¹N. Mangold et al. (>10)*
¹Laboratoire de Planétologie et Géodynamique de Nantes, CNRS, UMR6112, Université de Nantes, Nantes, France
*Find the extensive, full author and affiliation list on the publishers website

The Yellowknife Bay formation represents a ~5 m thick stratigraphic section of lithified fluvial and lacustrine sediments analyzed by the Curiosity rover in Gale crater, Mars [Grotzinger et al., 2014]. Previous works have mainly focused on the mudstones that were drilled by the rover at two locations. The present study focuses on the sedimentary rocks stratigraphically above the mudstones by studying their chemical variations in parallel with rock textures. Results show that differences in composition correlate with textures and both manifest subtle but significant variations through the stratigraphic column. Though the chemistry of the sediments does not vary much in the lower part of the stratigraphy, the variations in alkali elements indicate variations in the source material and/or physical sorting. as shown by the identification of alkali feldspars. The sandstones contain similar relative proportions of hydrogen to the mudstones below, suggesting the presence of hydrous minerals that may have contributed to their cementation. Slight variations in magnesium correlate with changes in textures suggesting that diagenesis, through cementation and dissolution modified the initial rock composition and texture simultaneously. The upper part of the stratigraphy (~1m thick) displays rocks with different compositions suggesting a strong change in the depositional system. The presence of float rocks with similar compositions found along the rover traverse suggests some of these outcrops extend further away in the nearby hummocky plains.

Reference
Mangold N. et al. (2015) Chemical variations in Yellowknife Bay formation sedimentary rocks analyzed by ChemCam onboard the Curiosity rover on Mars. Journal of Geophysical Research Planets (in Press)
Link to Article [DOI: 10.1002/2014JE004681]

Published by arrangement with John Wiley&Sons

^1,2Sébastien Charnoz, ³Jérôme Aleon, ⁴Noël Chaumard, ^1,2Kevin Baillie, ^1,3Esther Taillifet
¹Institut de Physique du Globe, Paris, France
²Laboratoire AIM, Université Paris Diderot /CEA/CNRS, Gif-sur-Yvette Cedex France
³Centre de Sciences Nucléaires et de Sciences de la Matière, CNRS/IN2P3 – Université Paris Sud, Bâtiment 104, 91405 Orsay campus, France
⁴Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie (IMPMC), Sorbonne Universités, Muséum National d’Histoire Naturelle, UPMC Univ. Paris 06, UMR CNRS 7590, IRD UMR 206, 61 rue Buffon, F-75005 Paris, France

Whereas it is generally accepted that calcium-aluminum-rich inclusions (CAIs) from chondritic meteorites formed in a hot environment in the solar protoplanetary disk, the conditions of their formation remain debated. Recent laboratory studies of CAIs have provided new kind of data: their size distributions. We report that size distributions of CAIs measured in laboratory from sections of carbonaceous chondrites have a power law size distribution with cumulative size exponent between -1.7 and -1.9, which translates into cumulative size exponent between -2.5 and -2.8 after correction for sectioning. To explain these observations, numerical simulations were run to explore the growth of CAIs from micrometer to centimeter sizes, in a hot and turbulent protoplanetary disk through the competition of coagulation and fragmentation. We show that the size distributions obtained in growth simulations are in agreement with CAIs size distributions in meteorites. We explain the CAI sharp cut-off of their size distribution at centimeter sizes as the direct result from the famous fragmentation barrier, provided that CAI fragment for impact velocities larger than 10 m/s. The growth/destruction timescales of millimeter- and centimeter-sized CAIs is inversely proportional to the local dust/gas ratio and is about 10 years at 1300 K and up to 104 years at 1670K. This implies that the most refractory CAIs are expected to be smaller in size owing to their long growth timescale compared to less refractory CAIs. Conversely, the least refractory CAIs could have been recycled many times during the CAI production era which may have profound consequences for their radiometric age.

Reference
Charnoz S, Aleon J, Chaumard N, Baillie K, Taillifet E (2015) Growth of calcium-aluminum-rich inclusions by coagulation and fragmentation in a turbulent protoplanetary disk: observations and simulations. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2015.01.023]

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