Potassium isotopic compositions and exposure ages of evolved and silica-rich achondritesOPEN ACCESS 

Z. Vaci, Z. Tian, P. Koefoed, M. Habermann, M. Humayun, K. Ziegler, H. Busemann, D. Krietsch, J. M. D. Day, K. Wang

MAPS, Version of Record online: 07 July 2026

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“Some of the oldest igneous rocks in the Solar System include evolved and silica-rich achondrites that originate from parent bodies less than 1000 km in diameter, referred to as planetesimals. While Earth was initially in a molten state and required continental crust formation and plate tectonics to generate andesite bulk compositions, evolved and silica-rich achondrites likely formed from smaller degrees of melting and differentiation on initially chondritic parent bodies. Petrographic descriptions, bulk and in situ chemical analyses, oxygen and potassium isotope measurements, and noble gas analyses are presented to constrain the petrogenesis and possible associations of a suite of evolved and silica-rich achondrites including a trachyandesitic clast from the Almahata Sitta fall (ALM-A), Northwest Africa (NWA) 6698, NWA 11119, its launch pair NWA 11558, NWA 11575, and Graves Nunataks 06128 and 06129. In addition, leaching experiments were conducted that included terrestrial samples to examine the effects potential weathering-induced alteration might have on potassium isotope compositions. The measured potassium isotopic compositions do not covary with volatile depletion, as found when comparing samples from Earth, the Moon, Mars, and the asteroid Vesta, indicating that the planetary depletion trend observed in larger bodies does not apply in the absence of complete planetary differentiation. Modeled noble gas retention ages confirm the ancient formation times of several of these achondrites, while cosmic ray exposure ages suggest separation from their parent bodies in the past ~25 million years.”