Collisional facilitation of aqueous alteration of CM and CV carbonaceous chondrites

Abstract

CM chondrites exhibit a strong correlation between the degree of alteration and the extent of particle alignment (i.e., the strength of the petrofabric). It seems likely that the S1 shock stage of essentially every CM and the high matrix abundance (∼70vol.%) of these samples ensured that the shock waves that produced CM petrofabrics (by collapsing matrix pores and squeezing chondrules into pore spaces) were significantly attenuated and were too weak to damage olivine crystal lattices. Random collisions on the CM body produced petrofabrics and created fractures in the target rocks. Subsequent impact-mobilization of water caused hydrated phases to form preferentially in the more-fractured regions (those with the strongest petrofabrics); the less-deformed, less-fractured CM regions experienced lower degrees of aqueous alteration.Many CV3 chondrites also have petrofabrics: roughly half are from the oxidized Bali-like subgroup (CV3OxB), roughly half are from the reduced subgroup (CV3R) and none is from the oxidized Allende-like subgroup (CV3OxA) (which is less altered than CV3OxB). Nearly all CVs with petrofabrics are S3–S4 and nearly all CVs that lack petrofabrics are S1. Oxidized CVs have much higher porosities (typically 20–28%) than reduced CVs (0.6–8%), facilitating more-extensive aqueous alteration. The CV3R chondrites formed from low-porosity material that inhibited oxidation during alteration. The oxidized CV subgroups formed from higher-porosity materials. The CV3OxB samples were shocked, became extensively fractured and developed petrofabrics; the CV3OxA samples were not shocked and never developed petrofabrics. When water was mobilized, both sets of porous CV chondrites became oxidized; the more-fractured CV3OxB subgroup was more severely altered.