NORTHWEST AFRICA 12379


Chondrite, ungrouped
(metal-rich)
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Purchased 2017/2018
no coordinates recorded

Five relatively fresh fragments lacking fusion crust, with a combined weight of 765 g, were acquired by L. Labenne in 2017/2018. Samples were sent for analysis and classification to the Institute for Geosciences, Germany (C.A, Jansen, A.N. Krot), the University of Hawaii, USA (A.N. Krot), Senckenberg Forschungsinstitut und Naturmuseum, Germany (J. Zipfel), and the Centre for Star and Planet Formation, Denmark (M. Bizzarro, M. Schiller), and NWA 12379 was determined to be a unique ungrouped metal-rich chondrite. A probable pairing, 280 g NWA 12273, was acquired by J. Piatek in October 2018 and classified at the University of New Mexico (C. Agee, K. Ziegler). Another paired mass weighing ~400 g was acquired by D. Dickens but has not yet been given a NWA series designation. A 32 g section of NWA 12379 was purchased from L. Labenne by a private collector and subsequently cut into smaller specimens for trade and sale.

Jansen et al. (50th LPSC, 2019, #2741) described NWA 12379 as consisting primarily of FeNi-metal grains (~70 vol%) 60–1200 µm (ave. ~400 µm) in diameter, silicates (~25–30 vol%) consisting of chondrules and chondrule fragments in a similar size range as the metal grains. Chondrules examined in NWA 12273 were found to be consistent with petrologic type 3 (Agee et al., 2019). Minor amounts of troilite (<5 vol%), Ti-bearing chromite (<1 vol%), chlorapatite, and merrillite are also present. Hydrothermal alteration led to the formation of the phosphates, chromite, and tetrataenite as well as the partial replacement of low-Ca pyroxene with ferroan olivine. No matrix component is present and no refractory phases were observed.

Although physical and textural similarities exist between NWA 12379 and the Isheyevo transitional CH/CB chondrite, several differences distinguish them from each other. Chondrules in NWA 12379 are mostly ferroan (type II) porphyritic (POP, PO, PP) with only rare barred, cryptocrystalline, and skeletal olivine types. By comparison, mostly magnesian cryptocrystalline and skeletal chondrules are present in the metal-rich lithology of Isheyevo, the latter considered to have formed as gas–melt condensates in an impact-generated plume. On the other hand, the metal-poor litholgy of Isheyevo contains porphyritic chondrules more similar to those in NWA 12379. The fayalite content in olivines in NWA 12379 (Fa18.1–28.5 mol%) is similar to unequilibrated ordinary chondrites, while those in the metal-rich CB and "G" chondrites have much lower Fa values (Fa1.5–4 and Fa0.3–1.3, respectively) indicating their formation under more reducing conditions (see diagram below).

Fe/Mn vs. Fa for Metal-rich Chondrites
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Diagram credit Agee et al., 50th LPSC, #1176 (2019)

Oxygen isotope analyses of silicate chondrules from both NWA 12379 and NWA 12273 demonstrate that these meteorites plot in the field associated with L/LL chondrites, consistent with formation in the inner, non-carbonaceous region of the Solar System (see diagram below). By comparison, the oxygen isotopes for Isheyevo place its formation in the carbonaceous region beyond Jupiter.

Oxygen Isotope Diagram for NWA 12379 and NWA 12273
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Diagram adapted from MetBull 107
NWA 12379: dark orange (1); NWA 12273: light orange (3)

A preliminary Fe-isotopic analysis of the metal component of NWA 12379 indicates a formation in the outer (CC) region of the protoplanetary disk (Jansen et al., 2019). Therefore, the formation of this meteorite could be analogous to that of metal-rich meteorite Isheyevo and the metal-rich "G" chondrite grouplet (Weisberg et al., 2015), represented by GRO 95551, NWA 5492, and possibly Sierra Gorda 009, by condensation from an impact-generated vapor plume resulting from a collision between a metal-rich (iron core?) body and an ordinary chondrite-like body.

It has been posited that after Jupiter had grown to a massive size (>50 M) by ~4 m.y. at an initial heliocentric distance of ~3 AU, it underwent a chaotic migration in a 3:2 (or 2:1) resonance with Saturn—first inward for ~100,000 years to ~1.5–2.0 AU while clearing the inner disk of planetesimals, an then outward for ~4–5 m.y. years to its current location near 5.2 AU ("Grand Tack" scenario of Walsh et al., 2011; Johnson et al., 2016; Brasser et al., 2016). Planetary modeling employed by Johnson et al. (2016) demonstrates that only during a relatively short timeframe within this migration period will dynamical excitement produce impact velocities that reach levels high enough (>18 [±5] km/s) to vaporize Fe in a planetesimal core. Their model is consistent with the late timing of the vapor plume from which the CB chondrites condensed, and in a similar manner, the G chondrites and NWA 12379/12273 may derive through condensation from an impact-generated vapor plume; however, chondrule ages for these latter meteorites have not yet been established. Further details about the "Grand Tack" scenario can be found in the Appendix Part III.

The specimen of NWA 12379 shown above is a 5.16 g partial slice sectioned from the 32 g slice. The photo below shows the main mass of NWA 12273 in the possession of J. Piatek.

NWA 12273 pairing
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Photo courtesy of Carl Agee