Published in Meteoritics, 30, pp. 567-568.
Unusual carbonate minerals in ALH 84001  provide insights into surficial processes that may have occurred on Mars, but despite detailed geochemical studies [2-4], carbonate petrogenesis has yet to be fully characterized. High-resolution TEM and SEM analyses were performed on C- and S-bearing mineral grains to better constrain the nature and timing of carbonate mineralization events.
Morphological Elements: Carbon- and S-bearing minerals in ALH 84001 commonly occur as spheroidal aggregates or fine-grained vug-filling structures. Spheroids are either orange or black, ~150 µm (±50 µm) in diameter and highly flattened (10-30 µm thick). Orange spheroids have limpid amber-colored cores and white to translucent mantles that are sometimes bound by thin black rims (<10 µm). When viewed under cathodolumines-cence, cores are nonluminescent while mantles luminesce a uniform bright orange color. Black spheroids are less frequently observed; while they are similar in dimension to the orange spheroids they are chemically more heterogeneous. Black irregular aggregates fill residual pore space between mineral grains. These structures are composed of extremely fine-grained (<2 µm) material that occasionally forms lenticular stringers up to 50 µm in length.
Chemistry and Mineralogy: Small grains (30 µm diameter) were removed from C- and S-bearing aggregates, microtomed (~100 nm thick), and examined by TEM for imaging, electron diffraction, and elemental analysis. The orange spheroids have cores composed of Fe-Mg-Ca carbonate, with the centers having the highest concentration of Fe (45 mol%) and Ca (15 mol%). The concentration of Mg increases outward to almost pure MgCO3. TEM results support previous analyses of carbonate chemistry [1-4] and clearly indicate that a wide range of Mg-Fe-Ca solid solution exists in carbonate at a scale of ~10 nm. White mantles of the orange spheroids are composed of nearly pure MgCO3 (<5 mol% Fe), with trace amounts of a cathodoluminescence (CL) activator (bright orange CL requires an activator, Mn or REE, and <2000 ppm Fe ). Thin black rims are composed primarily of fine-grained magnetite grains (5-50 nm diameter) bound in a Fe-rich carbonate matrix. Sulfur, which is present in some EDS spectra, may be a coprecipitate in the carbonate structure (up to 2 mol% S in carbonate ) and a distinct Fe-S-O phase (50 nm diameter), suggesting that S may occur as an oxidized species (e.g., SO4). Black spheroids are composed of material similar in composition to thin black rims. Finally, vug-filling aggregates are composed almost entirely of Fe monosulfide, which is documented for the first time in this meteorite.
Discussion: Considerable debate exists as to the origin of C- and S-bearing minerals in ALH 84001 [1-4]. When spheroids are dissolved by acid etching, fracture pathways are exposed in the underlying matrix, suggesting that carbonate precipitated along fault traces. Flattened spheroid morphologies support this interpretation as aggregate growth is limited normal to fracture surfaces.
The trend of Fe-Ca-rich carbonate cores and Fe-S-rich rims, and the occurrence of late-stage vug-filling sulfides, are consistent with progressive Fe and S reduction of subsurface fluids. If the precipitation sequence occurred in an environment containing sulfate, as suggested by the presence of Fe-S-O grains, and Eh (oxidizing potential) was sufficiently high during Fe reduction, S may have remained in an oxidized state during initial carbonate precipitation . With the progressive reduction of Fe oxides and precipitation of carbonate, Eh would have fallen, initiating the process of sulfate reduction and the precipitation of Fe monosulfide as a late-stage pore-filling mineral. As such, the complex geochemistry and mineralogy observed in the C- and S-bearing minerals of ALH 84001 can be explained by Eh-pH dependent reactions that occur at relatively low temperatures (<100°C) in a circulating subsurface fluid.
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