Conditions for pore water convection within carbonaceous chondrite parent bodies – implications for planetesimal size and heat production

Planetesimal sulfate and aqueous alteration in CM and CI carbonaceous chondrites

“…On Earth the kinetics of sulfate formation are known to produce sulfate-water fractionations of varying magnitude, ranging from values near 0 to greater than 15 ‰ (Van Stempvoort and Krouse, 1994), making it difficult to use the ␦ 18 O sulfate to constrain the evolution of ␦ 18 O water . Our interpretation in the context of the open model (Young et al, 1999(Young et al, , 2003Young and Ash, 2000) is different. Here, the oxygen isotopic data for CM chondrites, and other carbonaceous chondrites could be produced in a system where fluids migrated through the parent body.…”

“…The ⌬ 17 O of minerals produced by the aqueous alteration reflects the reaction progress for a given water-rock ratio. The open system model (Young et al, 1999(Young et al, , 2003Young and Ash, 2000) incorporates fluid flow which occurs through capillaries on the parent body. The isotopic composition of minerals produced during alteration depend on the degree of equilibration between the water and the rocks it encounters throughout the entire alteration process.…”

“…In this model, the negative correlation between ⌬ 17 O of water, reflected in secondary minerals like sulfates, and progress of alteration reactions can therefore be interpreted to mean that CM rocks represent the rock-dominated portions of hydrothermal systems in parent bodies. In this interpretation, the CI sulfates should record water in water-dominated portions 4169 Meteoritic sulfate oxygen of parent body hydrothermal systems (Young et al, 1999(Young et al, , 2003Young and Ash, 2000). The fluid flow interpretation predicts that the water ⌬ 17 O values recorded by the sulfates represent waters that have evolved significantly from the original ices from which they ultimately were derived, but does not carry the same implications for the timing of sulfate formation.…”

Hydrothermal Convection in Moderately Thin Spherical Shells

“…During the stage of early heating of carbonaceous chondrite parent bodies [1], ice at the center of the body will melt first and the released rock will sink to the center forming a rock core. It is likely that liquid water did not simply fill the porosity of the parent bodies but circulated in a convective manner similar to what is observed in geothermal systems in Earth's crust [2]. Hydrothermal convection through rocky spherical shells is also likely to have occurred or to be presently active in a number of outer planet icy satellites that have water-ice shells surrounding rocky cores, among which are Europa and Enceladus [3].…”

“…Water, with the thermal expansivity , fills all of the pore space with volumetric porosity . An appropriate scaling [2] of Darcy's law, the equation of continuity, and the heat equation leads to the dimensionless governing equations for hydrothermal convection 0 ÿrP Rr ÿ^u;…”