¹Z. Váci,²P.M. Kruttasch,¹M.J. Krawczynski,³R.C. Oglior,²K. Mezger
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2024.08.011]
¹Department of Earth and Planetary Sciences, Washington University in St. Louis, MO, USA
²Institut für Geologie, Universität Bern, Switzerland
³Department of Physics, Washington University in St. Louis, MO, USA
Copyright Elsevier

The ungrouped dunitic achondrite Northwest Africa (NWA) 12,217 contains symplectic spinel-pyroxene veins that are mineralogically identical to symplectites in other ultramafic planetary materials. The morphology and amount of chromite present in these features relative to the Cr in their olivine hosts suggest an exogenous origin. Petrological experiments show that a Cr laden sulfide liquid reacts with olivine to produce pyroxene by scavenging Mg and Fe from olivine to crystallize chromite. The liquid infiltrates cracks and grain boundaries within the olivine and produces a vein-like symplectic chromite-pyroxene mineralogy similar to that observed in NWA 12217. This process is likely responsible for forming the symplectites in the related ultramafic achondrites NWA 12217, 12319, 12562, and 13954, along with many other achondrites. The nucleosynthetic Cr isotopic composition of chromites appears to be in disequilibrium with that of silicates in NWA 12217, suggesting that the liquids responsible for the symplectite forming reaction are at least partially sourced from a different parent body and result from an impact .

¹Pascal M. Kruttasch,^1,2Aryavart Anand,³Paul H. Warren,⁴Chi Ma,¹Klaus Mezger
Geochimica et Cosmochimica Acta (in Press) Open Access Link to Article [https://doi.org/10.1016/j.gca.2024.08.012]
¹Institut für Geologie, Universität Bern, Baltzerstrasse 1+3, 3012 Bern, Switzerland
²Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
³Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, CA 90095, USA
⁴Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA
Copyright Elsevier

Establishing the temporal evolution of the ureilite parent body(ies) is crucial for understanding the quantitative timescale of planetesimal formation and evolution in the protoplanetary disk. In order to establish a timeline for these early processes, age constraints on the accretion, differentiation and secondary reduction were obtained with the short-lived ⁵³Mn-⁵³Cr chronometer to whole-rock and sequentially digested fractions of main group ureilites. A whole-rock isochron dates the reservoir-scale Mn-Cr fractionation in the ureilite parent body(ies), associated with magmatic differentiation, to 2.89-0.51+0.56 Ma after CAI formation. This age implies that the ureilite parent body(ies) accreted no later than ∼1.5 Ma after CAI formation, at a time when the NC-CC dichotomy was already established. The ⁵³Mn-⁵³Cr systematics of fractions from chromite-bearing ureilites yield an age of 4.29-0.45+0.49 Ma after CAI formation for a secondary reduction event on the parent body. This event is commonly associated with the catastrophic disruption of the ureilite parent body while still hot. The chromite model ages are consistent with the isochron ages obtained from chromite-bearing ureilites. Collectively these ages indicate that chemical differentiation processes were underway on the ureilite parent body(ies) during the time interval when undifferentiated meteorite parent bodies were forming, and may have paused at the peak of planetesimal formation when planetary collisions were common.