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Trachyandesitic volcanism in the early Solar System

Significance Volcanism is a fundamental geological process on planets and was substantial during crustal growth on planetary bodies in the early Solar System, as witnessed by ubiquitous rocks of basaltic composition, e.g., on Earth, Moon, Mars, and asteroids. Besides basaltic volcanism, trachyandesite lavas are generated on Earth. The first occurrence of a trachyandesite lava in the meteorite collections demonstrates that trachyandesitic, alkali-, and silica-rich volcanism takes place not only on Earth today but already occurred on a small planetesimal ∼4.56 billion years ago. It sets new constraints on mechanisms and styles of early Solar System volcanism. Volcanism is a substantial process during crustal growth on planetary bodies and well documented to have occurred in the early Solar System from the recognition of numerous basaltic meteorites. Considering the ureilite parent body (UPB), the compositions of magmas that formed a potential UPB crust and were complementary to the ultramafic ureilite mantle rocks are poorly constrained. Among the Almahata Sitta meteorites, a unique trachyandesite lava (with an oxygen isotope composition identical to that of common ureilites) documents the presence of volatile- and SiO2-rich magmas on the UPB. The magma was extracted at low degrees of disequilibrium partial melting of the UPB mantle. This trachyandesite extends the range of known ancient volcanic, crust-forming rocks and documents that volcanic rocks, similar in composition to trachyandesites on Earth, also formed on small planetary bodies ∼4.56 billion years ago. It also extends the volcanic activity on the UPB by ∼1 million years (Ma) and thus constrains the time of disruption of the body to later than 6.5 Ma after the formation of Ca–Al-rich inclusions.