NORTHWEST AFRICA 2775


Acapulcoite, FeO-rich
Acapulcoite–Lodranite Clan
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Purchased November 2005
no coordinates recorded

A 222 g [231 g] partially fusion-crusted stone, which was found in Algeria, was eventually marketed in Erfoud, Morocco. The stone was purchased by American collectors and a portion was submitted for analysis to both the Northern Arizona University (J. Wittke and T. Bunch) and the University of Washington in Seattle (A. Irving). Northwest Africa 2775 was classified as an acapulcoite (see MetBull 91).

While NWA 2775 exhibits the typical recrystallized texture of acapulcoites, its olivine and pyroxene have FeO contents among the highest measured thus far in other acapulcoites. Notably, grain sizes have a range of 0.35–0.8 mm (ave. 0.55 mm), which is consistent with lodranites (>0.5 mm) according to one of the parameters that help distinguish lodranites from acapulcoites. However, with the many new samples available for study, it is now evident that a continuum exists for the grainsizes of these two groups, and it has been proposed by Bunch et al. (2011) that an arbitrary group division is no longer justified; the term "acapulcoite–lodranite clan" should therefore be applied to all members of the combined group.

Oxygen isotopes were analyzed at the Carnegie Institute, Washington D.C. (D. Rumble III), and the Δ17O was found to be the lowest among all acapulcoites measured to date, but was determined to be consistent with an acapulcoite classification. A method to distinguish acapulcoites from winonaites was recently devised by Rumble III et al. (2005). Utilizing their diagram comparing the Fa mol% of olivine vs. the Δ17O‰, it is demonstrated that NWA 2775 (Fa14.5; ave. Δ17O –0.75‰) plots in a unique location at the edge of the acapulcoite field.

The reason for this observed correlation between the Fa content in olivine and the Δ17O value in acapulcoites was considered by Irving et al. (2007). The possibility was suggested that a metal-poor impactor with a Δ17O value plotting close to the terrestrial fractionation line, similar to a brachinite, was mixed into the regolith of a body having olivine and an O-isotopic composition similar to a CH chondrite; subsequent to the collision, the mixture was thermally equilibrated.

The closure of the Hf–W chronometer on the ACA–LOD parent body occurred 4.5621 (±0.0014) b.y. ago, or 6.4 (±1.3) m.y. after CAI formation (Touboul et al., 2007). A slightly older Hf–W age of 3.84 (+3.6/–3.1) m.y. after CAI formation was calculated by Schulz et al. (2010). With other factors considered, they concluded that the metal melting point, or the cooling point at which redistribution of Hf and W between metal and silicate ended, occurred 4.1 (+1.2/–1.1) m.y. after CAIs. Although the ACA–LOD parent body reached higher temperatures than did the H chondrite parent body by assimilating a higher abundance of radiogenic nuclides during its earlier accretionary period, it also cooled more rapidly at high temperatures, possibly reflecting a smaller-sized planetesimal and/or a near-surface residence for the acapulcoites (Kleine et al., 2007). Another possibility for its early rapid cooling could be the fact that it experienced a collisional disruption early in its history forming sub-km- to multi-km-sized fragments, which eventually succumbed to gravitational reassembly.

For more complete amd current formation scenarios of the acapulcoite–lodranite parent body, visit the Monument Draw and Lodran pages. The specimen of NWA 2775 shown above measures 35 mm × 19 mm and weighs 2.68 g. The photo below shows a magnified image of this acapulcoite.

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Photos courtesy of S. Turecki