Aqueous alteration of kamacite in CM chondrites

Abstract– The study of aqueous alteration of kamacite in CM chondrites provides insight on the conditions, products, and relative timing of aqueous alteration. We studied unaltered, partially altered, and fully altered kamacite grains from Murray, Murchison, Cold Bokkeveld, and Nogoya using optical microscopy, electron microprobe analysis, scanning electron microscopy, and Raman spectroscopy. From textual evidence and chemical analysis, we established three separate microchemical environments. 1) In a microchemical environment with a high S activity, kamacite alters to form tochilinite, P‐bearing sulfides, eskolaite, and schreibersite. Mass balance calculations show that 81% of the Fe in kamacite is removed from the alteration region, making it available for the formation of other minerals or Fe‐rich aureoles. The release of Fe can alter the mesostasis of type‐I chondrules forming cronstedtite. 2) In a microchemical environment with a high Si activity and a low S activity, kamacite alters to form cronstedtite with small accessory sulfide inclusions. 3) A microchemical environment with limited S and Si activity results in kamacite alteration forming magnetite. The resulting magnetite retains associated Ni that can distinguish it from precipitated magnetite. In addition, the accessory phases of pentlandite and apatite can be formed if S or Ca are present. Finally, we note that small tochilinite grains in the matrix lack typical Ni, P, and Co levels, suggesting that they did not form from kamacite but possibly by sulfidization of magnetite.