^1,2Stepan M. Chernonozhkin,³Lidia Pittarello,⁴Genevieve Hublet,⁵Philippe Claeys,⁴Vinciane Debaille,¹Frank Vanhaecke,⁵Steven Goderis
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.14258]
¹Atomic & Mass Spectrometry—A&MS Research Unit, Department of Chemistry, Ghent University, Ghent, Belgium
²Isotope Ratio Analysis Research Group, Chair of General and Analytical Chemistry, Montanuniversität Leoben, Leoben, Austria
³Naturhistorisches Museum Wien – NHMW, Vienna, Austria
⁴Laboratoire G-Time, Université Libre de Bruxelles, Brussels, Belgium
⁵Archaeology, Environmental Changes, and Geo-Chemistry (AMGC), Vrije Universiteit Brussel, Brussels, Belgium
Published by arrangement with John Wiley & Sons

The processes of planetary accretion and differentiation, whereby an unsorted mass of primitive solar system material evolves into a body composed of a silicate mantle and metallic core, remain poorly understood. Mass-dependent variations of the isotope ratios of non-traditional stable isotope systems in meteorites are known to record events in the nebula and planetary evolution processes. Partial melting and melt separation, evaporation and condensation, diffusion, and thermal equilibration between minerals at the parent body (PB) scale can be recorded in the isotopic signatures of meteorites. In this context, the acapulcoite–lodranite meteorite clan (ALC), which represents the products of thermal metamorphism and low-degree partial melting of a primitive asteroid, is an attractive target to study the processes of early planetary differentiation. Here, we present a comprehensive data set of mass-dependent Fe, Zn, and Mg isotope ratio variations in bulk ALC species, their separated silicate and metal phases, and in handpicked mineral fractions. These non-traditional stable isotope ratios are governed by mass-dependent isotope fractionation and provide a state-of-the-art perspective on the evolution of the ALC PB, which is complementary to interpretations based on the petrology, trace element composition, and isotope geochemistry of the ALC. None of the isotopic signatures of ALC species show convincing co-variation with the oxygen isotope ratios, which are considered to record nebular processes occurring prior to the PB formation. Iron isotopic compositions of ALC metal and silicate phases broadly fall on the isotherms within the temperature ranges predicted by pyroxene thermometry. The isotope ratios of Mg in ALC meteorites and their silicate minerals are within the range of chondritic meteorites, with only accessory spinel group minerals having significantly different compositions. Overall, the Mg and Fe isotopic signatures of the ALC species analyzed are in line with their formation as products of high-degree thermal metamorphism and low-degree partial melting of primitive precursors. The δ66/64Zn values of the ALC meteorites demonstrate a range of ~3.5‰ and the Zn is overall isotopically heavier than in chondrites. The superchondritic Zn isotopic signatures have possibly resulted from evaporative Zn losses, as observed for other meteorite parent bodies. This is unlikely to be the result of PB differentiation processes, as the Zn isotope ratio data show no covariation with the proxies of partial melting, such as the mass fractions of the platinum group and rare earth elements.