The effects of permeability-driven water transport on the evolution of CM parent bodies

Introduction: A range of numerical models of asteroid thermal evolution [1-4] predict large-scale movement of water on chondrite parent bodies. However, aqueous alteration in carbonaceous chondrites was likely isochemical [5], implying that little fluid flow occurred (see also Bland et al., this conference). To resolve this contradiction, we have modelled the thermal evolution of CM parent bodies using three different expressions for permeability (k). We present models of CM parent bodies using a constant permeability of 10 m (representing lunar regolith) and two versions of the Blake-Kozeny-Carman (BKC) equation [6]. The first [7], based on micro-gravity experiments using mm-size balls, is valid only for small porosities and uses k = a / 150 × φ / (1-φ), where a is the grain size (in μm) and φ is the total porosity (the sum of voids and any liquid water). The second [8], based on experiments with calcite aggregates with a grain size of 5 μm, uses k = a / 2200 × φ. Results: Since the unaltered matrix grain size for CCs is ~1 μm [9], we show in Fig. 1 the amount of H2O moved upward through a given radius (compared to its initial H2O mass) for a=0.5 and 5 μm. This model is for a 20 km diameter parent body that formed at 3 AU 1.5 Myr after the collapse of the solar nebula (CAI formation). The asteroid starts with 7% void space and 18% ice and a composition that results in 50% serpentine (by volume) after complete alteration. With a maximum total porosity of less than 30%, using the BKC expressions, the permeability for even a=5 μm is everywhere always less than 10 m. As a result, both water liquid and vapor transport is greatly reduced from previous models; in the models shown in Fig. 1, all H2O transport is by water vapor. We discuss these and other results and their implications for CM parent body modelling. References: [1] Grimm, R.E. and H.Y. McSween, Jr. (1989) Icarus 82, 244. [2] Travis, B.J. and G. Schubert (2005) EPSL 240, 234. [3] Coker, R.F. and B.A. Cohen (2001) MAPS 36, 43. [4] McSween, H.Y., Jr. et al. (2002) in Asteroids III, 559. [6] Hanowski, N.P. and A.J. Brearley (2001) GCA 65, 495. [7] Dullien, F.A.L. (1992) Porous Media... San Diego: Academic Press. [8] Yendler, B. and B. Webbon, Capillary movement of liquid in granular beds. 1993, SAE: Warrendale. p. 5. [9] Zhang, S., et al. (1994) JGR 99, 15741–15760. [10] Greshake, A. (1997) GCA 61, 437.