HR: 0800h
AN: P41A-0219    [Abstracts]
TI: Primary Ferric Iron-Bearing Rhönite in Plutonic Igneous Angrite NWA 4590: Implications for Redox Conditions on the Angrite Parent Body
AU: * Kuehner, S M
EM: kuehner@ess.washington.edu
AF: Dept. of Earth and Space Sciences, University of Washington, Seattle, WA 98195, United States
AU: Irving, A J
EM: irving@ess.washington.edu
AF: Dept. of Earth and Space Sciences, University of Washington, Seattle, WA 98195, United States
AB: Northwest Africa 4590 is a heterogeneous olivine gabbro with cumulate texture composed of Al-Ti-rich clinopyroxene, pure anorthite, Ca-rich olivine, kirschsteinite and ulvöspinel, with accessory troilite, merrillite, Ca silicophosphate, kamacite and glasses [1]. Rhönite now has been identified in this specimen (for the first time in any angrite) as (1) a large (0.65 mm long), blocky, anhedral grain adjacent to anorthite, kirschsteinite and troilite, (2) ca. 15 micron grains along grain boundaries of the major phases (in one case in contact with clinopyroxene and metal), and (3) ca. 30 micron grains within melt inclusions and veins composed of kirschsteinite, olivine, anorthite, troilite, hercynite and glass. The rhönite is nearly opaque in transmitted light, with a deep cinnamon-red color on thin grain edges. The average composition of the largest grain is (in wt.%): SiO₂ 23.6, TiO₂ 9.9, Al₂O₃ 16.3, Cr₂O₃ 0.1, FeOt 33.6, MnO 0.14, MgO 3.5, CaO 13.1. Stoichiometry (14 cations, 20 oxygen atoms) requires about 12% of the total iron to be in the ferric state, resulting in the nominal formula: (Ca_2.01Mn_0.02)(Fe²⁺_3.55Fe³⁺_0.45Mg_0.75Al_0.12Cr_0.15)Ti_{0.9 5}(Si_3.37Al_2.63)O₂₀ In the co-existing ulvöspinel about 18% of the iron must be ferric to achieve charge balance; likewise, Fe-Ti spinel coexisting with metal in Angra dos Reis contains ferric iron [2]. In contrast, the spinel (Cr-pleonaste) in metal-rich angrite NWA 2999 is stoichiometric without any apparent ferric iron. The coexistence of ferric iron- bearing silicate and oxide phases with Fe metal implies that the oxygen fugacity during crystallization of NWA 4590 was somewhat more oxidizing than that of the IW buffer. Compositions of primary (pre-exsolution) olivine and kirschsteinite in NWA 4590 record a minimum magmatic temperature of 910-950°C, based on the solvus of [3]. Previous experimental studies [4] also imply that other metal-bearing plutonic (AdoR, LEW 86010) and quench-textured (LEW 87051) angrites equilibrated at oxygen fugacities near QFM - 2 log units and relatively high temperatures. Although not previously known from angrites, rhönite has been reported from terrestrial alkalic rocks, CV chondrites and a lunar mare basalt [5]. [1] Irving A. et al. (2006) EOS, Trans. AGU 87, #P51E-1245; Kuehner S. and Irving A. (2007) LPS XXXVIII, #1344 [2] Prinz M. et al. (1977) EPSL 35, 317-330 [3] Mukhopadhyay D. and Lindsley D. (1983) Amer. Mineral. 68, 1089-1094 [4] Jurewicz A. et al. (1991) Science 252, 695-698; McKay G. et al. (1994) GCA 58, 2911-2919 [5] Treiman A. (2007) LPS XXXVIII, #1244.
DE: 3620 Mineral and crystal chemistry (1042)
DE: 3640 Igneous petrology
DE: 3662 Meteorite mineralogy and petrology (1028, 6240)
DE: 6240 Meteorites and tektites (1028, 3662)
SC: Planetary Sciences [P]
MN: 2007 Fall Meeting