¹Qian Fang,²Yan Li,¹Hongrui Ding,¹Liao Yang,¹Hanlie Hong,¹Zhong-Qiang Chen,¹Anbei Deng,¹Qile Geng,²Anhuai Lu
American Mineralogist 110,1343-1360 Link to Article [https://doi.org/10.2138/am-2024-9585]
¹State Key Laboratory of Geomicrobiology and Environmental Changes, School of Earth Sciences, China University of Geosciences, Wuhan 430074, China
²School of Earth and Space Sciences, Peking University, Beijing 100871, China
Copyright: The Mineralogical Society of America

Rock varnish is widely distributed across Earth’s various climatic zones, especially prevalent in arid environments similar to Mars. Its potential presence on Mars has made it a significant Mars analog for planetary research. The primary components of rock varnish are clay minerals and iron-manganese oxyhydroxides, with clay minerals possibly playing a crucial role in the enrichment of iron and manganese. However, there has been scarce in-depth and detailed research on these clay minerals within rock varnish. To better understand the formation and transformation mechanisms, as well as the influencing factors of clay minerals in rock varnish, we conducted X-ray diffraction (XRD) analyses on clay minerals isolated from rock varnish samples collected across different climatic regions in China. Additionally, in situ visible to near-infrared spectroscopy (Vis-NIR), scanning electron microscopy (SEM), and focused ion beam high-resolution transmission electron microscopy (FIB-HRTEM) were performed on the rock varnish samples. The results revealed the presence of illite in all rock varnish samples, while the selective occurrence of other clay minerals was closely correlated with climatic backgrounds. Furthermore, the crystallinity of illite was significantly influenced by climatic conditions. Illite found in rock varnish existed as both detrital and authigenic forms. Generally, the detrital illite in rock varnish was thicker than the nanometer-scale authigenic illite and exhibited distinct differences in chemical composition (e.g., Si/Al, K/Al ratios) and nanoscale morphology. In many cases, the possible transformation of illite to chlorite was observed, either internally within illite particles or through the formation of regular or irregular interstratified structures between illite and chlorite. Both interlayer brucitization and talc brucitization mechanisms may be involved in the chloritization (brucitization) of illite in rock varnish. Such transformations are generally uncommon in surface environments and are more frequently associated with low-grade metamorphism, suggesting that the environment at the illuminated rock surfaces, akin to metamorphic conditions, might provide the energy needed for these reactions. Considering the strong solar irradiance characteristic of Mars and its abundance of Mg- and Fe-rich rocks, it is plausible to expect the continued occurrence of chloritization on the martian surface and even within martian rock varnish. Our findings are significant for better understanding the formation and transformation of clay minerals on the martian surface and martian rock varnish, and climate-controlled water-rock interactions on Mars.

^1,2Riley Havel,^1,2Daniel E. Ibarra,³Rainer Bartoschewitz,¹Gerrit Budde
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.70039]
¹Department of Earth, Environmental and Planetary Sciences, Brown University, Providence, Rhode Island, USA
²The Institute at Brown for Environment and Society, Brown University, Providence, Rhode Island, USA
³Bartoschewitz Meteorite Laboratory, Gifhorn, Germany
Published by arrangement with John Wiley & Sons

Triple oxygen isotope analyses of meteorites are a fundamental tool for classifying meteorites and investigating early solar system processes. However, its utility can be significantly compromised by terrestrial oxygen contamination during weathering processes on Earth’s surface. Aiming to restore the original bulk oxygen isotope composition of meteorites through the removal of terrestrial weathering products, leaching procedures with hydrochloric acid (HCl) or ethanolamine thioglycollate (EATG) are often employed, but their effects remain poorly understood. Therefore, here we obtained high-precision triple oxygen isotope data for a comprehensive set of meteorites to systematically evaluate the efficacy and consequences of these leaching methods as a function of meteorite group, weathering grade, petrologic type, and find/fall location and status. Our data for untreated and leached bulk meteorite powders show that leaching can cause shifts of several permil in 18O/16O and 17O/16O in aqueously altered and pristine chondrites, and lower magnitude shifts in thermally metamorphosed chondrites and achondrites. Though some shifts can be explained by removal of terrestrial weathering products, many suggest the inadvertent removal of indigenous phases. As such, this study highlights the benefits and disadvantages of leaching methods for meteorites, which can be best assessed by analyses of both untreated and HCl/EATG-leached aliquots.