GSA Annual Meeting in Denver, Colorado, USA - 2016

Paper No. 197-7
Presentation Time: 9:40 AM


MCCANTA, Molly, Department of Earth and Planetary Sciences, University of Tennessee Knoxville, Knoxville, TN 37996, DYAR, M. Darby, Dept. of Astronomy, Mount Holyoke College, South Hadley, MA 01075, RUTHERFORD, Malcolm, Department of Earth, Environmental, and Planetary Sciences, Brown University, Providence, RI 02912 and LANZIROTTI, Antonio, Center for Advanced Radiation Sources, The University of Chicago, Argonne National Laboratory, 9700 S. Cass Ave., Bldg. 434A, Lemont, IL 60439,

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.