ERG ATOUILA 001

Two wind-abraded stones weighing 553 g and 240 g were found within a km of each other in the Sahara Desert region of northern Mali (Google Earth image). Both stones were purchased by Mohamed Aid who sent a sample to the University of New Mexico for analysis and classification (M. Spilde, A. Ross, C. Agee, and K. Ziegler). A paired individual weighing ~30 g was also recovered (photo courtesy of A. Habibi). The meteorite was designated Erg Atouila 001 and classified as an ungrouped achondrite having the characteristics of an albitite (95 vol% albitic alkali feldspar).

In addition to the predominant albite, Erg Atouila 001 contains ~3 vol% clinopyroxene (Ti-, Cr-, Na-rich augite) along with accessory phases including ferroan olivine, K-feldspar, fluorapatite, troilite, very low Ni Fe-metal (some oxidized), and ilmenite (Agee et al., 2022 #2442). The feldspars in Erg Atouila 001 form an alkali feldspar solid solution with albite and orthoclase endmembers (see a feldspar ternary diagram). In a comparison of volcanic rocks based on chemistry and plotted on a total alkali vs. silica diagram, Erg Atouila 001 is the first analyzed meteorite that falls within the trachyte field (see TAS diagram below). Alternatively, the relatively coarse texture of Erg Atouila 001 could be indicative of an igneous plutonic or shallow intrusive origin, in which case its mineralogical composition would be consistent with a syenite, or more specifically, a sodic felsic syenite as assessed by Irving et al. (2022 #2059) (see QAPF diagram below).

Erg Atouila 001 has an oxygen isotope composition and Fe/Mg and Fe/Mn ratios that are similar to those of the acapulcoite–lodranite clan meteorites. On the oxygen three-isotope diagram below, Irving et al. (2022) demonstrate that Erg Atouila 001 plots precisely along the least-squares regression line originally established by Clayton and Mayeda (1996) from data points for the acapulcoite–lodranite clan, and which has been further populated by many accepted and proposed (e.g., Choteau, NWA 468) related meteorites. However, such apparent O-isotopic correspondence has been shown in previous cases to be merely coincidental when employing coupled ε54Cr vs. Δ17O diagrams. Until Cr isotopes and other data are obtained for Erg Atouila 001, no reliable conclusions about parent body origin can be made.

In a study of the hydrogen systematics of many of the rare silica-rich meteorites, Rider-Stokes et al. (2024 #6324) utilized both phosphates and pyroxene to ascertain the water contents and δD of each meteorite. For Erg Atouila 001 and trachyandesite Néma 001, considered to be crustal samples from the ACA–LOD parent body, results show the water contents in phosphates are 5–18 and 40–135 ppm, respectively, and in pyroxene they are 6.5 (±2.7) and 4.6 (±4.0) ppm. This relatively low water content is consistent with the range determined previously for acapulcoites and lodranites of ~5 ppm. The δD values for phosphates in Erg Atouila 001 and Néma 001 are similar to each other and to other NC achondrites (right "δD vs. H2O" diagram in poster); however, the δD values for pyroxene in Erg Atouila 001 and Néma 001 differ significantly, presenting at 171 (±300) ‰ and –264 (±200) ‰, respectively (left "δD vs. H2O" diagram in poster).

After a determination of the U-Pb isotopic compositions of phosphates (apatite and merrillite) in Erg Atouila 001, Wu et al. (2024 #6113) derived a weighted mean PB–Pb age of 4.459 (±0.022) b.y., or ~100 m.y. after CAIs. As this relatively young age reflects the fact that cooling below the closure temperature of the U–Pb system occurred long after the time of radiogenic Al extinction, another heat source must be responsible for the melting and formation of silica-rich magma. Wu et al. (2024) suggest thermal metamorphism and/or impact events as the probable mechanisms, and they describe features in Erg Atouila 001 which are consistent with both. They also argue that the Moon-forming event is likely associated with this disruptive period in the early asteroid belt.

A diligent search of the area surrounding the Erg Atouila 001 find site has resulted in the recovery of five additional stones located in the near vicinity—between 40 and 900 meters (44 yards to half a mile) away. Analyses of these five stones, designated Erg Atouila 003 and weighing together 1442 g, were completed by Irving and Carpenter (2022 #6370). The data confirm that the component minerals in the new stones are chemically close to those in Erg Atouila 001; however, there are some obvious mineralogical differences between them, as well as some differences in both the phases observed and the modal abundances (see a photo of a 15 g slice of EA 003, courtesy of Mohamed Ali Loud). Nevertheless, it is easy to infer that these five stones were once incorporated together with EA001 in a single heterogeneous meteoroid that was earlier ejected from their parent body. It is now arguably even more likely that these meteorites originated on the acapulcoite–lodranite asteroid.

The photo of Erg Atouila 001 shown above is a 3.75 g slice that was cut from the piece shown in the excellent high-res photographs below, courtesy of Phil Schmitt. The natural surface lacks fusion crust and shows a light-orange desert patina, while the cut face exhibits coarse-textured grains of light pink to white albitic feldspar intermixed with dark green to black pyroxene and sparse Fe(Ni)-metal.