Early Partial Melting and Formation of Na-Rich Asteroidal Crust Revealed by the Albite-Rich Achondrite Erg Atouila 001

Abstract

Elucidating the extent of early magmatic diversity on asteroids is crucial for understanding the planetary formation processes. While our knowledge of early asteroidal differentiation is predominantly based on basaltic components due to the relatively large number of basaltic achondrites discovered, the recent identification of high-Si achondrites offers a new opportunity to investigate the origin and evolution of felsic magmas on asteroids. In this study, we conducted detailed petrological, mineralogical, and in situ phosphate chronological analyses on Erg Atouila (EA) 001, an ungrouped achondrite recognized for its high SiO₂ and alkali contents. Our findings suggest that EA 001 could derive from low degree partial melting of an oxidized parent body, involving extensive segregation of Fe–Ni-S melts and potential degassing of volatiles. Impact heating may play an important role in the thermal evolution of its parent body. Geochemical modeling of rare earth element concentrations indicates that EA 001 could have originated from <15% fractional melting of chondritic precursors (e.g., Acapulco-like, Lewis Cliff 88763-like), with apatite playing a crucial role. Although the oxygen isotopic composition of EA 001 is similar to that of the acapulcoites-lodranites clan and some FeO-rich achondrites, other petrological and geochemical features suggest formation from different source materials. The diversity among high-Si achondrites implies that partial melts of feldspar-rich components are more common than previously thought on asteroids and are important constituents of crustal components. This process likely occurred on various asteroidal parent bodies with different initial compositions, oxidation conditions, degassing scenarios, and differentiation extents.