Thermochemical evolution of the acapulcoite–lodranite parent body: Evidence for fragmentation-disrupted partial differentiation

Michael P. Lucas, Nicholas Dygert, Jialong Ren, Marc A. Hesse, Nathaniel R. Miller, Harry Y. McSween

Version of Record online: 21 November 2022


“Primitive achondrites of the acapulcoite–lodranite clan (ALC) are residues of partial melting that displays a continuum of thermal metamorphism and partial melting most likely set by burial depth within an internally heated, primordial acapulcoite–lodranite parent body (ALPB). New major and trace element data from eight ALC meteorites and the application of several thermometric methods suggest that the ALPB was affected by partial differentiation disrupted by rapid cooling from peak, magmatic temperatures. Application of rare earth element-in-two-pyroxene thermometry recovers temperatures of 1125–1250 °C for lodranites, while two-pyroxene solvus and Ca-in-olivine thermometry recover lower temperatures for ALC meteorites (941–1114 °C and 686–850 °C, respectively). Major and trace element disequilibrium in acapulcoite and transitional groups provides evidence for cryptic melt infiltration and melt rock reaction within these layers of the ALPB. From lodranites, we determined rapid cooling rates of ~1 to ~26 °C yr−1 from peak temperatures, consistent with collisional fragmentation of the parent body during differentiation. After this initial period of rapid cooling, cooling rates decreased by two to four orders of magnitude through Ca-in-olivine closure temperatures (~750 °C). We hypothesize that the primordial ALPB possessed an onion shell-type layered structure that was disrupted by collisional breakup during partial differentiation. Thermal modeling suggests that ALC samples originate from ~300 m to ~10 km radius collisional fragments that cooled rapidly over time scales of several to ~20,000 yr, then reaccreted to form a slower cooling, second-generation rubble-pile asteroid. The source of ALC meteorites is a second-generation (or later) rubble-pile body of S-type spectral class located near the Jupiter 3:1 mean motion resonance in the Main Belt of asteroids.”