NORTHWEST AFRICA 10019


Pyroxene-plagioclase pallasite, ungrouped
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Purchased January 2015
no coordinates recorded

Within the Northwest Africa dense accumulation area (DCA) a single 580 g fusion-crusted meteorite was found along with 26 g of related fragments. The meteorite was subsequently sold to meteorite dealer Steve Arnold who submitted a type sample to the University of New Mexico (C. Agee and N. Muttik) for analysis and classification. Northwest Africa 10019 is an ungrouped pyroxene–plagioclase-bearing pallasite that exhibits significant compositional heterogeneity. It is composed of olivine (up to 6 mm), orthopyroxene (up to 5 mm), kamacite, and taenite, along with minor Ca-plagioclase, troilite, chromite, schreibersite, and Ca–Mg-phosphates (farringtonite, stanfieldite, and merrillite).

Northwest Africa 10019 is unique among all pallasites in that it contains a low abundance (<1 vol%) of Ca-plagioclase (An50–84); the pyroxene pallasite Choteau is the only other pallasite found to contain plagioclase, but in that pallasite it is highly albitic (Ab85.6). Boesenberg et al. (2016) observed that plagioclase in NWA 10019 is present as fine (10–50 µm) to coarse (2 mm) grains within olivine and orthopyroxene. The angular olivines in NWA 10019 have a Fa content (~Fa16.5) that is appreciably higher than typical main-group pallasites, but similar to the subset of main-group pallasites with anomalous silicates (Springwater, Rawlinna, Phillips County, and Zaisho). Boesenberg et al. (2016) identified a unique enclave in NWA 10019 that has more primitive phases than the rest of the pallasite, including Mg-rich chromite, slightly more magnesian olivine, and plagioclase that has a broader compositional range. In addition, they reported that the pallasite has an Fe/Mn value of 28–37.

Five additional pyroxene-bearing meteorites having a pallasite-like composition have been characterized: Vermillion, Y-8451, Zinder, NWA 1911, and Choteau. Vermillion is composed of 86 vol% FeNi-metal and 14 vol% silicates, with the silicates consisting of 93% olivine and 5% pyroxene (4.9% opx and 0.1% cpx)—equivalent to a modal composition of ~0.7 vol% pyroxene. Wasson and Kallemeyn (2002) recognized that Vermillion might possibly be related to the IAB complex iron meteorites. The 54.8 g Y-8451 pallasite contains 57 vol% silicates consisting of 97% olivine, 2% orthopyroxene, 0.4% clinopyroxene, and 0.4% augite. The silicates in Y-8451 are modally equivalent to ~1.6 vol% pyroxene (Boesenberg et al., 2000). The 46 g Zinder pallasite has a high modal abundance of pyroxene more similar to that of NWA 1911, estimated to be 28 vol% (Wittke and Bunch, 2003). The modal abundance of silicates in NWA 10019 is ~60%, comprised of olivine (~43–51 vol%) and orthopyroxene (~9–17 vol%), with pyroxene accounting for ~1–5 vol% of this pallasite (Boesenberg et al., 2016).

In a study conducted by Gregory et al. (2016), it was ascertained that Choteau is compositionally and isotopically similar to both Vermillion and Y-8451, and it was concluded that these three pyroxene pallasites form a grouplet; they suggested that these meteorites should be termed 'Vermillion pallasites' (see the Vermillion page for additional details). The low-Ca pyroxene in Zinder, NWA 1911, and NWA 10019 is entirely composed of orthopyroxene (orthopyroxene in NWA 10019 contains ~100µm-sized clinopyroxene inclusions; Boesenberg et al., 2016), while that in the Vermillion pallasites comprises both orthopyroxene and clinopyroxene (Niekerk, 2005; Irving and Kuehner, 2013). Zinder contains a higher abundance of chromite compared to the Vermillion pallasites. The O-isotopic compositions of the Vermillion pallasites are distinct from the other three pyroxene pallasites, and many are associated with a number of established O-isotopic trends: Zinder plots on the terrestrial fractionation line (Bunch et al., 2005), the Vermillion pallasites plot near the field of acapulcoites and lodranites, and both NWA 1911 and NWA 10019 plot on the eucrite/mesosiderite fractionation line, which remains incompletely resolved from the bimodal fractionation trend of the main-group pallasites (Ziegler and Young, 2011; K. Ziegler, 2015).

A separate O-isotopic analysis for NWA 10019 was conducted by Boesenberg et al. (2016), and it provided values which plot on an extension of the main-group near the pyroxene pallasite NWA 1911. However, many mineralogical features distinguish NWA 10019 from the main-group pallasites, including the presence of plagioclase, a significantly lower abundance of volatile elements (e.g., Ga, Ge, As and Au), high Al content in chromite, and metal that is more evolved than in any other pallasite.

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Oxygen isotope composition of NWA 10019 compared to main-group pallasites and mesosiderites (left) and HEDs (right).
TFL = terrestrial fractionation line; EFL = eucrite fractionation line
Diagrams adapted from the Meteoritical Bulletin Oxygen Isotope Plots—The Meteoritical Society

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Diagram credit: Gregory et al., 47th LPSC, #2393 (2016)

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Diagram credit: Boesenberg et al., 47th LPSC, #2297 (2016)

Based on the results of their study, Boesenberg et al. (2016) determined that NWA 10019 and the main-group pallasites formed from a similar O-isotopic reservoir but under very different petrologic conditions, and they concluded that NWA 10019 and the main-group pallasites derive from distinct parent bodies. The pallasites in our collections could represent at least seven separate parent bodies as follows: 1) main-group, 2) Eagle Station group, 3) Vermillion pallasites (Vermillion, Y-8451, and Choteau), 4) Zinder, 5) Milton, 6) NWA 1911, and 7) NWA 10019. In addition, several pallasites with anomalous silicates (e.g., Springwater) and anomalous metal (e.g., Glorieta Mountain) could possibly increase the number of unique parent bodies. The specimen of NWA 10019 shown above is a 2.018 g partial slice. The photo shown below is a large slice from this unique pyroxene pallasite.

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Photo courtesy of Steve Arnold