^1,2Mohit Melwani Daswani, ³Susanne P. Schwenzer, ⁴Mark H. Reed, ¹Ian P. Wright, ¹Monica M. Grady
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12713]
¹Department of Physical Sciences, The Open University, Milton Keynes, UK
²Department of the Geophysical Sciences, University of Chicago, Chicago, Illinois, USA
³Department of Environment, Earth and Ecosystems, The Open University, Milton Keynes, UK
⁴Department of Geological Sciences, University of Oregon, Eugene, Oregon, USA
Published by arrangement with John Wiley & Sons

Clay minerals, although ubiquitous on the ancient terrains of Mars, have not been observed in Martian meteorite Allan Hills (ALH) 84001, which is an orthopyroxenite sample of the early Martian crust with a secondary carbonate assemblage. We used a low-temperature (20 °C) one-dimensional (1-D) transport thermochemical model to investigate the possible aqueous alteration processes that produced the carbonate assemblage of ALH 84001 while avoiding the coprecipitation of clay minerals. We found that the carbonate in ALH 84001 could have been produced in a process, whereby a low-temperature (~20 °C) fluid, initially equilibrated with the early Martian atmosphere, moved through surficial clay mineral and silica-rich layers, percolated through the parent rock of the meteorite, and precipitated carbonates (thereby decreasing the partial pressure of CO2) as it evaporated. This finding requires that before encountering the unweathered orthopyroxenite host of ALH 84001, the fluid permeated rock that became weathered during the process. We were able to predict the composition of the clay minerals formed during weathering, which included the dioctahedral smectite nontronite, kaolinite, and chlorite, all of which have been previously detected on Mars. We also calculated host rock replacement in local equilibrium conditions by the hydrated silicate talc, which is typically considered to be a higher temperature hydrothermal phase on Earth, but may have been a common constituent in the formation of Martian soils through pervasive aqueous alteration. Finally, goethite and magnetite were also found to precipitate in the secondary alteration assemblage, the latter associated with the generation of H2. Apparently, despite the limited water–rock interaction that must have led to the formation of the carbonates ~ 3.9 Ga ago, in the vicinity of the ALH 84001 source rocks, clay formation would have been widespread.

^1,2A.Lamali et al. (>10)*
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12715]
¹CRAAG, Alger, Algeria
²FSTGAT, Alger, Algeria
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Published by arrangement with John Wiley and Sons

The Maâdna structure is located approximately 400 km south of Algiers (33°19′ N, 4°19′ E) and emplaced in Upper-Cretaceous to Eocene limestones. Although accepted as an impact crater on the basis of alleged observations of shock-diagnostic features such as planar deformation features (PDFs) in quartz grains, previous works were limited and further studies are desirable to ascertain the structure formation process and its age. For this purpose, the crater was investigated using a multidisciplinary approach including field observations, detailed cartography of the different geological and structural units, geophysical surveys, anisotropy of magnetic susceptibility, paleomagnetism, and petrography of the collected samples. We found that the magnetic and gravimetric profiles highlight a succession of positive and negative anomalies, ones that might indicate the occurrence of a causative material which is at least in part identical. Geophysical analysis and modeling suggest the presence of this material within the crater at a depth of about 100 m below the surface. Using soil magnetic susceptibility measurements, the shallowest magnetized zone in the central part of the crater is identified as a recently deposited material. Paleomagnetic and rock magnetic experiments combined with petrographic observations show that detrital hematite is the main magnetic carrier although often associated with magnetite. A primary magnetization is inferred from a stable remanence with both normal and reverse directions, carried by these two minerals. Although this is supposed to be a chemical remagnetization, its normal polarity nature is considered to be a Pliocene component, subsequent to the crater formation. The pole falls onto the Miocene-Pliocene part of the African Apparent Polar Wander Path (APWP). Consequently, we estimate the formation of the Maâdna crater to have occurred during the time period extending from the Late Miocene to the Early Pliocene. Unfortunately, our field and laboratory investigations do not allow us to confirm an impact origin for the crater as neither shatter cones, nor shocked minerals, were found. A dissolved diapir with inverted relief is suggested as an alternative to the impact hypothesis, which can still be considered as plausible. Only a drilling may provide a definite answer.

¹George G. Managadze et al. (>10)*
Planetary and Space Science (in Press) Link to Article [http://dx.doi.org/10.1016/j.pss.2016.07.005]
¹Space Research Institute, Profsoyuznaya, st. 84/32, Moscow 117997, Russia
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