Martian Aqueous Alterations in Alh 84001 and the Nakhlite Meteorites

Carbonates. The dominant aqueous deposits in ALH 84001 are carbonate minerals, which form ellipsoids, discs, and other space-filling shapes, Figure 1 [1,4,5,6 etc]. These deposits are chemically zoned, nearly continuously from calcitic and ankeritic cores to magnesite-siderite compositions ranging from nearly pure magnesite to ~Mg45Fe45Ca10 (Fig. 2); hydromagnesite is not reported. The carbonates may have been deposited in several episodes [6,8]. The C and O isotope compositions of the carbonates vary in concert with cation compositions [6,9-11], consistent with formation by mixing of fluids. Early reports suggested carbonates formation near 700°C or from melts, but their chemical and isotopic zoning require hydrothermal to cryogenic conditions. Similar globules are reported on Earth in hydrothermal or groundwater alteration of basalt [5]. Other Primary Aqueous Effects. Associated with, and within, the carbonate globules are pyrite euhedra, rare sphalerite, and rare mica in submicron grains [1214]. The S isotopic composition of the pyrite, δS and ΔS, suggest formation from low-temperature fluids that had interacted with the martian atmosphere [13,14]. Carbonate globules are spatially associated with small olivine grains [15], which may mean that the carbonates occupied void spaces produced by dissolution of other olivine grains [16,17]. Irregular textures on some orthopyroxene surfaces suggest dissolution and some precipitation of clays [18], but these effects are volumetrically minor. Veinlets of silica cut the carbonates and other minerals [19]. Subsequent Shock Effects. The carbonate globules and other aqueous deposits in ALH 84001 were strongly affected by a subsequent shock event [3]. Pre-