¹Toru Matsumoto,¹Takaaki Noguchi,¹Yu Tobimatsu,²Dennis Harries,^2,3Falko Langenhorst,⁴Akira Miyake,⁴Hiroshi Hidaka
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2021.02.013]
¹Faculty of Arts and Science, Kyushu University, 744 Motooka, Nisi-ku, Fukuoka 819-0395, Japan
²Institute of Geoscience, Friedrich Schiller University Jena, Carl-Zeiss-Promenade 10, 07745 Jena, Germany
³Hawai’i Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawai’i at Manoa, Honolulu, HI 96822, USA
⁴Division of Earth and Planetary Sciences, Kyoto University, Kitashirakawaoiwake-cho, Sakyo-ku, Kyoto-shi 606-8502, Japan
⁵Department of Earth and Planetary Sciences, Nagoya University, Furo-cho, Chikusa-ku, Nagoya Science building E, 464-8601, Japan
Copyright Elsevier

Alteration of iron sulfides on the lunar surface by space weathering is poorly understood. We examined space weathering features of iron sulfides in lunar mature soil grains using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SEM observations reveal that iron sulfides have vesicular textures and iron whiskers on their surfaces. Iron sulfides observed using TEM are troilite and NC-pyrrhotite. The space-weathered rim on the iron sulfides is characterized by crystallographic misorientations and the disappearance of superstructure reflections of troilite in electron diffraction patterns. These crystallographic modifications are probably produced by solar wind irradiation. The rim contains opened vesicles that are aligned along the c-plane of the sulfides, as well as numerous tiny vesicles. The Fe/S ratio at the surface of the rim is higher than in non-altered regions, indicating selective sulfur loss from the surface. Iron whiskers protrude from the space weathered rim and consist of polycrystalline metallic iron. The sulfide rims and the iron whiskers are both coated with vapor-deposited materials rich in O and Si. The combined processes driven by the solar wind irradiation, heating during impact events, solar UV radiation, and the thermal cycling may cause vesicular textures, selective sulfur escape from the iron sulfides, and the formation of the iron whiskers. The rim textures support the notion that the enrichment of heavy sulfur isotopes in mature lunar soils is caused by space weathering of iron sulfides. The space weathered rims on lunar iron sulfides are similar to those observed in regolith samples from asteroid Itokawa. Therefore, alterations of sulfide surface might be common among airless bodies in the solar system.

¹Yoshinori Miyazaki,¹Jun Korenaga
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2021.114368]
¹Department of Earth and Planetary Sciences, Yale University, New Haven, CT 06511, USA
Copyright Elsevier

Chondrites are the likely building blocks of Earth, and identifying the group of chondrite that best represents Earth is a key to resolving the state of the early Earth. The origin of chondrites, however, remains controversial partly because of their puzzling major element compositions, some exhibiting depletion in Al, Ca, and Mg. Based on a new thermochemical evolution model of protoplanetary disks, we show that planetesimals with depletion patterns similar to ordinary and enstatite chondrites can originate at 1–2 AU outside where enstatite evaporates. Around the “evaporation front” of enstatite, the large inward flow of refractory minerals, including forsterite, takes place with a high pebble concentration, and the loss of those minerals results in depletion in Al, Ca, and Mg. The fractionation driven by the loss of forsterite would also create a complementary Mg-rich reservoir just inside the depleted region, creating two chemically distinct reservoirs adjacent to each other. The region around the evaporation front of enstatite has the highest dust concentration inside the snow line, and thus the streaming instability is most likely to be triggered therein. Planetesimals with two different major element compositions could naturally be created in the terrestrial region, which could evolve into parent bodies for Earth and chondrites. This can explain why Earth and enstatite chondrites share similar isotopic signatures but have different bulk compositions.