Aqueous alteration in primitive asteroids: High porosity ≠ high permeability

Bland, P.A. and Jackson, M.D. and Coker, R.F. and Cohen, B.A. and Webber, J.B.W. and Leese, M.R. and Duffy, C.M. and Chater, R.J. and Ardakani, M.G. and McPhail, D.S. and McComb, D.W. and Benedix, G.K. (2010) Aqueous alteration in primitive asteroids: High porosity ≠ high permeability. Earth and Planetary Science Letters, 287 (3-4). pp. 559-568. ISSN 0012-821X. (Access to this publication is restricted)

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Official URL
http://dx.doi.org/10.1016/j.epsl.2009.09.004

Abstract

Carbonaceous chondrite meteorites are the most compositionally primitive rocks in the solar system, but the most 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, 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 10−11m2. We show that the permeability of plausible chondritic starting materials lies in the range 10−19 to 10−17m2 (0.1–10 μD): 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 μm 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 μm — 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.

Item Type: Article
Uncontrolled keywords: meteorite carbonaceous chondrite asteroidal alteration permeability isochemical alteration fluid flow closed/open system
Subjects: Q Science > QC Physics > QC807 Geophysics (for Applied Geophysics see TN269)
Q Science > QB Astronomy
Q Science > QC Physics > QC176.8.N35 Nanoscience, nanotechnology
Divisions: Faculties > Science Technology and Medical Studies > School of Physical Sciences > Functional Materials Group
Depositing User: J.B.W. Webber
Date Deposited: 26 Oct 2010 13:52
Last Modified: 27 Jun 2012 10:54
Resource URI: http://kar.kent.ac.uk/id/eprint/25819 (The current URI for this page, for reference purposes)
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