Complex Redox Histories of Lunar Pyroclastic Beads Revealed by Spatially Correlated Chromium and Vanadium Valences

Abstract Chromium and vanadium valence measurements were obtained on 17 lunar glass beads from Apollo 15 and 17 regolith materials using microscale X‐ray absorption spectroscopy methods. Interior Cr valences ranged from 1.97 ± 0.02 to 2.88 ± 0.02 (Cr 2+ to Cr 3+ ). Interior V valences ranged from 2.82 ± 0.02 to 3.76 ± 0.10 (V 2+ /V 3+ mix to V 3+ /V 4+ mix). The interior valences of most beads cluster near V 3+ and Cr 2+ /Cr 3+ ≅ 0.6, that is, close to valences expected at IW‐1, but there is significant variability and several outliers exist. For main cluster beads, Cr valence‐inferred f O 2 ranged from IW‐1.5 to IW+1. These beads have V valence‐inferred f O 2 ranges from IW‐2 to IW. These ranges significantly overlap but V tends to be slightly more reduced than Cr, suggesting there could be some decoupling of the Cr and V barometers. Valences for the Apollo 15 glass beads are tightly clustered, as are the Apollo 17 bottom drive tube samples 73001. In contrast, the Apollo 17 upper‐drive tube samples 73002 are variable. Cr in the bead rims tended to be oxidized relative to the interiors, whereas V tended to show no redox difference between the rims and interiors. Processes responsible for establishing the redox states of the rims must be complex. Apparent f O 2 conditions inferred from Cr valence tended to be slightly more oxidized than those inferred from V. Parental magmas may have possessed variable compositions that in turn experienced varying degrees of assimilation of Cr 3+ ‐rich phase(s). Valence‐altering secondary processes may also have been significant.