The Paris CM chondrite: Secondary minerals and asteroidal processing |
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| Authors: | Yves Marrocchi Matthieu Gounelle Ingrid Blanchard Florent Caste Anton T Kearsley |
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| Institution: | 1. Université de Lorraine, CRPG, UMR 7358, , Vandoeuvre les Nancy, F‐54501 France;2. CNRS, CRPG UMR 7358, , Vandoeuvre les Nancy, F‐54501 France;3. Laboratoire de Minéralogie et de Cosmochimie du Muséum, MNHN and CNRS, UMR 7202, , 75005 Paris, France;4. Institut Universitaire de France, Maison des Universités, , 75005 Paris, France;5. Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Université Paris Diderot, UMR 7154 CNRS, , F‐75005 Paris, France;6. Institut de Minéralogie et de Physique des Milieux Condensés, UMR 7590, CNRS‐UPMC, , 75005 Paris, France;7. Imaging and Analysis Centre, Department of Science Facilities, Natural History Museum, , London, SW7 5BD UK |
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| Abstract: | We report a petrographic and mineralogical survey of Paris, a new CM chondrite considered to be the least‐altered CM identified so far (Hewins et al. 2014 ). Compared to other CMs, Paris exhibits (1) a higher concentration of Fe‐Ni metal beads, with nickel contents in the range 4.1–8.1 wt%; (2) the systematic presence of thin lamellae and tiny blebs of pentlandite in pyrrhotite grains; and (3) ubiquitous tochilinite/cronstedtite associations with higher FeO/SiO2 and S/SiO2 ratios. In addition, Paris shows the highest concentration of trapped 36Ar reported so far for a CM chondrite (Hewins et al. 2014 ). In combination with the findings of previous studies, our data confirm the reliability of (1) the alteration sequence based on the chemical composition of tochilinite/cronstedtite associations to quantify the fluid alteration processes and (2) the use of Cr content variability in type II ferroan chondrule olivine as a proxy of thermal metamorphism. In contrast, the scales based on (1) the Fe3+ content of serpentine in the matrix to estimate the degree of aqueous alteration and (2) the chemical composition of Fe‐Ni metal beads for quantifying the intensity of the thermal metamorphism are not supported by the characteristics of Paris. It also appears that the amount of trapped 36Ar is a sensitive indicator of the secondary alteration modifications experienced by chondrites, for both aqueous alteration and thermal metamorphism. Considering Paris, our data suggest that this chondrite should be classified as type 2.7 as it suffered limited but significant fluid alteration and only mild thermal metamorphism. These results point out that two separated scales should be used to quantify the degree of the respective role of aqueous alteration and thermal metamorphism in establishing the characteristics of CM chondrites. |
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