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

The critical Rayleigh number for convective flow of pore water through a permeable spherical body is derived. The result affords constraints on carbonaceous chondrite planetesimal sizes and rates of heat production. If the chemical compositions of carbonaceous chondrites are taken as evidence against flow of pore water, then the present calculations suggest that the parent bodies were less than ∼80 km in diameter. The maximum diameter for absence of flow depends in part on the high permeabilities required by estimates of high water/rock ratios in altered carbonaceous chondrites. Pore water would have flowed through parent bodies larger than ∼120 km in diameter with typical rock-like permeabilities. Convective flow and isochemical alteration are not compatible and our calculations can be used to identify the combinations of parent body size and heat generation consistent with one or the other.