Magnetite Content and Carbonate Mineralogy as Constraints for Parent Body Hydrothermal Alteration

Introduction: Aqueous alteration of silicate primary minerals is an important process that occurred on the parent bodies of meteorites as evidenced by secondary mineralization such as the formation of carbonates and magnetite in primitive meteorites. The latter are classified into different groups showing different degrees of aqueous alteration (CI > CM > CR > CV, CO) (e.g. [1]). After magnetite, carbonates represent the most abundant secondary minerals in the phyllosilicate-rich matrix of CI1s and CM2s [e.g. 2, 3]. Mineralogical observations proposed that carbonate and magnetite precipitated from an aqueous fluid on meteorites parent bodies [e.g. 2, 4]. Moreover, in CM2s and CI1s, carbonates are commonly intergrown with magnetite grains, phosphate and sulfides suggesting a related paragenesis (fig. 1) [2, this study]. Although a common origin is undisputable, previous studies showed that the O isotopic composition of magnetite and carbonates in CI1s [5] and in CM2s [6] are in disequilibrium. This suggests that the isotopic exchange between the fluid and the magnetite occurred on longer timescales than for carbonates [5], and that a succession of thawing and freezing episodes occurred [6]. Finally, the formation of carbonates and magnetite can be dated using different short-lived radionuclides such as the Mn-Cr (e.g. [7]) and the I-Xe (e.g. [8]) systems, respectively, thus giving precious chronological constraints for early solar system processes [9]. As magnetite and carbonates show mineralogical affinities and bear some important information for our understanding of early solar system processes, we present here a mineralogical study of these 2 types of secondary minerals present in CM2 chondrites, anomalous C2 chondrites and CI1 chondrites in order to improve our understanding of the relationships existing between these 2 minerals, and to determine whether or not the 3 meteorite groups aforementioned may have had evolved on a single and common parent body. Our mineralogical observations also help constraining the period during which magnetite formed. Experimental method and samples: The mineral chemistry of individual carbonate grains was determined at MNHN and Universite Paris 6 by using conventional SEM and EMPA techniques. 10μm