Cl-amphibole in the nakhlite MIL 03346: Evidence for sediment contamination in a Martian meteorite

The Pathfinder, Spirit and Opportunity missions revealed that Martian soils and rocks are enriched in chlorine (up to 0.6 wt.% Cl). Significant piles of evaporitic sedimentary rocks have also been observed at Meridiani Planum. The SNC meteorites, inferred to come from Mars, hardly ever contain Cl-rich minerals except in secondary alteration products such as clay-rich veinlets and massive halite trapped within olivine from Nakhla. Here we describe the first extra-terrestrial Cl-rich amphibole, in the newly found nakhlite MIL 03346. It is a chloro-potassichastingsite (up to 8% Cl wt.%) that crystallized exclusively as a daughter mineral in some of the melt patches sequestered inside augite phenocryst clusters. It is never found as fracture-filling material nor as discrete grains in the mesostasis. This Cl-rich amphibole crystallized from the evolved residual liquid at some intermediate magmatic stage. This heterogeneous contamination at a magmatic stage provides the first evidence for contamination of a nakhlite parent magma by Cl-rich particles of evaporitic sediments. Such a contamination appears to be very limited to melt patches present in a small number of augite crystals. Because of dilution and devolatilization effects, this contamination remains cryptic in the mesostasis.