NORTHWEST AFRICA 1911


Pyroxene Pallasite
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.

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; Vermillion has been included in the IAB iron-meteorite complex by Wasson and Kallemeyn (2002). 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 (~50 vol%) and orthopyroxene (~10 vol%), with pyroxene accounting for ~5 vol% of this pallasite.

All six currently recognized pyroxene pallasites (when including Vermillion) are compositionally distinct from each other. The low-Ca pyroxene in Zinder, NWA 1911, and NWA 10019 is entirely composed of orthopyroxene, while that in Y-8451 and Choteau comprises both orthopyroxene and clinopyroxene (Niekerk, 2005; Irving and Kuehner, 2013). Zinder contains a higher abundance of chromite compared to both Vermillion and Y-8451. The O-isotopic compositions of all six meteorites are distinct from each other, and are associated with a number of established O-isotopic trends: Zinder plots on the terrestrial fractionation line (Bunch et al., 2005), Choteau plots in 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).

Based on all of the data gathered so far, it could be presumed that the pallasites in our collections represent at least eight separate parent bodies: 1) main-group, 2) Eagle Station group, 3) Milton, 4) Y-8451, 5) Zinder, 6) NWA 1911, 7) Choteau, and 8) NWA 10019. In addition, the anomalous pallasites Springwater and Glorieta Mountain could possibly increase the number of unique parent bodies. Notably, the O-isotopic ratios for both Milton and the Eagle Station group pallasites plot on an extension of the trend line for the CV chondrites, and Choteau might be derived from the acapulcoite–lodranite parent body. Further information on the pyroxene pallasites can be found on the Vermillion page. The specimen of NWA 1911 shown above is a 6.47 g slice.