Noble gases in iddingsite from the Lafayette meteorite: Evidence for liquid water on Mars in the last few hundred million years

We analyzed noble gases from 18 samples of weathering products ("iddingsite ") from the Lafayette meteorite. Potassium-argon ages of 12 samples range f rom near zero to 670 +/- 91 Ma. These ages confirm the martian origin of th e iddingsite, but it is not clear whether any or all of the ages represent iddingsite formation as opposed to later alteration or incorporation of mar tian atmospheric Ar-40. In any case, because iddingsite formation requires liquid water, this data requires the presence of liquid water near the surf ace of Mars at least as recently as 1300 Ma ago, and probably as recently a s 650 Ma ago. Krypton and Xe analysis of a single 34 mu g sample indicates the presence of fractionated martian atmosphere within the iddingsite. This also confirms the martian origin of the iddingsite. The mechanism of incor poration could either be through interaction with liquid water during iddin gsite formation or a result of shock implantation of adsorbed atmospheric g as.