NORTHWEST AFRICA 176


Iron with silicate inclusions, ungrouped
(CO–CV clan related)
standby for northwest africa 176 photo
Found 1999
no coordinates recorded

A rare Saharan iron meteorite weighing 2 kg was found near the border of Morocco and Algeria. The mass was obtained by G. Cintron of Island Meteorites, and a portion was sent for study to several institutions, including the Hawaii Institute of Geophysics & Planetology, the Institute of Geophysics and Planetary Physics in Los Angeles, the Enrico Fermi Institute in Chicago, the Universitat Bern in Switzerland, and Washington University in St. Louis. This meteorite contains sub-mm- to cm-sized, rounded, greenish-yellow, silicate inclusions dispersed throughout the metal host constituting ~44 vol%. Some of the silicates are aligned along shear planes that were probably created during an impact event.

Northwest Africa 176 is nearly identical to the meteorite Bocaiuva in elemental abundances, oxygen isotope composition, and petrographic features, and both were likely derived from the same parent body—an asteroid with a composition similar to the CO–CV clan. Although the O-isotopic composition for both NWA 176 (Δ17O = –5.2‰) and Bocaiuva is similar to that of the IAB iron complex, they are clearly resolved from that group by their significantly lower Au and As values (Wasson, 2011). This O-isotopic value is also similar to the pallasites of the Eagle Station grouplet (see diagram below), but the Ge and Ga contents are higher in these silicated irons, which suggests an origin from similar chondritic material in the same region of the solar nebula. Northwest Africa 176 and Bocaiuva, along with the Eagle Station group pallasites, are resolved from the main-group pallasites and the major iron groups by their higher Ge/Ga ratios, higher Cu and Ir contents, and lower Au, As, and Sb contents. For those few irons that do have Ge/Ga ratios similar to NWA 176, in particular, the irons of group IIF and certain ungrouped irons such as the silicated iron Mbosi, their elemental abundance ratios rule against a genetic relationship with NWA 176 and its relatives. Nevertheless, it is likely they all originated in the same nebular region from similar chondritic precursor material.

standby for o-isotopic diagram
Diagram credit: Greenwood et al., Chemie der Erde, vol. 77, p. 21, (2017)
'Melting and differentiation of early-formed asteroids: The perspective from high precision oxygen isotope studies'
(open access: http://dx.doi.org/10.1016/j.chemer.2016.09.005)

A multi-stage formation history is proposed for NWA 176 in which an initial impact generated enough heat (~1100°C) to form a melt pool. This was followed by gravitational differentiation that was sustained above ~500°C for a significant time. Differentiation resulted in a lower metallic layer with a cumulate silicate layer above. A subsequent impact event shattered the silicate layer and mobilized metal forcing it into existing cracks in the silicate layer, while initiating a rapid cooling phase at a rate of 1000°C/m.y. This cooling data would be consistent with a small body of only a few km in radius, or possibly reflects the breakup phase of a much larger object. Extended annealing at depth led to the rounding of the corners on silicate grains, a thermodynamic process acting to minimize the surface energy.

Several factors support this origin rather than the core–mantle interface origin commonly envisioned for the main-group pallasites. The nearly chondritic silicate composition with its relatively low proportion of olivine is more consistent with an impact-melt model than for a core–mantle interface origin. Moreover, the heat-generating radiogenic isotopes present in the later-forming carbonaceous chondrite parent bodies would be insufficient to produce melts that could form pallasitic compositions. Furthermore, the shear forces that produced the deformation features, as well as the loss of alkali volatiles, can be reconciled through impact mixing processes. Finally, the low Ir content of NWA 176 and its relatives is not consistent with fractional crystallization processes necessary for a core–mantle origin. One possible alternative origin proposed for this meteorite is that the precursor was a metal-rich, highly metamorphosed, chondritic asteroid. Further details about possible origins of NWA 176 can be found on the Allende page.

Northwest Africa 176 has an absolute I–Xe age, calculated relative to Shallowater (4.5623 [±0.0004] b.y.), of ~4.544 (±0.007) b.y., indicating a relatively late formation, probably through impact processes (Bogard and Garrison, 2009). The meteorite has a remarkably high Ar–Ar age of 4.524 (±0.013) b.y. The CRE age of 41 (±12) m.y. calculated for NWA 176 is low compared to other iron meteorites.

A portion of the information above was gleaned from the paper "Bocaiuva—A Silicate-Inclusion Bearing Iron Meteorite Related To The Eagle-Station Pallasites" (Malvin, Wasson, Clayton, Mayeda, & Curvello), Meteoritics, vol. 20, #2, Part 1 (1985). The specimen of NWA 176 pictured above is an 8.07 g partial slice displaying rounded silicate inclusions. It's fresh dark fusion crust attests to a recent arrival.