads and comparing them to those found in impact derived glass can allow us to determine the effects of impacts on intrinsic fO₂ signatures.