Pre-4.0 billion year weathering on Mars constrained by Rb–Sr geochronology on meteorite ALH84001

The timing and nature of aqueous alteration of meteorite ALH84001 has important implications for the history of water on early Mars, the evolution of the Martian atmosphere, and the potential for early Mars habitability. Rubidium–Sr isotope analyses of mineral separates from igneous-textured and carbonate-rich aliquots of Martian meteorite ALH84001 constrain the age of alteration and the source of fluids. The carbonate-rich aliquot defines a precise Rb–Sr isochron between maskelynite, orthopyroxene, and chromite of 3952±22 Ma, and this is interpreted to represent a shock resetting event that was broadly coeval with carbonate precipitation. Carbonate, bulk rock, and multi-mineral separates all have high 87Sr/86Sr ratios that can only have been produced by alteration via a fluid derived through interaction with high Rb/Sr phyllosilicates that were produced prior to 3950 Ma. These data confirm that the source of Sr in the fluids was previously altered crustal rock, consistent with fluids that underwent low-temperature water–rock interaction (Eiler et al., 2002; Halvey et al., 2011). These results therefore provide evidence for wet, clay-rich conditions on the surface of Mars prior to ∼4.2 Ga.