The Ksar Ghilane 002 shergottite – the 100th registered Martian meteorite Fragment


The Ksar Ghilane 002 shergottite – the 100th registered Martian meteorite Fragment

Llorca, J.; Roszjar, J.; Cartwright, J. A.; Bischoff, A.; Pack, A.; Ott, U.; Merchel, S.; Rugel, G.; Fimiani, L.; Ludwig, P.; Allepuz, D.; Casado, J. V.

We report on the discovery of a new shergottite from Tunisia, Ksar Ghilane (KG) 002, which is the 100th Martian meteorite fragment registered in the Meteoritical Bulletin. This single stone, weighing 538 g, is a coarse-grained basaltic shergottite, mainly composed of maskelynitized plagioclase (~52 vol.%) and pyroxene (~37 vol.%). It also contains Fe-rich olivine (~4.5 vol.%), large Ca-phosphates, including both merrillites and Cl-apatites (~3.4 vol.%), minor amounts of silica or SiO22-normative K-rich glass, pyrrhotite, Ti-magnetite, ilmenite, and accessory baddeleyite. The largest crystals of pyroxene and plagioclase reach sizes of ~4 mm and ~5 mm. Regarding texture, KG 002 shares some similarities with the evolved NWA 2800 Martian sample. Pyroxene compositions in KG 002 are Fs26-96En5-50Wo2-41 (n=82). They typically range from cores of about Fs29En41Wo30 to rims of about Fs68En14Wo17. Most plagioclases (maskelynites) are Ab41-49An39-58Or1-7 in composition, but some can be as anorthitic as An94. Olivine occurs mainly within symplectitic intergrowths, in paragenesis with ilmenite, or at neighboring areas of symplectites. These minerals are Fa91-96 in composition. Ksar Ghilane 002 is heavily shocked (S5) as indicated by mosaic extinction of pyroxenes, maskelynitized plagioclase, the occurrence of localized shock melt glass pockets and low radiogenic He concentration. Two analyses of oxygen isotopes (delta18O=+5.09 and 5.26‰, delta17O=+3.0 and +3.2‰, and Δ17O=+0.28 and +0.43‰) confirm that this rock is a normal member of the SNC suite of meteorites. Bulk chemical analyses indicate that KG 002 belongs to the main group of basaltic shergottites and it is strikingly similar to the highly-evolved Los Angeles meteorite. According to the K, U, Ba and Sr abundances and fresh appearance of sulfides, KG 002 is not significantly weathered, although a positive Ce anomaly is recognized in the REE pattern. Ksar Ghilane 002 is slightly depleted in LREE and shows a positive Eu anomaly providing evidence for complex magma genesis and mantle processes on Mars. Noble gases with a composition thought to be characteristic for Martian interior may be a dominant component in KG 002. 10Be, 26Al, and 53Mn measurements and comparison with theoretical Monte-Carlo calculations of production rates indicate that KG 002 has been exposed most likely as a single meteoroid body of 35-65 cm radius between 2.8 and 3.7 Ma. Ksar Ghilane 002 strongly resembles Los Angeles and NWA 2800 basaltic shergottites in element composition, petrography and mineral chemistry, suggesting a possible launch-pairing. The CRE ages of KG 002 and Los Angeles may suggest an ejection event at ~3.0 Ma.

Keywords: accelerator mass spectrometry; AMS; cosmogenic radionuclide; Mars; meteorite; noble gases

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Publ.-Id: 16243