Mesosiderite, group B0
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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 by Bunch et al. (2004) 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. In a more detailed analysis of NWA 1878, Kimura et al. (2020) employed Raman spectroscopy to reveal that silica in igneous clasts of pyroxene and plagioclase is always cristobalite, whereas that occurring in pyroxene near metal is quartz. They also found that plagioclase has a bimodal composition with pure anorthite only occurring adjacent to metal grains. 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 such as 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 ~10–340 m.y.

Wang and Hsu (2019) used Pb–Pb chronometry to date 53 merrillite crystals associated with FeNi-metal in the Youxi mesosiderite. Based on the low REE abundances in the Youxi merrillite compared to that in eucrites, they contend that it was formed by oxidation of P in metal during the metal–silicate mixing event rather than during magmatic activity. They derived an age of 3.950 (±0.080) b.y. which they consider represents the timing of merrillite development during the mesosiderite-forming event. An equally plausible timing for the metal–silicate mixing event was ascertained by Haba et al. (2019) using high-precision U–Pb dating of zircons in several mesosiderites. Based on these results they contend that the metal–silicate mixing event occurred 4.52539 (±0.00085) b.y. ago. They propose a scenario in which a hit-and-run collision disrupted the northern hemisphere of Vesta leading to ejecta debris reaccreting to the opposite, southern hemisphere (see schematic diagram below). The deeply buried mesosiderite meteorites were ejected into Earth-crossing orbits by later impacts.

Schematic Illustration of Mesosiderite Formation
Crust (yellow); Mantle (blue); Core (red); Collisional debris (green)
standby for mesosiderite formation scenario diagram
Diagram credit: Haba et al., Nature Geoscience, vol. 12, #2, p. 512, (2019)
'Mesosiderite formation on asteroid 4 Vesta by a hit-and-run collision'

It is notable that the O-isotopic values of the mesosiderites are almost identical to those of the HED suite of meteorites, implying that a genetic link exists between these disparate groups (Greenwood et al., 2006). Conversely, multiple line of evidence presented by D.W. Mittlefehldt (2021), including petrological (e.g., modal, textural, and redox data), compositional (e.g., incompatible lithophile trace elements), and observational (Dawn at Vesta), indicate that separate parent bodies were probably involved.

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 B1. In further studies, 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 B1 ALHA77219 or the A1 Vaca Muerta mesosiderites, and accordingly, NWA 1878 was designated the first mesosiderite of metamorphic type B0. Further detailed analyses of NWA 1878 were conducted by Kimura et al. (2020). They proposed a new set of criteria for the establishment of a type 0 metamorphic category into which the group-B mesosiderite NWA 1878 has been classified:

  1. Texture: breccia clasts and isolated minerals do not show recrystallization or grain intergrowth; similar to subgroup 1
  2. Pyroxene zoning: unique occurrence of both normal igneous zoning (primary) and reverse zoning (secondary)
  3. Cristobalite: the presence of this silica polymorph reflects primary igneous conditions of high temperature and low pressure; occurrence of secondary quartz near metal
  4. Chromite: both the wide range of Cr/Al ratios of chromites and the enrichment in Mg and Al compared to other grains attest to a lack of thermal homogenization; the low Ti concentrations may also reflect a low degree of thermal metamorphism
  5. FeNi-metal: grains occur as lightly-sintered aggregates of small (ave. 120µm) spheroids indicative of minimal thermal metamorphism compared to the larger (~1 mm) and more irregular-shaped grains indicative of higher degrees of thermal metamorphism
  6. Pyroxene lamellae: no inversion of pyroxene and exsolution lamellae are extremely rare and very thin when present; some similarity to subgroup 1
  7. Olivine coronas: rather than the typical coronas consisting of orthopyroxene with abundant chromite and phosphate, only thin zones are present consisting of Mg-rich pyroxene and plagioclase with only rare chromite and phosphate
  8. Pyroxene near olivine: magnesian pyroxenes resulting from local secondary diffusion processes with magnesian olivine without larger-scale equilibration
  9. Plagioclase: unique bimodal occurrence of both albitic plagioclase and nearly pure anorthite attesting to a lack of thermal homogenization

The photo of NWA 1878 shown above is a 4.0 g partial slice acquired from M. Farmer. The photo below shows a beautiful slice of NWA 1878 courtesy of M. Graul, which exhibits the small spherical metal grains indicative of minimal thermal metamorphism and a type 0.

Photo source: Encyclopedia of Meteorites, shown courtesy of Mirko Graul