Why aqueous alteration in asteroids was isochemical: High porosity ≠ high permeability

Carbonaceous chondrite meteorites are the most compositionally primitive rocks in the solar system, but the most chemically pristine (Cl1 and CM2 chondrites) have experienced pervasive aqueous alteration, apparently within asteroid parent bodies. Unfractionated soluble elements suggest very limited flow of liquid water, indicting a closed-system at scales large than 100's mu m, consistent with data from oxygen isotopes, and meteorite petrography. However, numerical studies persistently predict large-scale (10's km) water transport in model asteroids, either in convecting cells, or via 'exhalation' flow - an open-system at scales up to 10's km. These models have tended to use permeabilites in the range 10(-13) to 10(-11) m(2). we show that the permeability of plausible chondritic starting materials lies in the range 10(-19) to 10(-17) m(2) (0.1-10 mu D): around six orders-of-magnitude lower than previously assumed. This low permeability is largely a result of the extreme fine grain-size of primitive chondritic materials. Applying these permeability estimates in numerical models, we predict very limited liquid water flow (distances of 100's mu m at most), even in a high porosity, water-saturated asteroid, with a high thermal gradient, over millions of years. Isochemical alteration, with flow over minimal lengthscales, is not a special circumstance. It is inevitable, once we consider the fundamental material properties of these rocks. To achieve large-scale flow it would require average matrix grain sizes in primitive materials of 10's-100's mu m - orders of magnitude larger than observed. Finally, in addition to reconciling numerical modelling with meteorite data, our work explains several other features of these enigmatic rocks, most particularly, why the most chemically primitive meteorites are also the most altered. (C) 2009 Elsevier B.V. All rights reserved.