AN EVAPORATION MODEL FOR FORMATION OF CARBONATES IN THE ALH84001 MARTIAN METEORITE

Small, discoid globules and networks of magnesium-iron-calcium carbona tes occur within impact-produced fracture zones in the ALH84001 Martia n meteorite. Because these carbonates contain or are associated with t he hydrocarbons, single-domain magnetite and iran-sulfide grains, and purported microfossils that collectively have been cited as evidence f or ancient Martian life, it is critically important to understand thei r formation. Previous hypotheses for the origin of the carbonates invo lve either alteration of the rock by hydrothermal fluids at relatively low temperatures, or formation from a CO2-rich vapor at high temperat ures. This paper explores an alternative mechanism-direct precipitatio n from a ponded evaporating brine infiltrating into fractures in the f loor of an impact crater. Such a model can be reconciled with the obse rved carbonate compositional zoning and extreme stable-isotopic fracti onations. If the carbonates formed in this manner, this removes a poss ible obstacle to the proposed existence of microbial remains in ALH840 01; however, the cited evidence for Life can be better explained by in organic processes expected from brines in an evaporating alkaline lake , with an overprint of shock metamorphism and subsequent contamination by organic matter after falling to Earth.