¹L.C. Chaves,¹M.S. Thompson,²M.J. Loeffler,³C.A. Dukes,⁴P.S. Szabo,¹B.H.N. Horgan 
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2023.115634]
¹Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, 550 Stadium Mall Drive, West Lafayette, IN 47907, United States of America
²Department of Physics and Astronomy, Northern Arizona University, 527 South Beaver Street, Flagstaff, AZ 86011, United States of America
³Materials Science and Engineering, University of Virginia, 395 McCormick Road, Charlottesville, VA 22904, United States of America
⁴Space Sciences Laboratory, University of California, Berkeley, 7 Gauss Way, Berkeley, CA 94720, United States of America
Copyright Elsevier

Magnetite is a relevant mineral component of asteroids as it has been identified in carbonaceous chondrites, on the surface of asteroid Bennu through remote sensing observations, and in samples returned from asteroid Ryugu. However, the effects of space weathering processes on magnetite have not yet been explored. To investigate how this mineral phase responds to space weathering, here we simulate micrometeoroid bombardment and solar wind irradiation of magnetite using pulsed laser and ion irradiation experiments. We performed X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and visible to near-infrared (VNIR) reflectance spectroscopy analyses to characterize the chemical, microstructural, and spectral response of magnetite to simulated space weathering. In addition, we carried out ion impact simulations using the SDTrimSP software to evaluate the calculated response of magnetite to 1 keV H⁺ and 4 keV He⁺ ions and compared these results to our XPS and TEM results. Ion irradiation simulated ~750 years on the surface of asteroid Bennu, with a solar-wind appropriate total H:He fluence ratio (~24). Within this time, depletion of O was observed with H⁺ and He⁺ ion irradiation, with significantly greater change via protons due to the larger fluence, where preferential sputtering promotes the formation of a metallic iron layer at the magnetite surface. This suggests that solar wind ions act as reducing agents on Fe oxides, with a fraction remaining implanted in these phases. Indeed, we observe elongated defects contained in a crystalline rim created by He⁺ implanted ions in the TEM. Pulsed laser irradiation, analogous to micrometeoroid impacts, generates melts on the surface of the magnetite grains. The impact melts and H⁺-generated metallic iron rims both result in increased VNIR spectral reflectance, but lower fluence He⁺ implantation has no significant spectral effect. These results suggest that space weathered magnetite could contribute to bright regions detected in remote sensing analyses of the Ryugu and Bennu surfaces by the Hayabusa2 and OSIRIS-REx missions and will contribute to the identification and interpretation of space weathered magnetite in returned samples retrieved from both asteroids.

¹Ziyan Han,^1,2,3Hejiu Hui,^1,2Haizhen Wei,^1,2Weiqiang Li
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2023.05.007]
¹State Key Laboratory of Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, China
¹CAS Center for Excellence in Comparative Planetology, Hefei 230026, China
³CAS Key Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
Copyright Elsevier

The Moon is depleted in volatile elements and compounds, and lunar samples exhibit a wide range of Cl isotopic compositions, which is believed to result from the volatilization of metal chlorides (e.g., NaCl, KCl, and FeCl2). However, the Cl isotopic fractionation behavior during volatilization is not well constrained, particularly for metal chlorides. Furthermore, the effect of metal chloride evaporation on metal isotopes is poorly known. In the present study, we performed NaCl and KCl sublimation experiments to study Cl and K isotopic fractionations at temperatures ranging from 923 K to 1061 K and at pressures of 7×10–5 bar to 1 bar in an N2 atmosphere. The isotope fractionation factors of 37/35Cl(αgas–solid) from NaCl sublimation experiments are 0.9985±0.0002, 0.9958±0.0004, and 0.99807±0.00004 at 1, 10–2, and 7×10–5 bar, respectively. Those of 41/39K(αgas–solid) and 37/35Cl(αgas–solid) from KCl sublimation experiments are 0.99884±0.00004 and 0.9988±0.0003 at 1 bar, 0.9977±0.0002 and 0.9972±0.0003 at 10–2 bar, and 0.9989±0.0002 and 0.9989±0.0001 at 7×10–5 bar, respectively. Chlorine and K isotopes fractionate more at 10–2 bar than at 7×10–5 bar and 1 bar. The saturation index in all the sublimation experiments was >95%, which resulted in near-equilibrium isotopic fractionation at the sublimation interface. Therefore, the isotopic fractionation was controlled by mass transfer processes in the gas and solid phases. The isotopic fractionation at 10–2 bar was controlled by the chemical diffusion of sublimated gas in an N2 atmosphere with almost no convection effect, (i.e., Pe number close to zero), whereas the isotopic fractionation at 1 bar was suppressed by atmospheric convection with a turbulence factor of 0.4±0.1 (i.e., Pe number >1). The extremely high sublimation rate and the very slow diffusion in the sublimating solid at 7×10–5 bar suppressed isotopic fractionations. Based on our experimental results, calculations using Cl/K and Na/K in lunar materials reveal that degassing of KCl contributed very little (<0.2‰) to the K isotopic fractionation (>0.58‰) during lunar magma ocean degassing. The Cl isotopic fractionation factor from lunar samples is similar to our results at 10–2 bar. This similarity of Cl isotope fractionation indicates that there may have been a transient atmosphere above the lunar magma ocean.