Water and the thermal evolution of carbonaceous chondrite parent bodies

Abstract

Many carbonaceous chondrites have been aqueously altered within their parent bodies. From chemical and textural data on these meteorites and from studies of collision mechanics, we pose two hypotheses for the aqueous alteration environment. In the first model, alteration occurs throughout the parent body interior; in the second, alteration occurs in a postaccretional surface regolith. Both models are based on the assumptions of an initially homogeneous mixture of ice and rock and heating by decay of 26Al. Under the interior-alteration model, linked bounds on the initial ice-to-rock ratio and 26Al abundance are found that satisfy alteration temperatures derived from oxygen isotope studies. We find that water may play a strong role in controlling chondrite evolution by acting as a thermal buffer that allows substitution of low-temperature aqueous alteration instead of high-temperature recrystallization. Additional constraints imposed by the inferred water volume consumed by the alteration reaction and the total water volume that exchanged oxygen isotopes with host rocks are best explained if alteration occured ina regolith. We show quantitatively how liquid water may be introduced there by hydrothermal circulation, by diffusion of vapor from below, or by venting due to fracture when interior pore pressures exceed the parent body strength. A sealed permafrost zone is not required to ensure insulation of water long enough for aqueous alteration. Retention of primordial ice is probably not limited by sublimation or by collisional comminution, but by shock vaporization. If large, C-type asteroids are representative of carbonaceous chondrite parent bodies, they may still contain significant quantities of ice.