NORTHWEST AFRICA 4969


Brachinite
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Found 2007
no coordinates recorded

A portion of a fresh, fusion-crusted stone weighing 180 g was found in Northwest Africa and subsequently purchased by M. Cimala in Erfoud, Morocco. A type specimen was submitted for analysis to Northern Arizona University (J. Wittke and T. Bunch), and NWA 4969 was classified as a new brachinite. This brachinite might be paired with other brachinites found in Northwest Africa during a similar timeframe (e.g., NWA 4042, 4872, 4874, 4882 [the largest brachinite mass known], and 6349).

Northwest Africa 4969 is an olivine-rich (~89 vol%) dunite with a protogranular texture typical of this group. It has a bimodal grainsize, ranging from fine- to medium-grained (0.1 mm to 1.2 mm), and contains minor clinopyroxene, K-poor sodic plagioclase, chromite, and FeNi-metal. Similar to some other brachinites, olivines contain very fine-grained assemblages of orthopyroxene and opaques (metal and sulfide) lining various olivine grain boundaries, features which have been attributed to fluctuations in redox conditions (e.g., Rumble III et al, 2008). Several methods for the reduction of primary olivine were reviewed by Goodrich et al., 2017), including its reaction with methane to form orthopyroxene + metal (Irving et al., 2013) and through its sulfurization by a S-rich fluid or gas to form orthopyroxene + sulfide (e.g., Singerling et al., 2013).

The formation age of the paired brachinite NWA 4882 was calculated by Dunlap et al. (2016) using the short-lived Mn–Cr chronometer. An absolute age relative to the D'Orbigny age anchor was determined to be 4.5502 (±0.0008) b.y. This is ~14.6 m.y. later than the formation of the type specimen for this group, Brachina, which has a Mn–Cr age of 4.5648 (±0.0005) b.y. (Dunlap et al., 2016). In addition, their study revealed that the ε54Cr value for this meteorite is similar to that of other brachinites, indicating that they all derive from a common parent body that experienced a long duration of impact-heating. In a subsequent study, Dunlap et al. (2017) determined a very ancient Al–Mg absolute age relative to D'Orbigny of 4.5680 (±0.0005) b.y., possibly reflecting the time of partial melting of the source lithology.

Many of the known brachinites have disparate cosmic-ray exposure ages, indicating that they represent numerous separate ejection events. According to a study by Patzer et al. (2003), the CRE ages of EET 99402/407, Hughes 026, and Eagles Nest form a cluster at ~48 m.y., and those of Reid 013 and ALH 84025 coincide at ~10 m.y. In a separate study by Ma et al. (2003), the cosmogenic nuclide calculations establish a range of CRE ages from 4 m.y. for Brachina to ~25.5 m.y. for Eagles Nest. From their noble gas analyses of 15 brachinite and brachinite-like meteorites, together with the literature values for seven others, Beard et al. (2018) identified three potential CRE age clusters. The oldest cluster reflects a possible ejection event that occurred ~49.9 (±4.1) m.y. ago, comprising the six brachinites Eagles Nest, EET 99402, Hughes 026, NWA 4969, NWA 7605, and NWA 10637. Importantly, two of these CRE age clusters include both brachinite and brachinite-like meteorites, which attests to a common parent body for all of these meteorites (see diagram below).

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click on image for a magnified view

Diagram credit: Beard et al., 81st MetSoc, #6170 (2018)

Information concerning the origin and petrogenesis of the brachinites and the meteorite GRA 06128/9, considered to be a possible representative of the feldspathic crust of the brachinite parent body, can be found on the NWA 3151, Reid 013, and Eagles Nest pages. The specimen of NWA 4969 shown above is a 0.93 g partial slice. The photo below shows the partial stone as found.

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Photo courtesy of Marcin Cimala—Polish Meteorite Laboratory