A Unique Sodic Mafic Igneous Achondrite From an Ancient(?) Differentiated Parent Body

Sodium-rich mafic igneous rocks are rare, even on planet Earth. Among meteorites, one of the few examples is Graves Nunataks 06128/06129 (paired stones). Albitic plagioclase also occurs in aubrites and other enstatite achondrites, but only as a minor component. A very different sort of plutonic igneous achondrite was found as multiple specimens in far southern Algeria in May 2020, and was classified as Erg Chech 002. This unique unbrecciated sodic gabbroic achondrite contains sporadic large (up to 9 cm) xenocrysts of orthopyroxene (Fs_14.4-31.1Wo_1.2-3.8, FeO/MnO = 16-26, Cr₂O₃ = 0.3-1.0 wt.%), pigeonite (Fs_21.3-40.7Wo_7.0-5.0, FeO/MnO = 17-20, Cr₂O₃ = 0.3-0.8 wt.%) and augite (Fs_14.7-24.7Wo_38.5-38.1, FeO/MnO = 15-18, Cr₂O₃ = 1.1 wt.%), exhibiting embayed shapes and reaction rims against the dominant groundmass. The groundmass (mean grainsize ~1.5 mm) is composed of clustered prismatic grains of exsolved pigeonite (host Fs_42.3-52.2Wo_2.9-4.6, exsolution lamellae Fs_18.1-25.9Wo_40.8-38.1, FeO/MnO = 18-23) and laths of zoned oligoclase-albite (Ab_68.8-85.9An_26.7-7.1Or_7.0-4.4), plus accessory Ti-chromite, ilmenite, cristobalite, merrillite, K-feldspar (Or_84.1Ab_11.3An_4.6), troilite and rare Ni-poor metal.

Whole rock abundances (in wt.%): SiO₂ 58.43, TiO₂ 0.40, Al₂O₃ 8.89, Cr₂O₃ 0.57, FeO 10.96, MnO 0.46, MgO 8.23, CaO 7.74, Na₂O 3.62, K₂O 0.60, P₂O₅ 0.11; Mg/(Mg+Fe) = 0.572, FeO/MnO = 24) and oxygen isotopic composition (D¹⁷O = -0.13±0.02 per mil) together establish this material as non-terrestrial and distinct from other achondrites. Although the oxygen isotopic composition is close to values for four anomalous eucrites (Bunburra Rockhole, Emmaville, Asuka 881394 and EET 92023), Erg Chech 002 differs in having highly sodic rather than highly calcic plagioclase. We propose that it derives from a previously unsampled differentiated, planet-like parent body, possibly one that suffered collisional destruction and dispersion of its crustal rocks early in solar system history.