Ferromagnetic resonance intensity (Is) : a rapid method for determining the mode of origin of lunar glass beads

Glass beads formed by lunar fire-fountaining have been recognized in soil samples returned by the Apollo missions to the Moon. These minute droplets represent frozen samples of lunar magmas which are relatively primitive to the mare basalts. The chemistries preserved in these glass particles are essential to any petrogenetic considerations of the early Moon.

Glass beads originating from impact processes (i.e., shock melting by meteorites) are abundant throughout the Apollo soil collections. The chemistries recorded by the impact beads can represent melts of a regolith (composed of grains and fragments of rock, mineral, and meteorite, and also previous impact melts), or the chemistry may reflect the melt from a rock unit or a combination of units.

Cursory inspection of volcanic and impact beads usually will not allow their discrimination on the basis of origin. The difficulty of discerning a bead's mode of origin has led to the test of a new criterion: the intensity of ferromagnetic resonance (I_s) of a bead. This technique can be employed for rapid screening to determine the origin of the glass beads.

The ferromagnetic resonance method relies on the detection of magnetically single-domain Fe formed during impact melting by auto-reduction of Fe²⁺ in the melt. In order to establish the usefulness of I_s, extensive scanning electron microscope (SEM) inspection of glass bead surfaces was conducted which provided reliable evidence of the mode of glass genesis. I_s was tested against the "type" Apollo 15 and Apollo 17 volcanic glass beads. Additional volcanics were also found during the course of the study. Of the volcanic glasses, 94% have Is values less than one, implying the absence of single—domain Fe. Moderate to high I_s values (10 to 270) were recorded in 75% of the impact beads. The remaining impact beads have Is values less than one. These samples lack single-domain Fe and could represent impact melts of bedrock with little or no contribution from the regolith. The ability to discriminate bedrock-derived impact melts as opposed to impact melts partially or completely from the regolith is an additional important potential for the ferromagnetic resonance technique.