Physics
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufm.p51e1245i&link_type=abstract
American Geophysical Union, Fall Meeting 2006, abstract #P51E-1245
Physics
1028 Composition Of Meteorites (3662, 6240), 3662 Meteorite Mineralogy And Petrology (1028, 6240), 6235 Mercury, 6240 Meteorites And Tektites (1028, 3662)
Scientific paper
Tamassint Angrite: A small fragmented stone found in June 2006 south of Tamassint oasis in the Morocco-Algeria border region represents a new type of angrite lithology, unlike the coarse grained metamorphic or fine grained "basaltic" to quench-textured examples known previously. This extremely fresh, fusion-crusted specimen has a coarse grained (0.6-12 mm) plutonic igneous cumulate texture, and is composed of Al-Ti-rich clinopyroxene (33.4%), pure anorthite (28.6%), Ca-rich olivine (18.7%) with prominent exsolution lamellae (10-50 μm wide) of kirschsteinite, ulvöspinel (18.5%), and accessory glass, troilite and metal. Subhedral anorthite grains are partially enclosed within larger ulvöspinel grains. Mineral compositions are as follows: clinopyroxene (Fs20.8-33.3Wo53-54.9, Al2O3 = 5.7 to 9.4 wt.%, TiO2 = 0.9 to 2.9 wt.%, FeO/MnO = 85-278), olivine (Fa72.6-74.7Ln3.5-3.6, CaO = 2.1 wt.%, FeO/MnO = 70-87), kirschsteinite (Fa44.7-45.4Ln46-47.2, FeO/MnO = 73-82), ulvöspinel (TiO2 = 27.6 wt.%, Al2O3 = 5.5 wt.%). Reintegration of the kirschsteinite lamellae gives a pre-exsolution olivine composition of Fa68.1Ln12.2 with 7.3 wt.% CaO. Present along grain boundaries (notably between anorthite and ulvöspinel) are narrow (5-20 μm) curvilinear zones of glass associated with secondary kirschteinite, clinopyroxene and olivine (which show similar curvilinear morphology and truncate kirschsteinite lamellae). Glass compositions plot close to a mixing line between anorthite and ulvöspinel. Replicate oxygen isotopic analyses of acid-washed minerals by laser fluorination gave δ18O = 3.881, 3.845, δ17O = 1.967, 1.927, Δ17O = -0.0745, -0.0956 per mil (for TFL slope = 0.526). Comparison With NWA 2999: We previously showed [1] that angrite Northwest Africa 2999 is a metamorphically annealed breccia with distinctive symplectites and coronas representing forward and reverse versions of the same solid state reaction. We suggested that these disequilibrium textures required burial followed by rapid cooling and uplift on a large differentiated planetary body (such as Mercury), and they may be related to the very high metal content (10 vol.%) in this meteorite. Textures and mineral homogeneity in the Tamassint angrite imply a prolonged period of cooling and equilibration, whereas the distinctive grain boundary glass implies a subsequent very rapid melting and cooling event (possibly triggered by decompression rather than heating). This glass is compositionally different from glass in the D'Orbigny angrite, which however contains a trapped solar noble gas component [2] typical of large differentiated planets with magma oceans. If the APB indeed is (or was) a large planet, then perhaps angrites derive from early collisional stripping of its outermost (possibly more ferroan) lithosphere. [1] Irving A. J. et al. (2005) EOS, Trans. AGU 86, #P51A-0898; Kuehner S. M. and Irving A. J. (2006) LPS XXXVII, #1344 [2] Busemann H. et al. (2004) LPS XXXV, #1705.
Irving Anthony J.
Kuehner S. M.
Rumble Doug
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