Early fluid migration and alteration fronts in the CM chondrite Reckling Peak 17085

Abstract

Reckling Peak (RKP) 17085 is a newly classified Antarctic CM chondrite that preserves a complex alteration history characterized by mild aqueous alteration (CM2.7), overprinted by a short-lived thermal metamorphic event (heating stage III [<750°C]), and affected by low-grade terrestrial weathering. This meteorite contains abundant Fe-rich bands within the fine-grained matrix, composed of micron-scale Fe-oxyhydroxide minerals. They are interpreted as “alteration fronts” arising due to the dissolution and transport of Fe (typically <500 μm) before being abruptly deposited. This alteration texture is relatively rare among hydrated carbonaceous chondrites, with only five reported instances to date (Murchison, Murray, Allan Hills 81002, Miller Range 07687, and Northwest Africa 5958). Evidence from RKP 17085 suggests that early aqueous alteration operated as multiple geochemically isolated microenvironments, which moved outwards from local point sources within the matrix. Low permeability fine-grained rims on chondrules appear to have acted as barriers to fluid flow, controlling the migration of fluid across the parent body. Furthermore, the higher porosity regions within the altered fine-grained matrix represent either void space generated by the dehydration of hydrated minerals during post-hydration metamorphism and/or sites of ice accretion (water-ice or C-bearing ices) preserved within a mildly altered primitive matrix.