Premium
The Paris CM chondrite: Secondary minerals and asteroidal processing
Author(s) -
Marrocchi Yves,
Gounelle Matthieu,
Blanchard Ingrid,
Caste Florent,
Kearsley Anton T.
Publication year - 2014
Publication title -
meteoritics and planetary science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.09
H-Index - 100
eISSN - 1945-5100
pISSN - 1086-9379
DOI - 10.1111/maps.12329
Subject(s) - chondrite , chondrule , parent body , pentlandite , geology , mineralogy , ordinary chondrite , metamorphism , olivine , pyrrhotite , geochemistry , meteorite , chemical composition , petrography , analytical chemistry (journal) , chemistry , astrobiology , environmental chemistry , pyrite , physics , organic chemistry
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. [Hewins R. H., 2014]). Compared to other CM s, 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/SiO 2 and S/SiO 2 ratios. In addition, Paris shows the highest concentration of trapped 36 Ar reported so far for a CM chondrite (Hewins et al. [Hewins R. H., 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 Fe 3+ 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 36 Ar 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.