Iron, IAB complex, NWA 468 duo
Purchased before January, 2000
no coordinates recorded
A fresh silicated iron meteorite (W1) weighing 6,100 g was purchased from a Moroccan dealer by Dr. D. Gregory in Tucson, Arizona. The Moroccan dealer had previously purchased the meteorite in Alnif, Morocco. Mineral analyses and classification of the type specimen of NWA 468 was completed at UCLA by J. Wasson and A. Rubin. The main mass remains with the purchaser, while a 185 g specimen is maintained at the Royal Ontario Museum in Toronto, Canada.
This iron is chemically similar to members of the nonmagmatic IAB iron-meteorite complex, with fine- to coarse-grained (0.3–3.5 mm) silicate inclusions comprising ~55 vol% of the meteorite (Rubin et al., 2002). The silicates are incorporated in plessitic FeNi-metal containing kamacite sparks and spindles. These silicates (ave. Fa5.3) exist as large, multi-cm-sized masses, along with small mm-sized and smaller grains. Other mineral phases present in NWA 468 include low-Ca pyroxene, diopside, and plagioclase, along with troilite, chromite and schreibersite. Troilite is prevalent as veins and veinlets surrounding and intruding the silicates, and also occurs as µm-sized grains forming curvilinear trails, resulting in some silicate darkening.
The IAB iron-meteorite complex constitutes one of the largest iron chemical groups, and many members contain silicate inclusions within the FeNi-metal host. These inclusions can be sulfide-rich, silicate-rich chondritic, silicate-rich nonchondritic, graphite-rich, or phosphate-bearing, consistent with an origin from a quenched impact-melt pool on a metal-rich, carbonaceous chondrite parent body. This event involved the segregation of an FeS melt from a silicate melt. The presence of low-Ca clinopyroxene and 5-mm-wide taenite crystals in NWA 468 provides evidence in support of crystallization from a rapidly cooled melt, reaching temperatures of ~660°C within a few hours. A much slower cooling rate occurred below this temperature as indicated by the nucleation of kamacite sparks and spindles within the plessite. A further shock event was responsible for the mobilization and reintroduction of sulfide.
On element–Au diagrams, NWA 468 plots in the high-Au field and is an ungrouped member of the IAB complex (Wasson and Kallemeyn, 2002). Another ungrouped iron, the Antarctic meteorite GRV 98003, plots close to NWA 468 on most element–Au diagrams, and the two been grouped together as a IAB-related duo. However, the two meteorites differ significantly in their Ir, Ga, Ge, Re, and Cr contents. Northwest Africa 468 also shows compositional similarities to Sombrerete and Lonaconing, both members of the high-Au, low-Ni subgroup (sHL), and to Ventura, an ungrouped IAB-related iron. The silicate mineralogy of the pyroxene pallasite Y-8451 exhibits similarities as well, and has relatively close O-isotopic ratios. Elemental abundance ratios of NWA 468 silicates are very similar to those of lodranites, and they may have experienced a similar petrogenesis on similar parent bodies.
The O-isotopic composition of NWA 468 silicates plots slightly outside of the range of IAB members, but is similar to that of Sombrerete, the acapulcoites and lodranites, and the CR and CH chondrites. These groups also contain abundant FeNi-metal like NWA 468, and are consistent with the type of material that was the precursor of NWA 468. The above specimen of NWA 468 is a 2.45 g partial slice. The photo below shows a full slice with a close-up of the lower left corner from which the above specimen was removed. The remainder of this slice is in the collection of UCLA.
