IN SITU MEASUREMENT OF FERRIC IRON IN LUNAR GLASS BEADS USING FE-XAS

The measurement of iron valence state in geologic samples is non-trivial. Synchrotron X-ray absorption spectroscopy (XAS) presents the opportunity to measure Fe³⁺ at microscales in situ, thereby allowing for resolution of potential variations within the sample. We have developed glass calibration standards and a software model to allow for accurate XAS measurements of glasses (Dyar et al., 2016) with accuracies of ±3.6% for %Fe³⁺. We apply that calibration here to analyses of glass Fe³⁺contents from multiple lunar glass beads from Apollos 11, 14, 15, and 17.

Fe-XAS analyses were performed on the glass beads at the Advanced Photon Source GSECARS beamline using a beam size ~2-5 µm. Average %Fe³⁺ values measured in the glasses ranged from 6.4 to 35.6. Beads exhibited both increasing and decreasing %Fe³⁺ from core to edge, however, many analyses were within the margin of analytical error suggesting sample Fe³⁺ homogeneity at this scale. %Fe³⁺values were not correlated with major element chemistry, bead diameter, or melt physical properties.

The range of Fe³⁺ contents observed in the glasses may result from degassing processes, i.e., rapid diffusive loss of H or OH from the melt droplet during magma ascent. Redox diffusivity measurements suggest the time required for complete oxidative equilibration of a melt droplet (diameter = 50-100 µm) during initial fire fountain eruption would be ~0.3 s. This could explain the high, homogeneous %Fe³⁺ values observed in some beads. If quenched prior to attaining equilibrium this would result in higher %Fe³⁺ values at the bead edge grading to lower %Fe³⁺values in the interior such as those measured in multiple beads.

The presence of low %Fe³⁺ values and beads zoned in redox concentrations may result from a combination of post-eruptive oxidation followed by subsequent reduction either in the lunar vacuum or in the dissipating gas cloud that may have been fairly reducing due to the addition of H from the degassing melt. This would result in a reduction front propagating from the bead exterior to the interior. If quenched prior to attaining equilibrium this would result in zoned beads with lower %Fe³⁺ values at the bead edge grading to higher %Fe³⁺ values in the interior where the reduction front had not yet reached such as those measured in multiple beads.