^1,2Yash Srivastava et al. (>10)
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.14357]
¹Planetary Science Division, Physical Research Laboratory, Ahmedabad, India
²Scripps Institution of Oceanography, University of California San Diego, San Diego, California, USA
Published by arrangement with John Wiley & Sons

Aubrites are rare meteorites from highly reduced differentiated parent bodies. The Rantila meteorite was recovered soon after falling on 17 August 2022 at Rantila and Ravel villages in Gujarat state, India. We report the petrography, mineralogy, chemical composition, oxygen- and chromium-isotope compositions, along with reflectance spectroscopy, all showing that Rantila is an aubrite. Coarse enstatite and diopside grains constitute the main mass of Rantila, while mm-wide fracture domains pervade the coarse enstatites. In the fractures, comminuted enstatite, diopside blebs, olivine, a plagioclase–silica assemblage, sulfides, and metals occur. Rantila consists of enstatite (>85 vol%), diopside (~8 vol%), forsterite, albite, and silica along with various sulfides and Fe-Ni alloys. The concentration of rare earth elements is ~1–2 × CI, consistent with main group aubrites. Noble gas and nitrogen isotopic analyses reveal young exposure ages (13.81 ± 6.47 Ma), a heterogeneous nitrogen isotopic composition, and a major K-Ar resetting event around 3.2 ± 0.4 Ga in the parent body of Rantila. The bulk oxygen isotope values are within the range of aubrites. The chromium isotopic values of Rantila are consistent with main group aubrites. The mineral assemblages, texture, and crystallization modeling suggest that Rantila had an igneous origin. The mineral assemblages in fractures indicate the involvement of external melt possibly during an impact-fracturing event, which aligns well with the heterogeneous N isotopic composition. Additionally, Rantila shows a wider range of oxygen isotopes than other aubrites suggesting some extent of O isotopic heterogeneity, likely stemming from exogenous processes. The variation in intra-sample bulk O and N isotope values implies inherent heterogeneity within the main group aubrites, potentially caused by late-stage impact contamination.

¹Brendan A. Anzures,^1,2Kathleen E. Vander Kaaden,³Francis M. McCubbin,^1,4Richard L. Rowland, II,^1,4Gordon M. Moore,¹Kelsey Prissel,³Richard V. Morris,⁵Rachel L. Klima,⁵Karen R. Stockstill-Cahill,⁶David G. Agresti
American Mineralogist 110, 570-581 Open Access Link to Article [https://doi.org/10.2138/am-2024-9400]
¹Jacobs, NASA Johnson Space Center, 2101 NASA Parkway, Houston, Texas 77058, U.S.A.
²NASA Headquarters, Mary W. Jackson Building, Washington, D.C. 20546, U.S.A.
³ARES NASA Johnson Space Center, 2101 NASA Parkway, Houston, Texas 77058, U.S.A.
⁴Los Alamos National Laboratory, Los Alamos, New Mexico 87545, U.S.A.
⁵The Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, Maryland 20723, U.S.A.
⁶Department of Physics, University of Alabama at Birmingham, 902 14th Street South, Birmingham, Alabama 35294, U.S.A.
Copyright: The Mineralogical Society of America

Results from X-ray remote sensing aboard NASA’s MErcury Surface Space ENvironment GEochemistry and Ranging (MESSENGER) spacecraft have demonstrated that Mercury has a low, but measurable, concentration of Fe on its surface. However, ultraviolet to near-infrared spectroscopic measurements of the mercurian surface do not show the 1 μm absorption band characteristic of ferromagnesian silicates. This observation is consistent across multiple Fe-bearing terranes with a range of ages, suggesting the Fe present on Mercury’s surface may not be stored within silicate phases. To further constrain the possible mineralogy and composition of Fe-bearing phases on Mercury, we used various spectroscopic techniques to characterize synthetic olivine with minor amounts of Fe (i.e., Fo99.62–Fo99.99) and more Fe-rich natural olivines. Our results indicate that the distinctive 1 μm absorption band of olivine is detectable in reflectance spectra of olivine at a concentration as low as 0.03 wt% FeO and 0.01 wt% in continuum removed data. Additionally, MESSENGER’s lack of a 1 μm absorption, taking into account Mercury Dual Imaging System (MDIS)’s limited spectral resolution and Mercury Atmospheric and Surface Composition Spectrometer (MASCS)’s high signal-to-noise ratio, suggests there is <0.38 wt%, and likely <0.01 wt%, FeO on the surface of Mercury. Because the 1 μm band is not observed in surface spectra, these results indicate that the Fe observed on the surface of Mercury is not bound in an olivine structure. Rather, we posit that Fe is present as nano-phase and macroscopic Fe-rich metal or Fe-sulfide that formed as a result of space weathering and igneous smelting processes. Looking forward to ESA/JAXA’s BepiColombo mission that has a planned Mercury orbit arrival time in December 2025, Mercury Radiometer and Thermal Infrared Imaging Spectrometer (MERTIS) mid-infrared spectra should provide a mineralogical detection or absence of olivine where MIR spectral features are still present even in synthetic olivines with minor amounts of Fe (Fo99.99).