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

Within the Northwest Africa (NWA) dense accumulation area 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 001, 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.

To date, seven pyroxene-bearing meteorites having a pallasite-like composition have been characterized: Choteau, LoV 263, NWA 1911, NWA 10019, Vermillion, Yamato 8451, and Zinder. 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. 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, similar to that in 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). The silicates in the 4.88 kg LoV 263 pallasite are comprised of approximately equal proportions of olivine and orthopyroxene.

The low-Ca pyroxene in Zinder, NWA 1911, NWA 10019, and LoV 263 is composed entirely of orthopyroxene (orthopyroxene in NWA 10019 contains ~100µm-sized clinopyroxene inclusions; Boesenberg et al., 2016), while that in Vermillion, Y-8451, and Choteau comprises both orthopyroxene and clinopyroxene (Niekerk, 2005; Irving and Kuehner, 2013). Zinder contains a higher abundance of chromite compared to Vermillion, Y-8451, and Choteau.

The O-isotopic compositions of 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). Although Zinder has been demonstrated to be associated with NWA 1911 (Boesenberg and Humayun, 2019), it plots on the terrestrial fractionation line due to a difference in δ17O values; however, terrestrial weathering may be the reason for this difference. Notably, the O-isotopic ratios for the ungrouped Milton pallasite and the Eagle Station group pallasites plot on an extension of the trend line for the CV/CK chondrites.

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

Oxygen Isotope Composition of Ungrouped Pallasites
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click on image for a magnified view

Diagram adapted from Gregory et al., 47th LPSC, #2393 (2016)

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

Better resolution of many of the pyroxene pallasites as well as others belonging to the main-group pallasites was obtained by Dey and Yin (2022) through the use of Cr isotope analysis. They discovered that although the three pyroxene pallasites Vermillion, Y-8451, and Choteau could be grouped together based on oxygen isotopes and other mineralogical similarities, nucleosynthetic 54Cr isotopes clearly resolve the Choteau pallasite from the Vermillion pallasite (see diagram below). The new coupled Δ17O–ε54Cr diagram shows that Vermillion plots in a unique space between the winonaite and acapulcoite–lodranite fields, while Choteau plots in the ureilite field.

Oxygen and Chromium Isotope Systematics for Pallasites
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click on image for a magnified view

Diagram credit: Dey and Yin, 53rd LPSC, #2428 (2022)

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. Boesenberg et al. (2018) utilized an Fe/(Fe+Mg) vs. Al/(Cr+Al) coupled diagram in an analysis of chromite for various pallasites. They demonstrated that NWA 10019 chromite contains a relatively high Al content and plots in a unique compositional space (see diagram below).

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Diagram credit: Boesenberg et al., 49th LPSC, #1556 (2018)

From all of the data gathered so far, it could be concluded that the pallasites in our collections represent at least ten separate parent bodies: (1) main-group high-Δ17O; (2) main-group low-Δ17O; (3) Eagle Station group; (4) Milton; (5) Vermillion + Y-8451; (6) Zinder + NWA 1911; (7) Choteau; (8) NWA 10019 ± Bordj Badji Mokhtar 001; (9) LoV 263; (10) Hassi el Biod 002. Although the two pyroxene pallasites NWA 10019 and Bordj Badji Mokhtar 001 have similar oxygen isotopes that plot in the mesosiderite field, other important differences could exclude the possibility of a common source object. The specimen of NWA 10019 shown above is a 2.02 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