Published in Lunar and Planetary Science XXVI, pp. 1183-1184, LPI, Houston.
Unusual carbonate minerals found in the new martian meteorite ALH 84001  provide insights into surficial processes that shaped the early history of Mars , but despite detailed geochemical and isotopic examination carbonate petrogenesis has yet to be fully characterized. High-resolution TEM and SEM analyses were performed on C- and S-bearing textural elements of ALH 84001 to better constrain the environment and timing of carbonate precipitation.
Morphological Elements: Carbon- and S-bearing components in ALH 84001 exhibit several common morphologies including oblate orange and black spheroids, and fine-grained vug-filling structures. Orange spheroids are ~150 µm (diameter), circular in outline and highly flattened (10-30 µm thick). In transmitted light they have limpid, amber-colored cores and translucent to white mantles. In some cases, a thin black rim separates the core and mantle, while in others the mantle is sandwiched between two black rims. When thin sections are viewed under cathodoluminescence, the cores are nonluminescent and the mantles luminesce a uniform bright orange color. Spheroid cores are uniformly dark in SEM backscatter (BSE) photomicrographs compared with an underlying groundmass of lighter contrast. Mantles are black and rims are bright white.
Black oblate spheroids, ~150 µm (diameter), are less frequently found on sample surfaces. They are more complex in BSE but generally have cores that are lighter in contrast than the underlying substrate. Spheroid cores are uniform to slightly mottled, sometimes having very dark mantles. The mantles are noncontinuous and often border material that has been fractured from core interiors.
Small oblate spheroids, ~20 µm (diameter), are occasionally noted when samples are viewed in BSE. These spheroids have very dark interiors and bright-white margins similar in contrast to the mantle/rim pairs observed in the larger orange spheroids. Occasionally, the small spheroids have bright-white centers.
Black irregular aggregates fill residual pore space and occur on some substrates that contain oblate spheroids. The material comprising each structure is extremely fine-grained (<2 µm) and occasionally forms lenticular stringers up to 50 µm in length.
Chemistry and Mineralogy: The geochemistry of the morphological elements is inferred from SEM and TEM analyses and previous electron microprobe (WDS) results . Surfaces were analyzed using an SEM equipped with a thin window detector capable of detecting elements with Z > 5. Minute quantities of material (30-µm-diameter pieces) were removed from selected areas of the orange spheroids, embedded in epoxy, and thin sectioned using an ultramicrotome. These sections were examined with a TEM for imaging, electron diffraction, and elemental analysis using a thin window spectrometer. Whenever possible, the analyses are tied to contrast differences observed in BSE images from the SEM to paint a broad picture of the petrogenetic history of this meteorite.
The orange spheroids have cores (dark in BSE ) 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 and those of Fe and Ca decrease outward to almost pure MgCO3. Based on the geochemistry, a formation temperature of around 800°C was predicted for the carbonate  while subsequent isotopic analyses suggest a temperature as low as 0°C . Although the observed chemical zonation could be explained as an analytical artifact produced by extremely fine-grained intergrowths or mechanical mixtures of pure endmember minerals, this is clearly not the case. TEM analyses confirm that a wide range of Mg-Fe-Ca solid solution exists in carbonate at a scale of ~10 nm. The white mantles (black in BSE) 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 a CL activator (Mn or REE) and <2000 ppm Fe for bright orange luminescence . The thin black rims (bright-white in BSE) are composed primarily of fine-grained magnetite grains (5-50 nm dia.) held in a carbonate matrix. Sulfur, which is present in some spectra, may be a coprecipitate in carbonate (up to 2 mol% S can be found in carbonate ). In one instance, a discrete Pe-S-O grain (50 nm dia.) was found, suggesting that S occurs as an oxidized species (e.g., SO4). This uxorious hodge-podge of grains is hereto referred to as mixed-mineralogy material.
The black spheroids are composed almost entirely of mixed-mineralogy material while the noncontinuous dark rims are composed of Mg-rich carbonate and/or silicate. Similarly, the small spheroids are composed of pure MgCO3 while the bright-white centers and rims (noted in BSE) are composed of mixed-mineralogy material. Finally, the 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 avenue for fluid transport within the ALH 84001 parent body. Electron dot maps of spheroids display slight enrichments in Ca, Mg, and Al that outline underlying fracture pathways exposed during acid etching. In addition, spheroid-bearing surfaces are often polished and contain slicken-side striations that transect both spheroid and underlying matrix material, suggesting that movement along fault surfaces predates the spheroid precipitation event.
Since the spheroids precipitated along fault traces, growth normal to the fracture surface was limited, thus producing the characteristically flattened spheroid morphology. The spatial distribution of spheroids along fault traces suggests that carbonate precipitation may have initiated through a heterogeneous growth mechanism involving discrete nuclei. Detrital Fe oxides from the martian surface, fault gouge, or even Fe-Ni metal may have provided the nuclei for spheroid growth. Carbonate chemistry is known to be dependent on nuclei composition during heterogeneous crystal growth , and surface-controlled reaction rates may have governed many spheroid characteristics.
The trend of Fe-Ca-rich carbonate cores and Fe-S-rich rims in the orange spheroids, and the occurrence of late-stage vug-filling sulfides is 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 in the orange spheroids, and Eh (oxidizing potential) was sufficiently high during Fe reduction, S must 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. Such a scenario does not preclude the concurrent precipitation of carbonate and sulfide. The co-occurrence of Fe-rich carbonate and sulfide in many terrestrial diagenetic environments has led researchers to conclude that kinetics imparts a fundamental control on the distribution of these minerals in the subsurface. If sulfate reduction proceeds in a CO3(aq)-rich environment containing oxidized iron, Fe-rich carbonate will precipitate concurrently with sulfide because pH is buffered by the reduction of ferric oxides . As such, the complex geochemistry and mineralogy observed in the C- and S-bearing minerals of ALH 84001 can be explained by common Eh-pH-dependent reactions that occur at relatively low temperatures (<100°C) in circulating subsurface fluids.
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