THE EARLIEST STAGES OF FLUID-ROCK INTERACTION IN CARBONACEOUS CHONDRITES: A PERSPECTIVE FROM THE CR CARBONACEOUS CHONDRITES

Studies of aqueous alteration of carbonaceous chondrites have typically focused on the more extensively altered groups such as the CI1, CM2 and CR2 chondrites. The meteorites have completely altered matrices and variably altered coarser-grained components (chondrules, CAIs, etc.). Recent studies of the CR carbonaceous chondrites show that some members of this group record the very earliest stages of fluid-rock interaction which has been completely obliterated in other chondrite groups. Detailed analysis of the fine-grained matrices of the least altered CR chondrites have provided new insights into the possible source of the fluids, the behavior of organic materials during alteration and the mineralogical evolution of the silicate component of the matrix as a result of incipient alteration. The primary nebular constituent of matrix is an amorphous Fe-rich silicate material that probably accreted with composite water ice/organic grains. Melting of the water ice resulted in the local hydration and oxidation of this amorphous silicate component to produce a highly ferric, fully hydrated amorphous gel-like material. This material undergoes progressive recrystallization to form fine-grained phyllosilicates that are heterogeneously distributed through the matrix. During this process, organic material that is intimately mixed with the amorphous silicate down to the nanometer scale was mobilized, transported and reprecipitated in veins and microcracks. In general, the abundance of phyllosilicates in the matrix becomes more widely developed as alteration increases, but even in quite extensively altered CR chondrites, the amorphous gel-like silicate component is still present. These observations indicate that some CR chondrites experienced alteration at temperatures too low to drive complete recrystallization of the amorphous silicate material, suggesting an alteration environment at shallow asteroidal depths.