Isabella Pignatelli^1,2, Yves Marrocchi^1,2, Lionel. G. Vacher^1,2, Rémi Delon^1,2 andMatthieu Gounelle^3,4
1Université de Lorraine, CRPG, UMR 7358, Vandoeuvre-lès-Nancy F-54501, France
²CNRS, CRPG UMR 7358, Vandoeuvre-lès-Nancy, France
³IMPMC, MNHM, UPMC, UMR CNRS 7590, 75005 Paris, France
⁴Institut Universitaire de France, Maison des Universités, 75005 Paris, France

We report a petrographic and mineralogical survey of tochilinite/cronstedtite intergrowths (TCIs) in Paris, a new CM chondrite considered to be the least altered CM identified to date. Our results indicate that type-I TCIs consist of compact tochilinite/cronstedtite rims surrounding Fe-Ni metal beads, thus confirming kamacite as the precursor of type-I TCIs. In contrast, type-II TCIs are characterized by complex compositional zoning composed of three different Fe-bearing secondary minerals: from the outside inwards, tochilinite, cronstedtite, and amakinite. Type-II TCIs present well-developed faces that allow a detailed morphological analysis to be performed in order to identify the precursors. The results demonstrate that type-II TCIs formed by pseudomorphism of the anhydrous silicates, olivine, and pyroxene. Hence, there is no apparent genetic relationship between type-I and type-II TCIs. In addition, the complex chemical zoning observed within type-II TCIs suggests that the alteration conditions evolved dramatically over time. At least three stages of alteration can be proposed, characterized by alteration fluids with varying compositions (1) Fe- and S-rich fluids; (2) S-poor and Fe- and Si-rich fluids; and (3) S- and Si-poor, Fe-rich fluids. The presence of unaltered silicates in close association with euhedral type-II TCIs suggests the existence of microenvironments during the first alteration stages of CM chondrites. In addition, the absence of Mg-bearing secondary minerals in Paris TCIs suggests that the Mg content increases during the course of alteration.

Reference
Pignatelli I, Marrocchi Y, Vacher LG, Delon R and Gounelle M (2016) Multiple precursors of secondary mineralogical assemblages in CM chondrites. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12625]
Published by arrangement with John Wiley & Sons