(metal-rich with affinities to ordinary chondrites)
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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 Research Institute and Natural History Museum, Germany (J. Zipfel), and the Centre for Star and Planet Formation, Denmark (M. Bizzarro, M. Schiller). After in-depth studies it was determined that NWA 12379 is 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 probable 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 #2741; Geochemistry, 2019) described NWA 12379 as consisting primarily of FeNi-metal grains (~70 vol%) 601200 µm (ave. ~370 µm) in diameter and silicates (~2530 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 by Agee et al. (2019) and 3.8 by Jansen et al., (2019). Minor abundances 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. The latter ferroan olivine plots on an extension of the trend line for aqueous alteration products (fayalite and magnetite) in unequilibrated ordinary chondrites (see O-isotopic diagram below). No matrix component is present and no refractory phases were observed. A small metaltroilite impact-melted region was identified in NWA 12379 (Jansen et al., 2019).
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 olivine, skeletal olivine, radial pyroxene, and cryptocrystalline (some bleached) types. By comparison, mostly magnesian cryptocrystalline and skeletal olivine chondrules are present in the metal-rich lithology of Isheyevo, the latter considered to have formed as gasmelt condensates in an impact-generated plume. On the other hand, the metal-poor lithology of Isheyevo contains porphyritic chondrules more similar to those in NWA 12379. The fayalite content in olivines in NWA 12379 (Fa18.128.3 mol%) is similar to unequilibrated ordinary chondrites, while those in the metal-rich CB and "G" chondrites have much lower Fa values (Fa1.54 and Fa0.31.3, respectively) indicating their formation under more reducing conditions (see diagram below).
Fe/Mn vs. Fa for Metal-rich Chondrites
Diagram credit Agee et al., 50th LPSC, #1176 (2019)
Oxygen isotope analyses of silicate chondrules from both NWA 12273 and NWA 12379 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.
Diagram credit: Jansen et al., Chemie der ErdeGeochemistry, vol. 79 #4 (2019 open accesslink)
'Mineralogy, petrology, and oxygen isotopic composition of Northwest Africa 12379, metal-rich chondrite with affinity to ordinary chondrites'
A preliminary Fe-isotopic analysis of the metal component of NWA 12379 indicates an accretion in the outer (CC) region of the protoplanetary disk, while the silicate component is consistent with an accretion in the inner (NC) disk region (Jansen et al., 2019). The ultimate 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), the latter represented by GRO 95551, NWA 5492, and Sierra Gorda 009. All of these meteorites are thought to have formed 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. Further stable isotope analyses on the metal grains in NWA 12379 should provide a better picture of the event.
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 Saturnfirst inward for ~100,000 years to ~1.52.0 AU while clearing the inner disk of planetesimals, and then outward for ~500,000 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 on the Protoplanetary Disk page.
The NWA 12379 meteorite has experienced minor terrestrial weathering consistent with grade W12. The specimen of NWA 12379 shown above is a 5.16 g partial slice from the 32 g section. The top photo below shows the main mass of the paired NWA 12273 in the possession of J. Piatek, while the bottom photos show a large section of one of the two pieces from the paired NWA 13202 (396 g) in the possession of M. Lyon.
NWA 12273 Main Mass
Photo courtesy of Dr. Carl Agee
NWA 13202 69 g Mass
click on photos for a magnified view
Photos courtesy of Mark Lyon and Paul SwartzMPOD 9 Feb 2020