T197 Mineral Spectroscopy—Harnessing Energy to Probe Solid Bodies in the Solar System: The G.K. Gilbert Award Session

Tuesday, 27 September 2016
8:00 AM - 12:00 PM
Room 201

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

Paper number: T197-8
Program: Technical Sessions
Time: 9:40 AM – 9:55 AM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 48, No. 7

doi: 10.1130/abs/2016AM-284822

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