NORTHWEST AFRICA 468

A relatively fresh (W1) silicated iron meteorite 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.

The silicate fraction of Northwest Africa 468 (~55 vol%, quantifiably a stony-iron) consists of angular, chondritic, fine- to coarse-grained (0.3–3.5 mm) inclusions (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.

It was previously shown by Wasson and Kallemeyn (2002) that NWA 468 plots in the high-Au, low-Ni field on element–Au diagrams, relatively close to the sHL subgroup members, and was tentatively considered to be an ungrouped member of the IAB iron-meteorite complex. It was associated with another ungrouped iron, the Antarctic meteorite GRV 98003, which plots close to NWA 468 on most element–Au diagrams, and the two were 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 (ungrouped silicated iron), Lonaconing (IAB complex, subgroup sHL), and Ventura (ungrouped IAB-related iron). The silicate mineralogy of the pyroxene pallasite Y-8451 exhibits similarities as well, and has relatively close O-isotopic ratios.

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. Their formation involved the segregation of an FeS melt from a silicate melt. The presence of low-Ca clinopyroxene (a small portion as clinobronzite) 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.

While it might be true that the FeNi-metal component of NWA 468 is similar to that of the IAB iron complex, elemental abundance ratios of NWA 468 silicates are very similar to those of lodranites. Importantly, NWA 468 silicates have more negative Δ17O values than those for IAB irons, with values of –1.39‰ and –0.3 to 0.6‰, respectively (Rubin et al., 2002 and references therein). The plot for NWA 468 on an oxygen three-isotope diagram is a significant distance from the IAB field. Interestingly, the O-isotopic composition of NWA 468 is very similar to that of Sombrerete, to meteorites of the acapulcoite–lodranite clan, and to the CR and CH chondrites. In a similar manner to NWA 468, each of these meteorite groups contain abundant FeNi-metal, and their bulk compositions are consistent with the type of material that was precursor to NWA 468.

Research on this meteorite has been ongoing (e.g., Rubin et al., 2002 [GCA vol. 66, #20]; Bunch et al., 2005, [#2308]; Floss et al., 2005, [MAPS vol. 40, #3]; Irving et al., 2014 [#2465]; Sanborn et al., 2014 [#2032]; A. Ruzicka, 2014 [Chemie der Erde–Geochemistry]). As recognized in the Sanborn et al. (2014) abstract, a Δ17O vs. ε54Cr diagram is one of the best diagnostic tools for determining genetic (parent body) relationships among meteorites. Moreover, Sanborn et al. (2015) demonstrated that ε54Cr values are not affected by aqueous alteration. Utilizing both the ε54Cr and Δ17O values calculated for NWA 468 (Sanborn et al. [2014] and Irving et al. [2014], respectively), the plot on a Δ17O vs. ε54Cr diagram shows that NWA 468 lies within the field of the acapulcoite–lodranite clan, in further support of a common parent body for these meteorites.

In their study of the fossil meteorite Österplana 065, Schmitz et al. (2016) included oxygen and chromium isotopic data for NWA 468 in a coupled Δ17O vs. ε54Cr diagram (shown below). This higher resolution diagram also clearly demonstrates that this meteorite plots within the acapulcoite–lodranite region in isotope space.

A comprehensive study of the H4 chondrite Grove Mountains (GRV) 020043 was conducted by Li et al. (2018). They demonstrated that the mineralogy, geochemistry, and O- and Cr-isotopic compositions of this meteorite support the reclassification to "Acapulcoite chondrite", representing the chondritic, unmelted, outermost layer of the acapulcoite–lodranite parent body. They also provided clear evidence for the origination of NWA 468 on the ACA–LOD parent body, which, along with the metal-rich lodranite GRA 95209 (NASA lab photo), are considered to represent the deepest lithology (early core segregation) of the parent body. A schematic representation of the acapulcoite–lodranite parent body was presented by Li et al. (2018):

The specimen of NWA 468 shown above 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.