NWA 1878

Mesosiderite, group 0B
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Purchased June 2003
no coordinates recorded A complete, fusion-crusted, stony-iron meteorite weighing 1,374 g was purchased in Morocco on behalf of A. and G. Hupé. A sample was sent to the University of Washington in Seattle (A. Irving and S. Kuehner) for analysis and classification, and NWA 1878 was determined to be a type-B mesosiderite. It was further ascertained that the mass was paired with the 728 g mesosiderite NWA 1817 (A. Irving and S. Kuehner, UWS), purchased in Morocco in January of 2003 for N. Oakes, as well as being paired with several other independently classified masses (e.g., NWA 1979 and NWA 2042) for a total combined weight of at least 6.4 kg. Continued research was conducted by Bunch et al. (2014) on these specimens and a large number of mesosiderite samples previously considered to represent a separate fall. It was eventually determined that all of these mesosiderites represent a single strewn field (totaling at least 80 kg) comprising mesosiderites of differing subgroups (see also NWA 1827).

Northwest Africa 1817 (=1878) was described as a coarse-grained, unbrecciated, plutonic igneous-textured assemblage of spheroidal FeNi-metal–silicate clusters together with a larger component of silicate material (predominantly orthopyroxene with lesser amounts of plagioclase), along with minor silica, troilite, chromite, and merrillite, plus rare olivine grains and clasts of eucritic and diogenitic composition (Bunch et al., 2004). Oxygen isotope values for NWA 1817 were obtained at Carnegie Institution, Washington D.C. (D. Rumble, III), and the meteorite plots in the field of the mesosiderites on an oxygen three-isotope diagram (see below).
Diagram courtesy of the Meteoritical Bulletin: Oxygen Isotope Plots Direct Link Ample petrographic evidence exists to support the hypothesis for a two-stage irradiation history for mesosiderites (Hidaka and Yoneda, 2011). In the first stage, occurring >4.4 b.y. ago, the silicate component of a large (~200–400 km diameter) parent body was irradiated near the surface, prior to mesosiderite formation. Subsequent to differentiation of this planetesimal, a low velocity collision occurred with a large (~50–150 km diameter) iron projectile ~4.4 b.y. ago, melting and mixing the cool silicate layer of the planetesimal with the molten FeNi-metal of the projectile forming complex breccias. The partial or total collisional disruption and gravitational reassembly of the target body is considered a strong likelihood by some investigators (Haack et al., 1996), while others favor a scenario in which a severe impact caused molten metal from the differentiated, molten core of the planetesimal itself to be mixed with the cooler silicates from the mantle (Scott et al., 2001).

After a brief period of rapid cooling resulting from the mixing of cold and hot material, NWA 1878 experienced very slow cooling at ~0.01 °C/year consistent with deep burial of the mesosiderite precursor material under an extensive debris blanket and/or within lava flows (Sugiura and Kimura, 2015). Other mesosiderites including NWA 1242, Crab Orchard, ALH 77219, and A-882023 also cooled much more slowly from peak temperatures down to intermediate temperatures, while others including Estherville, Vaca Muerta, NWA 2924, and Dong Ujimqin Qi experienced rapid cooling over the same temperature range indicative of a residence nearer the surface.

Over time, reduction processes were initiated, while episodic impact events on this large, slowly cooling body caused remelting, metal–silicate mixing and brecciation, formation of quench textures, mixing of deep silicates and near-surface silicates of eucritic and diogenitic compositions, regolith gardening, and degassing, ultimately resetting the Ar–Ar chronometer to reflect an age of ~3.6–3.9 b.y. Thereafter, impact excavation and ejection from the mesosiderite meteoroid occurred, with calculated CRE ages of various mesosiderites reflecting multiple excavations over the past 10–150 m.y.

Mesosiderites have been historically classified from type 1 to 4 in order of increasing degrees of thermal metamorphism due to impact-generated reheating. An in-depth analysis of the mesosiderite ALHA77219 was conducted by Agosto et al. in 1980, and they determined that the matrix is fine-grained and that the pigeonite grains are anhedral rather than coarsely poikiloblastic, features consistent with minimal recrystallization and a classification as type 1B. Recently, Sugiura (2013) developed criteria for establishing the most primitive mesosiderite, the study of which could elucidate the earliest history of the mesosiderite parent body. He determined that NWA 1878 was the most primitive mesosiderite known based primarily on the following indicators:

  1. a wider range of silicate compositional heterogeneity (especially for plagioclase)
  2. a smaller pyroxene lamellae width
  3. a smaller spheroidal metal grain size
  4. lack of corona formation in olivine (or coronal onset lacking chromite)

Additional discrimination criteria for primitiveness were identified and investigated by Sugiura et al. (2013), such as the Al/(Al+Cr) ratio and the Ti concentration in Cr-spinel. It was determined that NWA 1878 was more primitive than either the 1B ALHA77219 or the 1A Vaca Muerta mesosiderites, and accordingly, NWA 1878 was designated the first mesosiderite of metamorphic type 0B. The photo of NWA 1878 shown above is a 4.0 g partial slice acquired from M. Farmer. The top photo below shows a slice of NWA 1817 exhibiting the spherical metal grains (courtesy of M. Graul), while that below shows a portion of the main mass (courtesy of N. Oakes).
Photo source: Encyclopedia of Meteorites

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