NORTHWEST AFRICA 1911


Pyroxene pallasite, ungrouped
standby for nwa 1911 photo
Purchased March 2003
no coordinates recorded

A fresh (W1), complete stone, weighing 53.07 g, was retrieved by M. Farmer from a batch of meteorites shipped to him from Rissani, Morocco; this is the first pallasite recognized to be found in Northwest Africa. Northwest Africa 1911 was analyzed and classified at Northern Arizona University (Wittke and Bunch, 2003) and was found to have a modal composition of 24.3% FeNi-metal and 75% silicates, with the silicates consisting of 40.2% olivine and 34.5% orthopyroxene—the highest pyroxene content recorded for a pallasite. Minor troilite and chromite are also present, as well as trace merrillite.

In their study of NWA 1911, Boesenberg and Humayun (2019) found that the metal composition is extremely similar to that of the Zinder pyroxene pallasite. Utilizing a coupled Fe/(Fe+Mg) vs. Al/(Cr+Al) diagram for various pallasites, they demonstrated that chromite in both NWA 1911 and Zinder contains a relatively high Al content and plots in a common unique compositional space (see diagram below).

standby for chromite fe vs. al diagram
Diagram credit: Boesenberg and Humayun, 50th LPSC, #1438 (2019)

It is noteworthy that the pyroxene pallasite Zinder, and by association NWA 1911, were previously found to contain metal with a composition that is chemically identical to that of group IIIF irons, particularly Cerro del Inca, Moonbi, and St. Genevieve County (Boesenberg et al., 2017; Humayun et al., 2018; Zhang et al., 2021 #1640, 2022 [see Fig 5]). However, considerable data indicates that the IIIF irons formed in the carbonaceous region beyond Jupiter (e.g., Kruijer et al., 2017; Worsham et al., 2019), whereas the negative ε54Cr and δ26Mg* (‰) values (Wimpenny et al., 2019, Fig. 8b) as well as the O-isotopic composition of Zinder indicate it was formed in the non-carbonaceous region within the inner Solar System. See the Protoplanetary Disk page for further details about the NC and CC regions. Interestingly, Rubin (2018, Fig. 1) has shown that group IIIF irons are poorly resolved from the NC-iron groups with respect to Ni and Ir contents. Continued investigations will help resolve whether or not a genetic relationship exists between the IIIF irons and the Zinder and NWA 1911 pallasites.

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.

Oxygen Isotope Composition of Ungrouped Pallasites
standby for oxygen isotope diagram
click on image for a magnified view

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

In a study conducted by Gregory et al. (2016), it was ascertained that Choteau is similar to both Vermillion and Y-8451 in mineralogy, composition, and with respect to oxygen isotopes, and they concluded that these three pyroxene pallasites form a grouplet; it was suggested that these three meteorites be termed 'Vermillion pallasites'. However, 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 nucleosynthetic 54Cr isotopes clearly resolve the Choteau pallasite from the Vermillion pallasite (see the Vermillion page for additional details). Notably, on a coupled Δ17O–ε54Cr diagram Choteau plots in the ureilite field. Other investigations of pyroxene pallasites found that 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; Sharma et al., 2019). Zinder contains a higher abundance of chromite compared to Vermillion, Y-8451, and Choteau.

Many of the pyroxene-bearing pallasites are associated with a number of established O-isotopic trends: Vermillion, Y-8451, and Choteau have O-isotopic compositions similar to that of the ungrouped olivine pallasite Hassi el Biod 002 and plot in the field of the acapulcoite–lodranite clan on an oxygen three-isotope diagram (see plot 1 and 2); however, it was determined they are not related to that primitive achondrite group. 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. The O-isotope values for silicates in LoV 263 plot in a space distinct from all other known pallasites.

Based on 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. 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 1911 shown above is a 6.47 g slice.