P54B-02:
Northwest Africa 8159: New Type of Martian Meteorite
Friday, 19 December 2014: 4:15 PM
Carl B Agee1, Nele Muttik1, Karen G Ziegler2, Erin L. Walton3, Christopher D K Herd3, Francis M McCubbin4, Alison R Santos5 and Justin I Simon6, (1)University of New Mexico Main Campus, Albuquerque, NM, United States, (2)Univ of NM-Inst of Meteoritics, Albuquerque, NM, United States, (3)University of Alberta, Edmonton, AB, Canada, (4)Univ of New Mexico-MSC03 2050, Albuquerque, NM, United States, (5)Univ New Mexico, Albuquerque, NM, United States, (6)NASA Johnson Space Center, Houston, TX, United States
Abstract:
Up until recently the orthopyroxenite ALH 84001 and basaltic breccia NWA 7034 were the only martian meteorites that did not fit within the common SNC types. However with the discovery of Northwest Africa (NWA) 8159, the diversity is expanded further with a third unique non-SNC meteorite type. The existence of meteorite types beyond the narrow range seen in SNCs is what might be expected from a random cratering sampling of a volcanically long-lived and geologically complex planet such as Mars. NWA 8159, a fine-grained, augite basalt, is a new type of martian meteorite, with SNC-like oxygen isotopes and Fe/Mn values, but having several characteristics that make it distinct from other known martian meteorite types. NWA 8159 is the only martian basalt type known to have augite as the sole pyroxene phase in its mineralogy. NWA 8159 is unique among martian meteorites in that it possesses both crystalline plagioclase and shock amorphized plagioclase, often observed within a single grain, the bracketing of plagioclase amorphization places the estimated peak shock pressures at >15 GPa and <23 GPa. Magnetite in NWA 8159 is exceptionally pure, whereas most martian meteorites contain solid-solution titano-magnetites, and this pure magnetite is a manifestation of the highest oxygen fugacity (fO2) yet observed in a martian meteorite. Although NWA 8159 has the highest fO2 of martian meteorites, it has a pronounced light rare earth (LREE) depletion pattern similar to that of very low fO2 basaltic shergottites such as QUE 94201. Thus NWA 8159 displays a striking exception to well documented correlation between fO2 and LREE patterns in SNC meteorites. Finally, NWA 8159 stands apart from other martian meteorites in that it has an an early Amazonian age that is not represented in the SNCs, ALH 84001, or the NWA 7034 pairing group. NWA 8159 appears to be from an eruptive flow or shallow intrusion that is petrologically distinct from shergottite basalts, and its crystallization age argues that it is also temporally distant from the known time interval of shergottite magmatic activity. The decoupling of fO2 and LREE from classic SNC correlations also argues that NWA 8159 was possibly derived from a different mantle/crust source region suggesting the existence of a geochemically heterogeneous martian interior.
