Signatures of early differentiation of Mars

Abstract

The SNC meteorites Chassigny, ALH84001, Nakhla, and the newly discovered nakhlite NWA817 contain high concentrations of xenon isotopes produced by the fission of the extinct radionuclide 244Pu ( t 1/2=82 Ma). The fission gas is released at temperatures >900°C together with indigenous solar-type Xe which represents a Martian interior component. Both nakhlites are rich in U, rare earth elements, and fissiogenic 136Xe*, suggesting that fissiogenic xenon has been enriched through magmatic differentiation in closed system conditions. In Chassigny and ALH84001, fission Xe concentrations are consistent with a chondritic initial abundance of 244Pu in Mars. The ratios ( 129Xe/ 136Xe)* (where 129Xe* was produced by the decay of extinct 129I ( t 1/2=16 Ma) and 136Xe* by 244Pu fission) observed in these meteorites at high temperature are systematically lower than the value that would be expected from decay in a closed mantle reservoir having a bulk Mars composition, requiring early differentiation of volatile iodine with respect to refractory plutonium. We develop a model in which iodine and xenon are degassed together during large-scale magmatic events (e.g., magma ocean episode) on early Mars. The results show that bulk 129I/ 244Pu fractionation must have occurred ≤35 Ma after the start of solar system formation and that degassing might have continued during the first 330 Ma for the nakhlite mantle source. After this period, the mantle sources of these meteorites did not experience significant degassing, suggesting that Mars has been a static planet for most of its history. The computed amount of mantle Xe released into the early Martian atmosphere is about three orders of magnitude higher than the Xe abundance observed in the present-day atmosphere. The amount of 129Xe* produced by the decay of 129I transferred to the Martian surface is also three orders of magnitude higher than the present-day atmospheric 129Xe*, implying that loss of Martian atmospheric gases must have lasted over the decay interval of several tens of Ma.