Congratulations to Dr. Daniel Dunlap, who successfully defended his doctoral thesis August 28, 2019! Dr. Dunlap will next undertake a post-doctoral research position at NASA Johnson Space Center.
This research investigated the formation ages of three distinct achondrite types using the relative 26Al-26Mg and 53Mn-53Cr chronometers and the absolute Pb-Pb chronometer. Brachinites are ultramafic, wehrlitic-dunitic achondrites whose petrogenesis is debated, but are thought to represent partial melt residues. The chronology of two brachinites indicates that partial melting began contemporaneously with formation of the earliest formed solids in our Solar System (CAIs) and continued for at least 17 Ma. These findings have important implications for the evolution of the brachinite parent body(ies). Eucrites are basaltic crustal rocks from the asteroid Vesta and are key samples to understand how igneous activity progressed on a differentiated parent body. Due to their emplacement in the crust of Vesta, many eucrites experienced post formation thermal and shock metamorphism which obfuscates the chronology of their formation. Northwest Africa (NWA) 8661 is a pristine, unequilibrated and unshocked eucrite with intact isotope systematics providing unequivocal evidence for basalt formation on Vesta 3 Ma after CAIs. This is the first reported formation age of any eucrite. NWA 11119 is an ungrouped achondrite with an andesitic-dacitic bulk composition and has the highest bulk SiO2 of any known achondrite. The chronology of NWA 11119 indicates that evolved high SiO2 melts were being produced on planetesimals ~ 2 Ma after CAIs and may inform andesite production on other planetary bodies including Earth.