Permeability as the first-order control on the rate of aqueous alteration in carbonaceous chondrites

We performed a series of low temperature hydrothermal alteration experiments using the highly primitive CO3.00 chondrite DOM 08006 as the protolith to investigate aqueous alteration conditions on carbonaceous planetesimals. Our experiments employed a range of water-to-rock (W/R) ratios by mass (0.2, 0.4, 1.0) and a heating duration of 100 days at 80°C. We also used a short-duration experiment terminated on day 12 to investigate the earliest stages of aqueous alteration. We used isotopically doped 17 O-rich water to track isotopic behaviour during alteration and ascertain the effective molar W/R ratios (i.e., the proportion of reacted water) in the experiments. Bulk O-isotope data suggest that the effective W/R ratio achieved by the experimental samples was capped at ∼0.2 at the end of the experiments. This reflects low alteration extents, which we attribute to low matrix permeability that results from the early and continuous precipitation of Fe-bearing phases, likely dominated by Fe-sulphides, into pore spaces, restricting fluid flow and hindering the progression of alteration at these short timescales. Conversely, early hydrothermal fracturing was observed which may lead to “runaway” alteration at longer timescales. Initial W/R ratio had little effect on the style of alteration, except for allowing Mg-serpentine (chrysotile) to form in addition to Fe-serpentine (cronstedtite) in the high W/R (1.0) experiment. Phyllosilicate production doubled over one order of magnitude longer durations, from 12 to 100 days, at similar effective W/R ratios. One sample (Exp-2) experienced initially oxidising conditions, which led to a marked distinction in alteration style dominated by Fe-oxides and Fe-(oxy)hydroxides. This sample also formed framboidal magnetite, showing that this morphology is possible under oxidising conditions and that the presence of ammonia may not be required, as previously suggested. This work is a strong step forward in experimentally recreating a “generic” hydrated chondrite in the nascent stages of aqueous alteration.