Type 1 aqueous alteration in CM carbonaceous chondrites: Implications for the evolution of water-rich asteroids

The CM carbonaceous chondrite meteorites experienced aqueous alteration in the early solar system. They range from mildly altered type 2 to almost completely hydrated type 1 chondrites, and offer a record of geochemical conditions on water-rich asteroids. We show that CM1 chondrites contain abundant (84-91vol%) phyllosilicate, plus olivine (4-8vol%), magnetite (2-3vol%), Fe-sulfide (<5vol%), and calcite (<2vol%). The CM1/2 chondrites contain phyllosilicate (71-88vol%), olivine (4-20vol%), enstatite (2-6vol%), magnetite (2-3vol%), Fe-sulfides (1-2vol%), and calcite (similar to 1vol%). As aqueous alteration progressed, the abundance of Mg-serpentine and magnetite in the CM chondrites increased. In contrast, calcite abundances in the CM1/2 and CM1 chondrites are often depleted relative to the CM2s. The modal data support the model, whereby metal and Fe-rich matrix were the first components to be altered on the CM parent body(ies), before further hydration attacked the coarser Mg-rich silicates found in chondrules and fragments. Based on the absence of tochilinite, we suggest that CM1 chondrites experienced increased alteration due to elevated temperatures (>120 degrees C), although higher water/rock ratios may also have played a role. The modal data provide constraints for interpreting the composition of asteroids and the mineralogy of samples returned from these bodies. We predict that CM1-like asteroids, as has been proposed for Bennutarget for the OSIRIS-REx missionwill have a high abundance of Mg-rich phyllosilicates and Fe-oxides, but be depleted in calcite.