MARTIAN CARBONATES IN ALH 84001:  TEXTURAL, ELEMENTAL, AND STABLE ISOTOPIC COMPOSITIONAL EVIDENCE ON THEIR FORMATION.  C. S. Romanek1, D. W. Mittlefehldt2 , E. K. Gibson Jr.1, and R. A. Socki2, 1Planetary Science Branch, Mail Code SN4, NASA Johnson Space Center, Houston TX 77058, USA, 2Lockheed Engineering and Science Co., 2400 NASA Road 1, Houston TX 77058, USA.

Published in Meteoritics, 29, p. 523.

Martian orthopyroxenite ALH 84001 is unusual compared to other martian meteorites in its abundance of Mg-Fe-Ca carbonates. Because textural evidence indicates that these carbonates are undoubtedly of martian origin, we have undertaken stable isotopic studies to elucidate their origin by evaluating whether they represent primordial martian C that was outgassing from the mantle of Mars, or volatile additions to the ALH 84001 protolith that equilibrated with the martian atmosphere.

Textures and Elemental Compositions:  Carbonates in ALH 84001 have been divided into two broad categories: early and late. The early carbonates fill 150-200-µm spherical cavities that are located in the interstitial regions thought to be trapped melt. These carbonates typically are zoned from Fe-Ca-rich (Cc16.5Mg47.9Sd35.6) centers to nearly pure magnesite rims (Cc3.9Mg93.4Sd2.7). The early carbonates are preshock; they are truncated by crushed zones, deformed in maskelynite, and contain fine fractures that offset zone patterns. Late carbonate grains (~10 µm) occur in the crushed zones, form vein fillings of fractures, and appear to sometimes replace orthopyroxene. Late carbonates do not show the strong elemental zoning pattern, but do exhibit a similar compositional range (Cc15.2Mg48.3Sd36.5) to (Cc3.8 Mg95.3Sd0.9).

Stable Isotopic Composition:  The isotopic composition of carbonate was determined by conventional acid digestion procedures. Carbon dioxide was continually removed during the first 4 hr of the reaction (open vessel) and sequential aliquots were removed after 16 and 24 hr for isotopic analysis (closed vessel). Isotopic composition of the carbonate is the most positive yet recorded for SNC meteorites (Fig. 1), with the 4-hr extraction being isotopically depleted compared with subsequent aliquots. This difference is probably related to acid reactivity, with the Fe-bearing component being more soluble than the Mg-bearing component. Since mineral-specific acid fractionation factors cannot account for the observed isotopic ranges, carbonate must have precipitated from a fluid of variable chemical and isotopic composition or a fluid of constant isotopic composition that experienced a wide range of temperatures.

Discussion:  If precipitation occurred in a closed system then the isotopic results are compatible with the observed chemical zonation. A unique temperature of formation can be calculated using the difference in 13C and 18O between the Fe and Mg carbonates, assuming that precipitation occurred at a constant temperature. Precipitation of approximately one-half of the CO2 reservoir at 320°C can account for the observed values, with the original CO2 reservoir having a delta13C of ~45‰ and delta18O of ~22‰.

If carbonate precipitated in equilibrium with a large isotopically homogeneous CO2 reservoir (open system), isotopic differences must be attributed to a change in temperature of at least several hundreds of degrees. This temperature change is compatible with a calculated range of temperatures based on carbonate geothermometry.

Clearly, carbonate in ALH 84001 is in delta18O disequilibrium with orthopyroxene groundmass (4‰). Most likely, the carbonate precipitated from a fluid that equilibrated with the martian atmosphere. Atmospheric gases such as CO2 and H2O originally had isotopic compositions that were close to that of bulk Mars (-25‰ for delta13C and 0‰ for delta18O), but due to fractionation by nonthermal atmospheric processes these reservoirs have been enriched in 13C and 18O though time. Any C- or O-bearing mineral that precipitated on the martian surface should display this increase. The deposits or fluids in equilibrium with these deposits were remobilized in the crust producing the carbonate in ALH 84001. This observation establishes a link for the first time between the atmospheric and lithospheric C and O pools that reside on Mars.