^1,2Xiaojia Zeng, ¹Shijie Li, ³Ingo Leya, ¹Shijie Wang, ³Thomas Smith, ¹Yang Li, ⁴Peng Wang
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13073]
¹Center for Lunar and Planetary Sciences, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang, China
²University of Chinese Academy of Sciences, Beijing, China
³Physics Institute, University of Bern, , Bern, Switzerland
⁴Division of Mines and Geology, Sixth Geological Brigade, Hami, China
Published by arrangement with John Wiley & Sons

The Kumtag 016 strewn field was found in the eastern part of the Kumtag desert, Xinjiang Province, China. In this study, 24 recovered meteorites have been characterized by a suite of different analytical techniques to investigate their petrography, mineralogy, bulk trace elements, noble gas isotopic composition, density, and porosity. We attribute to the strewn field 22 L5 chondrites with shock stage S4 and weathering grade W2–W3. Two different meteorites, Kumtag 021, an L4 chondrite and Kumtag 032, an L6 chondrite, were recognized within the strewn field area. Moreover, Kumtag 003, an H5 chondrite, was previously found in the same area. We infer that the Kumtag 016 strewn field most likely consists of at least four distinct meteorite falls. The effects of terrestrial weathering on the studied meteorites involve sulfide/metal alteration, chemical changes (Sr, Ba, Pb, and U enrichments and depletion in Cr, Co, Ni, and Cs abundances), and physical modifications (decrease of grain density and porosity). Measurements of the light noble gases indicate that the analyzed Kumtag L5 samples contain solar wind‐implanted noble gases with a 20Ne/22Ne ratio of ~12.345. The cosmic‐ray exposure (CRE) ages of the L5 chondrites are in a narrow range (3.6 ± 1.4 Ma to 5.2 ± 0.4 Ma). For L4 chondrite Kumtag 021 and L6 chondrite Kumtag 032, the CRE ages are 5.9 ± 0.4 Ma and 4.7 ± 0.8 Ma, respectively.

¹Christopher T. Adcock, ¹Arya Udry, ¹Elisabeth M. Hausrath, ¹Oliver Tschauner
American Mineralogist 103, 502-516 Link to Article [DOI: https://doi.org/10.2138/am-2018-6193]
¹Department of Geoscience, University of Nevada, 4505 Maryland Parkway, Las Vegas, Nevada 89154-4010, U.S.A.
Copyright: The Mineralogical Society of America

The availability of terrestrial sites that are martian analogs allows researchers to investigate Mars using knowledge gained on Earth. Among the terrestrial analog sites for Mars is Craters of the Moon National Monument (COTM) in Idaho, U.S.A. Craters of the Moon National Monument is home to over 60 basalt lava flows, many of which have been dated from 2050 to 18 340 years before present (y.b.p.). Following previous authors, we examined the chemistry and petrogenesis of COTM basalts compared to basaltic martian rocks, martian meteorites, and meteorite clasts, and then examined the results of chemical weathering of the basaltic flows. Results of our comparative chemical analysis suggest COTM basalts are generally more evolved than the martian materials, with a few notable exceptions. Several COTM flow basalts, including rocks of the >18 000 year old Kimama flow, have high FeO, TiO2, and P2O5 contents similar to the Wishstone and Watchtower class rocks analyzed at Gusev Crater, Mars, by the Mars Exploration Rover Spirit. The youngest basalts of COTM, such as those of the Minidoka (3890 y.b.p.) and Blue Dragon (2050 y.b.p.) flows have similarities in SiO2, alkali contents, and mineralogical norms with select clasts in meteorite Northwest Africa (NWA) 7034. These similarities over a range of flow ages therefore suggest that COTM basalts have the potential to shed important light on specific igneous processes occurring on Mars.

Many of the basaltic rocks measured by rovers on Mars are thought to have experienced chemical weathering during aqueous interactions; however, few basalt weathering rates exist for terrestrial Mars-relevant field environments to help interpret these processes. COTM, which has important similarities to some martian rocks discussed above, also represents a basalt flow chronosequence, and therefore allows for the investigation of basalt weathering as a function of time. We measured the depth of developed porosity in a suite of basalt flows ranging from 2050 to 18 340 y.b.p., and compared field weathering relationships at COTM to weathering rinds developed on the Gusev Crater martian rocks Humphrey, Champagne, Mazatzal, and Wooly Patch. Our results indicate that depths of incipient weathering in COTM rocks increase with time at a rate of 2.32 × 10−2 to 3.04 × 10−2 μm/yr, which is comparable to other terrestrial advance rates. Interestingly, this rate also indicates that chemical weathering strongly outpaces physical weathering even in this arid to semi arid environment. Weathering primarily of the matrix glass indicates that glass may be functioning as the profile-controlling mineral, which may have implications for chemical weathering in glass-rich rocks on Mars. Weathering rates of glass and other minerals can also help constrain the conditions (pH, temperature) of alteration on Mars. Of the altered martian rocks we compared to COTM (Humphrey, Champagne, Mazatzal, and Wooly Patch), altered surfaces of Mazatzal rock at Gusev Crater show the most similarities to weathered surfaces at COTM. Comparisons of chemical weathering in COTM basalts with altered surfaces of rocks in Gusev Crater, Mars, indicate Gusev Crater martian rocks have undergone significantly more aqueous alteration than that experienced by basaltic flows at COTM.