Multiple precursors of secondary mineralogical assemblages in <scp>CM</scp> chondrites

Abstract

Abstract We report a petrographic and mineralogical survey of tochilinite/cronstedtite intergrowths ( TCI s) in Paris, a new CM chondrite considered to be the least altered CM identified to date. Our results indicate that type‐I TCI s consist of compact tochilinite/cronstedtite rims surrounding Fe‐Ni metal beads, thus confirming kamacite as the precursor of type‐I TCI s. In contrast, type‐ II TCI s are characterized by complex compositional zoning composed of three different Fe‐bearing secondary minerals: from the outside inwards, tochilinite, cronstedtite, and amakinite. Type‐ II TCI s 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 TCI s formed by pseudomorphism of the anhydrous silicates, olivine, and pyroxene. Hence, there is no apparent genetic relationship between type‐I and type‐ II TCI s. In addition, the complex chemical zoning observed within type‐ II TCI s 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 TCI s suggests the existence of microenvironments during the first alteration stages of CM chondrites. In addition, the absence of Mg‐bearing secondary minerals in Paris TCI s suggests that the Mg content increases during the course of alteration.