PINK SPINELS ON THE MOON: SAMPLES, REMOTE SENSING, ANALOGS, EXPERIMENTS, THERMOCHEMISTRY, AND FUTURE EXPLORATION (Invited Presentation)

Pink spinel, MgAl₂O₄ with small proportions of Cr and Fe, is rare in lunar rocks, but could have huge implications for the rocks’ origins and the history of the Moon. Pink spinel is rare in all lunar sample sets (returned and meteoritic), and abundant in only a few rocks and clasts (e.g., 15295, 67435, ALH81005, NWA16400). Being so rare, lunar pink spinel was treated as a space oddity until orbital remote sensing showed outcrop-scale exposures of rock containing 20-30% pink spinel at Moscoviense basin. Those discoveries, in VNIR spectral maps from the M3 instrument on the Chandraya’an spacecraft, sparked a reexamination of the significance of lunar pink spinel.

Hypotheses for the origin of lunar pink spinel include: high pressure (in the lunar mantle); by impact melting of troctolitic (olivine + anorthite) protoliths; and/or by assimilation of crustal anorthosite into ultrabasic magmas. Evaluating these mechanisms has involved the full range of petrologic/geochemical analyses. The high-pressure hypothesis relies on analogous Earth materials, i.e. mantle spinel peridotites, and the lab experiments and geochemical modeling used to understand them. The impact melting hypothesis relies on the petrography of shocked lunar samples, and models of heat budgets and melt crystallization. The assimilation hypothesis relies on the petrography of spinel-bearing lunar samples, lab experiments of analogous systems and on crystallization path modeling.

For returned samples and meteorites, distinguishing among these hypotheses is difficult because the samples lack geologic contexts. Meteorites lack lunar source locations, and nearly all returned samples are fragments from regolith or impact breccias. To understand the origin(s) of lunar rocks with pink spinel, it will be critical to visit the spinel-rich outcrops in person (by humans or robots) to assess their geological contexts. These future explorers will examine the spinel-bearing outcrops for structures and textures relevant to their emplacement and origin, gather reflectance spectra to ground-truth the orbital data, and collects samples for detailed analysis in lunar labs and eventual return to Earth. Understanding this enigmatic rock type will help us constrain geologic processes that shaped the Moon, and ultimately improve our understanding of the formation of the Earth-Moon system and its evolution in space and time.