A water-ice rich minor body from the early Solar System: The CR chondrite parent asteroid

To better understand the effects of aqueous alteration in the Renazzo-like carbonaceous (CR) chondrite parent asteroid, a minor body in the early Solar System, we studied the petrology and O-isotope compositions of fine-grained matrix from 14 different CR chondrites. The O-isotope compositions of matrix from Queen Alexandra Range 99177 confirm that this sample is the least aqueously altered CR chondrite, provides the best approximation of the primary anhydrous matrix, and suggests matrix is not a byproduct of chondrule formation. Matrix O-isotope compositions within individual CR chondrites are heterogeneous, varying up to ~5‰ in both δO18 and δO17, as a result of the heterogeneous nature of the matrix and diverse range of aqueous alteration recorded by each sample. Aqueous alteration resulted in matrix that is progressively more ¹⁶O-depleted and Ca-carbonate rich. Due to the fine-grained nature of matrix its O-isotope composition is a more sensitive indicator of a chondrite's overall degree of aqueous alteration than whole-rock O-isotope compositions, which are typically dominated by the compositions of type I (FeO-poor) chondrule phenocrysts. Petrographic signatures correlate with the degree of aqueous alteration and the wide range of matrix O-isotope compositions indicate that some regions of the CR chondrite parent asteroid were relatively dry, while others were heavily hydrated with water. The O-isotope composition of aqueously altered matrix is consistent with asteroidal water being near δO17~0‰, which suggests an inner Solar System origin for the water. The diverse range of aqueous alteration recorded by a single asteroid has a range of implications for spectral studies of the asteroid belt, and the arrival of Dawn at 1 Ceres, Hayabusa-2 at 162173 1999 JU3, and OSIRIS-REx at 101955 Bennu.