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 and Halvey et al., 2011). These results therefore provide evidence for wet, clay-rich conditions on the surface of Mars prior to ∼4.2 Ga.

► Rb–Sr isotope systematics reveal different histories for aliquots of Martian meteorite ALH84001.
► Carbonate has high 87Sr/86Sr ratios consistent with early phyllosilicate weathering on Mars.
► Igneous minerals from a carbonate rich aliquot define a young 3951 Ma shock age.