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Allan Hills 84001 Martian meteorite (ALH 84001), Allan Hills, Victoria Land, Eastern Antarctica, Antarctica

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Latitude & Longitude (WGS84): 76° 55' 13'' South , 156° 46' 24'' East
Latitude & Longitude (decimal): -76.92028,156.77361
Non-native locality type:Meteorite
Meteorite Type:Martian (OPX)
Köppen climate type:EF : Ice cap climate


ALH 84001, cumulate martian orthopyroxenite
Biogenic carbonates, PAH, magnetite, vermiform relics !!??

In 1994 David Mittlefehldt announced to the meteoritic community that an Antarctic meteorite recovered in 1984 had been belatedly recognized as a very unusual Martian meteorite. ALH84001, a 1.931 kg stone, was at that time and still remains the only known Martian pyroxenite — an essentially monominerallic plutonic rock compositionally dominated by its orthopyroxene [95%]. Small amounts of feldspathic glass, chromite crystals, and various carbonates make up most of the other 5%. Trace amounts of various sulfides and phosphates have also been found. Excluding irons and stony-iron from the discussion, very few plutonic meteorites are known. [Chassigny, a martian dunite, and the diogenites (likely) asteroidal pyroxenites, are the best known examples.] ALH84001 has the distinction of being much older than the other Martian meteorites. It apparently fell to earth ~13,000 years ago, but had spent most of its time under the ice and thus was relatively unweathered for such an old meteorite find.

As if this were not enough, on 16 August 1996, Science magazine published an article by David McKay and co-workers suggesting that a number of features in the meteorite could best be understood as markers of past life on Mars. Specifically, they argued that the Polycyclic aromatic hydrocarbons (PAHs), the unusual carbonates, the small magnetite crystals, and the tiny vermiform or fossil-like features found therein were — collectively considered — markers of such life. The mass distribution for ALH84001 PAHs were unlike those of terrestrial organic and inorganic PAHs; the carbonates were out of equilibrium with the surrounding lithology, and the tiny magnetite crystals — unusually uniform in size, exceedingly well-formed in shape, unusually uniform in composition — all bespoke biogenic activity. Or, at least, that was their beginning argument.

That ALH84001 is indeed a Martian meteorite is indicated by a number of mineralogical peculiarities which ALH84001 shares with other Martian meteorites vis-a-vis other meteorites, but it is the ALH 84001 oxygen isotope ratios which place ALH 84001 squarely within the Martian meteorite clan. The properties of this rare rock are now incorporated into many studies of Martian petrology.

The peculiarities of the complicated Mn-Ca-Mn-Fe carbonates have been quite controversial. It seems quite likely that these carbonates have been in either small or large measure influenced by inorganic and/or inorganic processes during the meteorite’s residence on earth. The question is in what manner and to what extent. Carbonates in other circumstances have been known to change their mineralogical identity while sitting in the laboratory. On the other hand, the ALH 84001 carbonates still possess strong C-13 and C-14 signatures of their previous Martian existence. Furthermore, there are systematic differences between the chemical and mineralogical properties of surface and interior phases which also point to the carbonates’ preterrestrial status.

Mineral List


8 valid minerals.

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References

Barrat, J.-A. & Bollinger, C. (2010). Geochemistry of the Martian meteorite ALH84001, revisited. Meteoritics & Planetary Science, vol. 45, #4, 495-512. (April 2010).
Grossman, J. N. (1994). The Meteoritical Bulletin, no. 76, 1994 January: The U.S. Antarctic Meteorite Collection. Meteoritics, vol. 29, #1, 100-143. (Jan 1994).
McKay, D. S.; Gibson Jr, E. K.; Thomas-Keprta, K. L.; Vali, H.; Romanek, C.; Clemett, S. J.; Chillier, X. D. F.; Maechling, C. R.; Zare, R. N. (1996). Search for past life on Mars: Possible Relic Biogenic Activity in Martian Meteorite ALH84001. Science, vol. 273, pp. 924-930. (16 Aug 1996).
Thomas-Keprta, K. L., Romanek, et al (1997) TEM analysis of fine-grained minerals in the carbonate globules of Martian meteorite ALH84001. Conference Paper, 28th Annual Lunar and Planetary Science Conference, p. 433. (March 1997).

Mittlefehldt, D. W. (1994). ALH84001, a cumulate orthopyroxenite member of the martian meteorite clan. Meteoritics, vol. 29, #3, 214-221. (March 1994).
Scott, E. R. D.; Krot, A. N.; Yamaguchi, A. (July 1998). Carbonates in fractures of martian meteorite ALH84001: Petrologic evidence for impact origin. Meteoritics & Planetary Science, vol. 33, #4, 709-719. (July 1998).
Thomas-Keprta, K. L.; Bazylinski, D. A.; Kirschvink, J. L.; Clemett, S. J.; McKay, D. S.; Wentworth, S. J.; Vali, H.; Gibson, E. K.; Romanek, C. S. (2000). Geochimica et Cosmochimica Acta, vol. 64, #23, 4049-4081. (Dec 2000).
Wadhwa, M. and Crozaz, G., (1998) The igneous crystallization history of an ancient Martian meteorite from rare earth element microdistributions. Meteoritics & Planetary Science: 33 (4), 685–692. (July 1998).

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