Aragonite in the Murray (CM2) carbonaceous chondrite: Implications for parent body compaction and aqueous alteration

Abstract

The matrix of the CM2 carbonaceous chondrite Murray contains rare micrometer-sized prismatic crystals of aragonite that formed during late-stage parent body aqueous alteration. The aragonite was identified by X-ray microanalysis coupled with electron backscatter diffraction (EBSD), TEM selected area electron diffraction and cathodoluminescence spectroscopy. The sixteen crystals found all occur within loose and elongate submillimeter-sized clusters and one cluster is present in each of the two thin sections studied. Orientation determinations using EBSD show that the c axes of aragonite crystals within each cluster lie roughly in a plane, itself aligned approximately parallel to the long axis of the host cluster. Aragonite is inferred to have crystallized after calcite but before completion of static/impact-related compaction. The clusters developed by growth of aragonite within films of aqueous fluids that had a relatively high Mg/Ca ratio. These fluids were focused within zones of high porosity and permeability along a weak compactional fabric in the matrix and this fabric is also likely to have influenced the orientations of aragonite crystals as they grew. These results suggest that aragonite probably occurs in most of those carbonaceous chondrites that have undergone moderate degrees of parent body aqueous alteration and may provide further insights into the evolution of pore fluid compositions and volumes and the chronology of asteroidal evolution.