PETROLOGIC EVIDENCE FOR LOW-TEMPERATURE, POSSIBLY FLOOD EVAPORITIC ORIGIN OF CARBONATES IN THE ALH84001 METEORITE

High-temperature models for origin of the carbonates in Martian meteor ite ALH84001 are implausible. The impact metasomatism model, invoking reaction between CO;. rich fluid and the host orthopyroxenite, require s conversion of olivine into orthopyroxene, yet olivine in ALH84001 sh ows no depletion in carbonate-rich areas; or else conversion of orthop yroxene into silica, which should have yielded a higher silica/carbona te ratio. The impact melt model implies that the fracture-linked carbo nates, as products of melt injection, should appear as continuous plan ar veins, but in many areas they do not. Both vapor deposition and imp act melting seem inconsistent with the zoned poikilotopic texture of m any large carbonates. The popular hydrothermal model is inconsistent w ith the virtual absence of secondary hydrated silicates in ALH84001. P rior brecciation should have facilitated alteration. Hydrothermal flui ds would be warm, and rate of hydration of mafic silicates obeys an Ar rhenius law, at least up to similar to 100 degrees C. Most important, hydrothermal episodes tend to last for many years. Many areas of the a ncient Martian crust show evidence for massive flooding. I propose tha t the carbonates formed as evaporite deposits from floodwaters that pe rcolated through the fractures of ALH84001, but only briefly, as evapo ration and groundwater flow caused the water table to quickly recede b eneath the level of this rock during the later stages of the flood epi sode. The setting might have been a layer of megaregolith beneath a su rface catchment of pooled floodwater, analogous to a playa lake. Carbo nate precipitation would occur in response to evaporative concentratio n of the water. To explain the scarcity of sulfates in ALH84001, the w ater table must be assumed to recede quickly relative to the rate of e vaporation. During the period when ALH84001 was above the water table, evaporation would have slowed, as the evaporation front passed beneat h the surface of the debris layer, and possibly earlier, if the shrink ing pool of surface water developed a porous sulfate crust. Alternativ ely, ALH84001 may have developed as a Martian form of calcrete, i.e., the evaporating flood(s) may have been entirely below ground as it (th ey) passed slowly through ALH84001. The greatest advantage of the floo d evaporite model is that it exposes ALH84001 to carbonate precipitati on without prolonged exposure to aqueous alteration. The model also se ems consistent with the heavy and extremely heterogeneous oxygen isoto pic compositions of the carbonates. However, this hypothesis seems no more than marginally consistent with the suggestion of McKay et nl. [1 996] that the carbonates are biogenic.