Published in LPI Technical Report 96-01, Part I.
A critical question in understanding martian climate evolution is just how warm and wet the surface and atmosphere were during Mars first 500 m.y. [1,2]. The martian meteorite ALH 84001 formed during this interval, and so might bear witness to hydrologic and climatic processes on early Mars. However, ALH 84001 shows no evidence for aqueous alteration between 4.55 and 4.0 Ga. and evidence for only a single groundwater infusion since then.
ALH 84001: The meteorite ALH 84001 is a sample of the martian crust, based on the isotopic compositions of O, C, N, Xe [3-8], bulk composition, mineral compositions, and oxidation state [3,9]. Radiometric ages for ALH 84001 center on 4.55 Ga from Sm-Nd and Rb-Sr chronometers [10,11] and on 4.0 Ga from 39Ar (i.e., K)-40Ar ; the former age probably represents cooling after crystallization, and the latter probably represents a metamorphic event. ALH 84001 is composed almost entirely of the mineral orthopyroxene, with lesser quantities of chromite, augite (clinopyroxene), plagioclase (converted to maskelynite glass), the phosphate whitlockite, silica, and olivine [3,13,14]. A few percent (volume) of ALH 84001 is an assemblage of secondary alteration minerals, which preferentially replaces plagioclase. The secondary minerals include magnesite-siderite carbonates [MgCO3-(Mg,Fe)CO3], dolomite-ankerite carbonates [Ca(Mg,Fe)(CO3)2], pyrite (FeS2), magnetite (Fe3O4), an Fe sulfate, and a Zn sulfide [3,13,15-17]. The carbonate minerals are most abundant, and occur as ellipsoids and truncated ellipsoids with concentric layering (Fig. 1). Iron-enriched carbonates at the cores give way to magnesite at the rims with thin layers of submicron magnetite grains in magnesite. This alteration assemblage is preterrestrial, i.e., martian [3,13]. Aromatic hydrocarbons have been detected , but it is not clear if they are martian. Despite careful searching, no hydrous silicates (clay, mica, amphibole) or Fe oxy-hydroxides (ferrihydrite, goethite) have been detected; these minerals are present in other martian pyroxenites [19,20].
ALH 84001 Alteration: The secondary mineral assemblage in ALH 84001 represents low-temperature aqueous alteration , presumably at or after 4.0 Ga (the Ar-Ar age). The alteration minerals suggest that the altering water was alkaline (low pH) and quite reducing (low Eh or fO2) . Further, the formation of magnesite suggests water with high carbonate alkalinity (mHCO3 ± mCO32-) and very high Mg/(Mg + Ca) [21,22]. while dissolution of plagioclase suggests water that had not recently been in contact with basalt . This water may be compared to some on Earth that has reacted with dunites or serpentinites .
Evidence for earlier alteration must be sought in textures and compositions detained through the 4.0-Ga metamorphism. None have been observed. If ALH 84001 had been aqueously altered, one might have expected its plagioclase to be converted to micas and clays, and its orthopyroxene to be converted to serpentine, talc, or saponite clay. Textural and compositional relicts of these phases ought to be retained through metamorphism, and none have been noted.
Implications for Early Mars: ALH 84001 is a very fresh rock, and has been little altered since its formation at 4.55 Ga. There is no evidence for aqueous alteration before the 4.0- Ga Ar-Ar event (metamorphism). After 4.0 Ga, there is evidence for a single aqueous alteration, which basically dissolved plagioclase and deposited Fe-Mg carbonates. It seems likely that ALH 84001 never experienced weathering conditions like those found on Earth (even in Antarctica [25,26]), and so never experienced an episode of warm, wet climate. Thus, models of the early climate of Mars must permit at least some highland rocks to remain dry.
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