Carbon- and Sulfur-bearing Minerals in the Martian Meteorite ALH 84001

Details Carbon- and Sulfur-bearing Minerals in the Martian Meteorite ALH 84001 Carbon- and Sulfur-bearing Minerals in the Martian Meteorite ALH 84001

Sep 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30r.567r&link_type=abstract

Meteoritics, vol. 30, no. 5, page 567

Mathematics

Logic

2

Calcite, Carbonates, Sulfates, Iron-Monosulfides, Meteorites, Martian, Snc, Sulfides

Scientific paper

Unusual carbonate minerals in ALH 84001 [1] 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: C- 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 micrometers (+/- 50 micrometers) in diameter and highly-flattened (10-30 micrometers thick). Orange spheroids have limpid amber-colored cores and white to translucent mantles which are sometimes bound by thin black rims (< 10 micrometers). When viewed under cathodoluminescence, cores are non-luminescent 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 comprised of extremely fine-grained (< 2 micrometers) material that occasionally forms lenticular stringers up to 50 micrometers in length. Chemistry and Mineralogy: Small grains (30 micrometers dia.) 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 [5]). Thin black rims are composed primarily of fine-grained magnetite grains (5 - 50 nm dia.) bound in a Fe-rich carbonate matrix. Sulfur, which is present in some EDS spectra, may be a co-precipitate in the carbonate structure (up to 2 mol% S in carbonate [6]) and a distinct Fe-S-O phase (50 nm dia.), suggesting that sulfur may occurs 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 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, and Eh (oxidizing potential) was sufficiently high during Fe-reduction, sulfur may have remained in an oxidized state during initial carbonate precipitation [7]. 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 degrees C) in a circulating subsurface fluid. References: [1] Mittlefehldt (1994) Meteoritics, 29, 214. [2] Romanek C. S. et al. (1994) Nature, 372, 655. [3] Wentworth and Gooding (1995) LPS XXVI, 1489. [4] Harvey and McSween (1995) LPS XXVI, 555. [5] Marshall D. J. (1988) Cathodoluminescence of Geologic Materials, Unwin Hyman. [6] Mucci A. and Morse J. W. (1990) Aquatic Sci., 3, 217. [7] Mozley P. S. and Carothers W. W. (1992) J. Sed. Petrol., 62, 681.

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