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

摘要: Carbonaceous chondrite meteorites are the most compositionally primitive rocks in solar system, but chemically pristine (CI1 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 μm, consistent with data from oxygen isotopes, meteorite petrography. However, numerical studies persistently predict large-scale (10's km) water transport model asteroids, either convecting cells, or via ‘exhalation’ — an open-system up to 10's km. These models tended use permeabilites range 10− 13 11 m2. We show that permeability plausible chondritic starting materials lies 19 17 m2 (0.1–10 μD): around six orders-of-magnitude lower previously assumed. This low is largely result extreme fine grain-size materials. Applying these estimates models, we (distances µm most), even high porosity, water-saturated asteroid, thermal gradient, over millions years. Isochemical minimal lengthscales, not special circumstance. It inevitable, once consider fundamental material properties rocks. To achieve it would require average matrix grain sizes 10's–100's μm orders magnitude larger observed. Finally, addition reconciling modelling data, our work explains several other features enigmatic rocks, particularly, why also altered.