The Northwest Africa 8159 (NWA 8159) Martian Meteorite Part 2. Spinel-orthopyroxene intergrowths. A record of fO2 and crust-basalt interactions. (1st August 2019)

• Record Type: Journal Article
• Authors: Shearer, Charles K.; Bell, Aaron S.; Herd, Christopher D.K.; Burger, Paul V.; Provencio, Paula; Sharp, Zachary D.; Papike, James J.
• Is Part Of: Geochimica et cosmochimica acta. Volume 258(2019)
• Page Start: 242
• Page End: 257
• Publication Date: 2019-08-01
• Subjects: Mars -- meteorites -- Cl isotopes -- XANES -- TEM; Geochemistry -- Periodicals; Meteorites -- Periodicals; Géochimie -- Périodiques; Météorites -- Périodiques; Geochemie; Astrochemie; Electronic journals; 551.905
• DOI: 10.1016/j.gca.2019.05.034
• Languages: English
• ISSNs: 0016-7037

Abstract

Abstract: In part 1 of our examination of Martian meteorite Northwest Africa 8159 (NWA 8159) we illustrated many interesting mineralogical and textural attributes that make this martian basalt unique. Unlike the shergottites that illustrate a clear relationship between the extent of trace element and isotopic characteristics and oxygen fugacity (reduced, depleted magmas; oxidized, enriched magmas), NWA 8159 illustrates a decoupling of this relationship as it has oxidized and depleted signatures. In part 2, using a series of new observations and measurements (Cl isotopes, XANES, TEM, empirical modeling) we use NWA 8159 to explore the interaction between mantle-derived magmas and the martian crust. The magnetite-orthopyroxene intergrowths associated with olivine are a product of a martian subsolidus oxidation event near the QFM buffer and not a magmatic reaction in an oxidizing magma (>QFM + 3). This subsolidus event is further supported by Cr valence in the olivine, alteration of P-rich olivine, and end-member magnetite in the matrix of the meteorite. Although this subsolidus alteration makes it extremely difficult to determine the original f O2 of the parental magma for NWA 8159, there is evidence that during the initial stages of crystallization the f O2 was modestly reducing (∼IW + 1). Potential manifestations of more reducing magmatic conditions include P-rich cores in the olivine and low Fe 3+ in silicates (plagioclase, pyroxene). Further, if analogous to all other depleted shergottites, NWA 8159 initially crystallized under reducing conditions. This decoupling between oxygen fugacity and isotopic-trace element characteristics suggests that basalts derived from the martian mantle interacted with the crust in ways that significantly influenced redox history and volatile element isotopic composition (Cl, S), without dramatically modifying many of its radiogenic isotope and trace element mantle fingerprints.