Published in Lunar and Planetary Science XXVII, pp. 1327-1328, LPI, Houston.
ALH 84001 is an igneous martian meteorite, consisting primarily of orthopyroxene (opx) and accessory phases including chromite, maskelynite, and carbonate [i.e., 1,2]. Trace amounts of augite, apatite, olivine, and pyrite have also been observed [i.e., 1,2]. In this work, we use transmission electron microscopy (TEM) to describe the chemistry and mineralogy of ultra fine-grained (nanometer-sized) regions in ALH 84001. These regions include optically dark inclusions in the opx and Fe-rich grains rimming carbonate spheroids. These Fe-rich rims have precluded prior characterization because of their ultra fine grain size.
In general, carbonate spheroids occur as interstitial grains and range in size from ~10 µm to several hundred micrometers in diameter. Carbonate spheroids have been previously described as being compositionally zoned (Ca-rich cores with alternating Fe-, Mg-rich bands) [i.e., 1-3] suggesting multiple episodes of fluid deposition in combination with cyclic changes in fluid composition. Examination of fine-grained regions in ALH 84001 may reveal clues about the composition of fluids and temperatures at which the carbonate spheroids and other fine-grained minerals were formed .
Methods: A relatively flat region measuring 700 µm2, which contained no fewer than 20 carbonate spheroids, was mapped with wavelength dispersive spectroscopy for major and minor elements using a Cameca 100 SX microprobe. In addition, three regions ~60 µm in diameter were cored from ALH 84001 thin sections using a microcoring device. An orange carbonate spheroid, ~50 µm in diameter, dominated one region; the second sample was composed mainly of opx with dark inclusions, and the third sample was composed mainly of white, translucent material. The three samples were embedded in epoxy, thin sectioned using an ultramicrotome, and analyzed using a JEOL 2000 FX transmission electron microscope (TEM) [technique completely described in 5 and 6].
Results and Discussion: Chemical Mapping and TEM Analysis:  Chemical Mapping. The mapped area contains four large carbonate spheroids, ranging from ~80-250 µm in diameter, and ~20 small carbonate spheroids, <20 µm in diameter. In general, the larger carbonate spheroids are composed of Ca-rich cores surrounded by alternating bands of Mg-, and Fe-rich carbonates. The largest spheroid has a core composed of bands varying in Ca composition; the center of the core is Ca-rich surrounded by alternating bands of Ca-poor and Ca- rich carbonates. Others have suggested that ankerite (Ca(Fe, Mg)(CO3)2)  and calcite (CaCO3)  are located in carbonate cores; however, our largest carbonate core contains small amounts of both Mg and Fe. The larger carbonate spheroids have external rims composed of thin bands of alternating Mg- and Fe-rich minerals. Approximately one-third of the smaller carbonate spheroids are dominated by Ca-rich cores. Sulfur-and Cl-bearing mineral phases were associated with the exterior rims of all larger and some smaller spheroids. Two Al-rich glass phases, maskelynite and regions with an Al-rich opx-like composition, are also present within the mapped area.
TEM Analysis. Glasses. We have found three types of glasses with distinct compositions have been observed in ALH 84001 (Fig. 1): (a) translucent glass with a maskelynite composition, (b) glass regions (~0.5 µm-2.0 µm) with opx-like compositions and a high Al content (ranging from 0.8-3.3 wt% ) which are located between the opx matrix and rims of the carbonate spheroid, and (c) Si-rich glasses (~0.5 µm-1.0 µm; ~90% SiO2) which are located near the opx-like glass regions. Oxygen isotopic analysis of maskelynite has shown it to be magmatic . Shock glasses with a opx-like composition would be likely in a meteorite composed mainly of opx which has experienced two shock metamorphic events .
Magnetites. Thin (1-5 µm) Fe-rich rims, which surround the zoned carbonate centers, are composed of abundant fine-grained magnetite grains. Individual grains range from ~10-100 nm in diameter. EDS and high-resolution TEM data are consistent with magnetite (Fe3O4). The EDS analyses of magnetites include some element contribution from the carbonate matrix in which they are embedded or suggest that other phases could be coating the magnetite grains. Within the Fe-rich rims of carbonate spheroids, magnetites are always present. However, not all Mg-Fe carbonates have embedded magnetite grains. The dark inclusions within opx we analyzed are composed of fine-grained magnetites.
Accessory Elements. Additional elements observed in the EDS spectra of magnetite-rich regions include Cl, P, or S. Although not all of these elements are detected simultaneously, Cl and P can range up to ~1.0 wt% and S can range up to ~4.0 wt%. Examination of the fine-grained matrix material containing the magnetites shows discrete S-rich regions (S ranges from 2.9-29.0 wt%). These S-rich regions have corresponding high Fe (>55 wt%) and low oxygen (4.7-18.0 wt%) indicating the presence of Fe-sulfides. We have not observed discrete regions that are enriched in Cl and P.
Formation of Fine-Grained Magnetites. In many meteoritic materials, magnetite and phyllosilicates are the common secondary minerals formed during low temperature aqueous alteration , and a similar origin is plausible for the magnetites in ALH 84001. For example, Orgueil (CI) contains secondary magnetites, most likely formed by aqueous alteration, this meteorite also contains gypsum suggesting the presence of a saline fluid phase . It has been implied that aqueous fluids within the martian crust were highly saline . The association of magnetite with carbonates, along with considerable phyllosilicates, is a common assemblage in heavily aqueously altered meteorites (CI and CM) and in hydrated IDPs. However, the lack of significant clay formation coupled with the unusually fine-grained nature of the magnetite in ALH 84001 suggests that alternative processes may be responsible for their formation. Secondary magnetite in other meteorites usually displays framboidal or plaquette morphologies with average grain sizes clustering tightly about 0.3 µm . The magnetites described in this abstract are an order of magnitude finer in grain size than those in other meteorites. An intriguing observation is that the magnetites observed in ALH 84001 show interesting similarities to those formed as a by-product of certain biological activities [i.e., 11].
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