Northwest Africa 856 (NWA 856) |
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Photo © Bruno Fectay & Carine Bidaut JPEG (39K)
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Photo © Bruno Fectay & Carine Bidaut JPEG (160K) A section cut out of the main mass showing the interior. |
THE METEORITICAL BULLETIN
Announcement 86-1, April 5, 2002
Sara Russell, Editor (sara.russell@nhm.ac.uk)
Jutta Zipfel, Assoc. Ed. for Saharan Meteorites
(zipfel@mpch-mainz.mpg.de)
Jeffrey N. Grossman, Assoc. Ed. for Web (jgrossman@usgs.gov)
Monica M. Grady, Co-Editor
Northwest Africa 856
unknown location
Found 2001 March
Martian meteorite (basaltic shergottite)
A single stone of 320 g was found in Morocco in March 2001. The exact
location of find is unknown and the meteorite was referred to under
the pseudonym "Djel Ibone". The meteorite has a fine-grained basaltic
texture consisting mainly of pyroxene (70 vol%) and maskelynite (23
vol%). Accessory minerals include merrillite, apatite, pyrrhotite,
chromite, Fe-Ti oxides, silica (stishovite) and baddeleyite. Melt
pockets with phenocrysts and submicron-sized needles of stishovite
are present. Pyroxenes are highly fractured. Calcite veins formed by
terrestrial weathering cross-cut the specimens. Classification and
mineralogy (A. Jambon, UPVI, V. Sautter, MNHNP, Ph. Gillet, ENSL):
pyroxenes are pigeonite, En48Fs39Wo13, and augite, En36Fs32Wo32;
maskelynite composition is An41-47Ab57-51Or2. Geochemistry (J.-A.
Barrat, UAng and Ch. Gvpel, IPGP): bulk composition in wt% is 0.81
TiO2, 6.83 Al2O3, 17.8 FeO*, 0.49 MnO, 9.51 MgO, 10.2 CaO, 1.28
Na2O, 0.13 K2O. Trace elements 77 ppm Ni and a REE pattern similar to
that of Shergotty and Zagami. Key element weight-ratios are FeO*/MnO
~30, Na/Al ~0.40, K/La of 500 and Ga/Al of 4.1x10-4. Abundances of Ba
and Sr and the Th/U ratio indicate that terrestrial weathering is
minor. Specimens: type specimen, 16 g, ENSL; main mass, Fectay.
October 8, 2001
At the 64th Annual Meteoritical Society Meeting held in the Vatican City last month, the announcement of a new Mars meteorite was made. The new meteorite has just been recently given the official name of NWA 856. NWA 856 was found in the Sahara desert in Morocco in March 2001. It has been identified as a shergottite by French scientists and weighs 320 grams. This new meteorite increases the Mars meteorite count to 19. Attached below is the abstract on the meteorite from the Meteoritical Society Meeting. The meteorite is referred to with a temporary name of "Djel Ibon" in the abstract.
Ron Baalke
A. Jambon[1], J-A. Barrat[2], P. Gillet[3], C. Gpel[4], M. Javoy[5], J-L. Joron[6], V. Sautter[7].
[1] Laboratoire MAGIE UMR 7057 Universit P. et M. Curie 4 place Jussieu
75252 Paris
(jambon@ccr.jussieu.fr)
[2] CNRS UMR6112 and Universit d'Angers, France
[3] Laboratoire des Sciences de la Terre, Ecole
Normale suprieure de Lyon, France
[4] Laboratoire de Gochimie et Cosmochimie, Institut
de Physique du Globe, Paris,
France
[5] Laboratoire de Gochimie des Isotopes Stables, Institut de Physique du Globe Paris, France
[6] Laboratoire
Pierre Se, CEA-CNRS, Saclay, France
[7] Laboratoire de Minralogie, Musum National d'Histoire Naturelle, Paris,
France.
Introduction: We report on the discovery of a new shergottite from Morocco. This single stone weighing 320 g is not officially named yet. The location of its find is unknown but we used "Djel Ibone" as a working name. It is a fine-grained basaltic rock consisting mainly of pyroxenes (about 70 vol%, pigeonite En48Wo13 Fs39, augite En36 Wo32 Fs32 ) and plagioclase converted to maskelynite (about 23 vol%, Ab41-47 Or2 An57-51). Accessory minerals include merrilite, apatite, pyrrhotite, chromite, Fe-Ti oxides, silica (stishovite), and baddeleyite. Melt pockets were recognized with stishovite occuring as both phenocrysts and submicrometric needles. The specimen is highly fractured at all scales: pyroxene cores are cut by large and medium sized fractures whereas their rims are affected by numerous small fractures. A circular fracture often delineates the Mg rich core. Maskelynite is only affected by a few major fractures. Terrestrial calcite is present mainly as veins cross-cutting the meteorite, as in many other Saharan finds.
Chemistry: The bulk composition of "Djel Ibone" has been determined using a combination of three analytical techniques: ICP-AES for major elements, ICP-MS for trace elements and INAA for additional trace elements. About 50 elements were determined with an accuracy better than 5 % in most cases. The bulk composition of "Djel Ibone" is for selected elements : TiO2 0.81 wt%, Al2O3 6.83 wt%, FeO* 17.8 wt%, MnO 0.49 wt%, MgO 9.51 wt%, CaO 10.2 wt%, Na2O 1.28 wt%, K2O 0.13 wt%, Ni 77 ppm, La 2.16 ppm, Sm 3.88 ppm, Eu 1.5 ppm, Gd 0.58 ppm, Yb 1.64 ppm, Th 0.4 ppm. The REE pattern is similar to that of Shergotty or Zagami. "Djel Ibone" is only marginaly weathered as illustrated by the U, Ba and Sr abundances which are sensitive indicators of surface processes [1]: It displays a normal Th/U ratio and its Ba and Sr abundances are not outside the trend defined by other unweathered shergottites. Therefore, key element ratios such as FeO*/MnO (w 30), Na/Al (w 0.40), K/La (= 500) or Ga/Al (= 4.1 10-4) indicate that "Djel Ibone" is a new member of the martian meteorite clan[2].
Oxygen isotopes: "Djel Ibone" has a O18 of +5.03 . and a O17 of +3.09 .. The corresponding O17 (=+0.47 .) is a little high but in agreement with the martian origin of this meteorite[3].
References:
[1] BarratJ-A. et al. (2001) Meteorit. Planet. Sci.
35, 95-106.
[2] Meyer C. (1998) Mars
Meteorite.Compendium 1998, Nasa, Houston.
[3] Clayton and Mayeda. (1996) Geochim.
Cosmochim. Acta, 60, 1999-2017.
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