^1,2Eric M. MacLennan,^2,3Joshua P. Emery,²Michael P. Lucas,^3,4Lucas M. McClure,^2,4Sean S. Lindsay
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.14150]
¹Department of Physics, University of Helsinki, Helsinki, Finland
²Earth and Planetary Sciences Department, Planetary Geosciences Institute, 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
Published by arrangement with John Wiley & Sons

The exposure to irradiation from high-energy particles alters the reflectance properties of asteroid surfaces and is referred to as space weathering. This process leads to an increase in spectral slope in visible and near-infrared wavelengths. However, changes in the regolith particle size, which can vary dramatically among the asteroid population, are known to influence the spectral properties of meteorites and asteroids. In this context, we investigate the changes in spectral slope and absorption band depths of fresh and irradiated ordinary chondrite meteorites to quantitatively compare the effects of space weathering and grain size variations. To do so, we develop and employ the Spectral Analysis for Asteroid Reflectance Investigation routine that calculates the band parameters of reflectance spectra. We then formulate a parameter called the Space Weathering Index (SWI) that is designed to encapsulate spectral changes due to space weathering. We find that the SWI is strongly dependent on the spectral slope which complicates the interpretation of asteroid spectra in the context of grain size variations and space weathering. We also show that a second parameter, the Band Depth Index, is indicative of petrologic type. Finally, we use a linear discriminant analysis to classify asteroid reflectance spectra into H, L, LL, and unequilibrated ordinary chondrites.

^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.

¹William Herbst,²James P. Greenwood
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.14153]
¹Department of Astronomy, Wesleyan University, Middletown, Connecticut, USA
²Department of Earth and Environmental Sciences, Wesleyan University, Middletown, Connecticut, USA
Published by arrangement with John Wiley & Sons

Chondrules probably formed during a small window of time ~1–4 Ma after CAIs, when most solid matter in the asteroid belt was already in the form of km-sized planetesimals. They are unlikely, therefore, to be “building blocks” of planets or abundant on asteroids, but more likely to be a product of energetic events common in the asteroid belt at that epoch. Laboratory experiments indicate that they could have formed when solids of primitive composition were heated to temperatures of ~1600 K and then cooled for minutes to hours. A plausible heat source for this is magma, which is likely to have been abundant in the asteroid belt at that time, and only that time, due to the trapping of ²⁶Al decay energy in planetesimal interiors. Here, we propose that chondrules formed during low-speed (≲1kms−1) collisions between large planetesimals when heat from their interiors was released into a stream of primitive debris from their surfaces. Heating would have been essentially instantaneous and cooling would have been on the dynamical time scale, 1/(Gρ) ~30 min, where � is the mean density of a planetesimal. Many of the heated fragments would have remained gravitationally bound to the merged object and could have suffered additional heating events as they orbited and ultimately accreted to its surface. This is a hybrid of the splash and flyby models: We propose that it was the energy released from a body’s molten interior, not its mass, that was responsible for chondrule formation by heating primitive debris that emerged from the collision.

¹Aaron J. Cavosie,¹Phil A. Bland,²Noreen J. Evans,²Kai Rankenburg,³Malcolm P. Roberts,^4,5Luigi Folco
Geochimica et Cosmochimica Acta (in Press) Open Access Link to Article [https://doi.org/10.1016/j.gca.2024.02.016]
¹Space Science and Technology Centre and The Institute for Geoscience Research, School of Earth and Planetary Science,Curtin University, Perth, WA 6102, Australia
²John de Laeter Centre, Curtin University, Perth, WA 6102, Australia
³Centre for Microscopy, Characterisation, and Analysis, University of Western Australia, Perth, WA 6009, Australia
⁴Dipartimento di Scienze della Terra, Università di Pisa, Pisa, 56126, Italy
⁵Centro per l’Integrazione della Strumentazione dell’Università di Pisa, Pisa 56126, Italy
Copyright Elsevier

Detection of extra-terrestrial geochemical components in melt generated during meteorite impact provides diagnostic evidence that can be used to confirm a hypervelocity impact event, and in some cases, classify the projectile. However, projectile contamination is often present at sub-percent levels, and can be difficult to detect. In contrast, meteoritic abundances in glass from small impact craters (<1 km diameter) formed by iron meteorites can be anomalously high, which has been attributed to glass originating from the projectile-target interface. Emulsion textures, immiscible liquids, metal spherules, and non-meteoritic siderophile element ratios have been cited as evidence that the projectile component is typically fractionated in impact glass. Here we present compositional data for impact glass from the Henbury crater field in Australia, where the largest crater is 145 m in diameter and the subgreywacke target rock and IIIAB iron projectile are geochemically distinct. Mixing models (Fe-Si, Ni-Co, Cr-Ir) and high platinum group element abundances indicate average projectile contributions ranging from 3 to 13 % in Henbury glass, comparable to ranges reported in glass from the Kamil (Egypt) and Wabar (Saudi Arabia) impact craters. However meteoritic siderophile element ratios (Fe:Ni, Fe:Co, Ni:Co) in Henbury glass appear nearly unfractionated, whereas Wabar and Kamil glasses have more fractionated ratios. Observed variations are attributed to fractionation of meteoritic Ni by formation of immiscible Ni-rich spherules during oxidation of meteoritic iron, and subsequent separation of Ni-rich spherules from glass during ejection. The Henbury glass sample analyzed is interpreted as an example of an interface melt that quenched prior to extensive oxidation and phase separation, and thus may represent one of the least fractionated samples of melt from the projectile-target interface described thus far, whereas Wabar and Kamil glasses record more evidence of fractionation processes. These results further highlight the influence of metal spherule formation on the composition of ejected glass from small impact structures formed by iron meteorites and provide new insights that explain textural features observed in natural impact glasses.