¹Catherine M.Weitz,²Janice L.Bishop,³John A.Grant,³Sharon A.Wilson,³Rossman P.IrwinIII,⁴Arun M.Saranathan,⁴Yuki Itoh,⁴Mario Parente
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2022.115090]
¹Planetary Science Institute, 1700 E Fort Lowell, Tucson, AZ 85719, USA
²SETI Institute, Carl Sagan Center, 339 Bernardo Ave., Mountain View, CA 94043, USA
³Center for Earth and Planetary Studies, National Air and Space Museum, Smithsonian Institution, 6th at Independence SW, Washington, DC 20560, United States of America
⁴University of Massachusetts Amherst, Dept. Electrical & Computer Engineering, 151 Holdsworth Way, Amherst, MA 01003, United States of America
Coypright Elsevier

The morphology and mineralogy of light-toned layered sedimentary deposits were investigated using multiple orbital datasets across the Ladon basin region, including within northern Ladon Valles, southern Ladon basin, and the southwestern highlands of Ladon basin. Light-toned layered deposits are particularly widespread in Ladon Valles and Ladon basin, ranging laterally for distances over 200 km, with the thickest exposure (54 m) located at the mouth of Ladon Valles. The restriction of layered sediments below a common elevation (−1850 m) in Ladon Valles and Ladon basin and their broad conformable distribution with bedding dips between 1 and 4° favor a lacustrine environment within this region during the Late Noachian to Early Hesperian. The Ladon layered deposits have spectral signatures consistent with Mg-smectites, even when the morphology of the layering varies considerably in color and brightness. These phyllosilicates were most likely eroded from the highlands upstream to the south, but the lacustrine environment may have also been favorable for in situ alteration and formation of clays. The southwestern highlands also display light-toned layered deposits within valleys and small basins. These sediments predominantly have signatures of Mg-smectites, although we also identified Fe/Mg-smectites and additional hydrated phases in some deposits. One of these altered deposits was found within a younger Holden crater secondary chain, possessing a Late Hesperian to Early Amazonian age for valleys and sediments that postdate the deposits within Ladon Valles and Ladon basin. Phyllosilicate signatures were also detected in the ejecta from two fresh craters that exposed highland materials upstream of Arda Valles, revealing that the highlands are clay-bearing and may be the most plausible source of the clay-bearing fluvial-derived sediments found within the valleys and basins downstream. Some of the highland deposits are likely coeval to similar clay-bearing sediments found to the south within Holden and Eberswalde craters, indicating late, widespread fluvial activity and deposition of allochthonous clays within the broader Margaritifer Terra region when Mars was thought to be colder and drier.

^1,2Ai-Cheng Zhang,¹Jie-Ya Li,¹Jia-Ni Chen,³Yuan-Yun Wen,⁴Yan-Jun Guo,^2,3Yang Li,⁵Naoya Sakamoto,^5,6Hisayoshi Yurimoto
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2022.05.017]
¹State Key Laboratory for Mineral Deposits Research, School of Earth Science and Engineering, Nanjing University, Nanjing 210023, China
²CAS Center for Excellence in Comparative Planetary, Hefei 230026, China
³Center for Lunar and Planetary Sciences, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550081, China
⁴CAS Key Laboratory of Standardization and Measurement for Nanotechnology, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing, China
⁵Isotope Imaging Laboratory, Creative Research Institution, Hokkaido University, Sapporo 010-0021, Japan
⁶Department of Natural History Sciences, Hokkaido University, Sapporo 060-0810, Japan
Copyright Elsevier

Impact is a fundamental process shaping the formation and evolution of planets and asteroids. It is inevitable that some materials on the surface of planets and asteroids have been impacted for many times. However, unambiguous petrological records for multiple post-formation impact events are rarely described. Here, we report that the thin shock melt veins of the shocked eucrite Northwest Africa 8647 are dominated by a fine-grained intergranular or vermicular pigeonite and anorthite assemblage, rather than compact vacancy-rich clinopyroxene. Vacancy-rich clinopyroxene in the veins instead is ubiquitous as irregularly-shaped, relict grains surrounded by intergranular or vermicular pigeonite and anorthite assemblage. The silica fragments entrained in shock melt veins contain a coesite core and a quartz rim. The occurrences of vacancy-rich clinopyroxene and coesite can be best explained by two impact events. The first impact event produced the shock melt veins and lead to the formation of vacancy-rich clinopyroxene and coesite. The second impact event heated the fine-grained melt veins and lead to the widespread partial decomposition of vacancy-rich clinopyroxene and the partial back-transformation of coesite. This paper is the first report of the decomposition reaction of shock-induced vacancy-rich clinopyroxene in extraterrestrial materials. We propose that widespread decomposition and/or back-transformation of high-pressure minerals in shocked meteorites can be considered as important records of multiple impact events.