^1,2Eric M. MacLennan,^2,3Joshua P. Emery,^3,4Lucas M. McClure,²Michael P. Lucas,^2,4Sean S. Lindsay,^2,5Noemi Pinilla-Alonso
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.14151]
¹Department of Physics, University of Helsinki, Helsinki, Finland
²Earth and Planetary Sciences Department, The University of Tennessee, Knoxville, Tennessee, USA
³Department of Astronomy and Planetary Science, Northern Arizona University, Flagstaff, Arizona, USA
⁴Department of Physics and Astronomy, The University of Tennessee, Knoxville, Tennessee, USA
⁵Florida Space Institute, University of Central Florida, Orlando, Florida, USA
Published by arrangement with John Wiley & Sons

Spacecraft missions to asteroids have revealed surfaces that have variations in albedo and spectral properties. Such variations are also detected across the asteroid population with ground-based observations, and are controlled by the physical characteristics of the regolith and by processes such as space weathering. Here, we investigate how space weathering and regolith grain size influence the spectra of ordinary chondrite-like asteroids observed from ground-based spectroscopy. The estimation of diagnostic band parameters from asteroid visible and near-infrared reflectance spectra allow us to estimate the degree of space weathering and their compositions, using results from an accompanying study (MacLennan et al., 2024). We use grain size estimations gleaned from the thermal inertia to show that regolith particle size differences have similar effect as space weathering on asteroid spectra. Finally, we quantify changes in spectral slope and band depth among asteroids using the space weathering index developed by MacLennan et al (2024), and reassess the importance of previously-proposed surface freshening mechanisms.

¹Lan F. Xie,¹Hong Y. Chen,¹Bing K. Miao,²Wen L. Song,¹Zhi P. Xia,¹Chuan T. Zhang,¹Guo Z. Chen,¹Jin Y. Zhang,^1,3Si Z. Zhao,¹Xu K. Gao
American Mineralogist 109, 457-470 Link to Article [http://www.minsocam.org/msa/ammin/toc/2024/Abstracts/AM109P0457.pdf]
¹Key Laboratory of Planetary Geological Evolution of Guangxi Provincial Universities, Collaborative Innovation Center for Exploration of Nonferrous Metal Deposits and Efficient Utilization of Resources in Guangxi, and Guangxi Key Laboratory of Hidden Metallic Ore Deposits Exploration, College of Earth Sciences, Guilin University of Technology, Guilin 541006, China
²State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi’an 710069, China
³Center for Lunar and Planetary Sciences, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550081, China
Copyright: The Mineralogical Society of America

Pink spinel anorthosite (PSA) and pink spinel troctolite (PST) are two lunar lithologies known to
contain Mg-rich spinel. PSA rich in spinel and lacking mafic minerals, was detected by the visible and
near-infrared reflectance spectroscopy. PST clasts were found in returned lunar samples and meteorites.
NWA 13191 is a recently approved lunar meteorite that contains a large amount of spinel-bearing clasts
and provides an opportunity to discuss its origin. Sixty-four spinel-bearing clasts were studied in this
research. These clasts are dominated by anorthitic feldspars (20.8–80.9 vol%, An90.9–96.8), mafic-rich
and aluminum-rich glass (14.7–72.1 vol%) quenched from a melt, and spinels (0.19–5.18 vol%). Fortynine of these clasts appear to have unusually low modal abundances of mafic silicates (avg. olivine
± pyroxene, 1.87 vol%), which distinguishes them from known spinel-bearing lunar samples (e.g.,
PST). The spinel compositions (avg. Mg# = 90.6, Al# = 97.4) and mafic minerals contents are basically
consistent with those of PSA. The absorption characteristics of glass in the reflection spectrum are not
obvious, so it is not clear if the PSA contains melt. The simulated crystallization experiment clearly
shows that it contains a large amount of melt at the spinel crystallization stage. These phenomena
provide experimental and sample evidence for the existence of glass in the lunar spinel-bearing lithologies. NWA 13191 records the highest known bulk Mg# (avg. 89.8), and the spinel records the highest
Al# (98.8) and Mg# (93.1) of lunar samples to date. The chemical properties of spinel-bearing clasts
in NWA 13191 are consistent with the slightly REE-enriched and alkali-poor Mg-suite rocks, such as
PST, magnesian anorthosites (MANs), and olivine-enriched Mg-suite rocks. These phenomena and
previous simulated crystallization experiments indicate that a Mg-Al-rich melt may be produced by
impact melting of Mg-rich anorthosite precursors. The spinel is a metastable crystallization product
along with plagioclase and vitric melt near the Moon’s surface. This realization provides observational
evidence for previous simulated crystallization experiments and theoretical speculations.