Early formation of primitive achondrites in an outer region of the protoplanetary disc
Ning Ma, W. Neumann, A. Néri, W.H. Schwarz, T. Ludwig, M. Trieloff, H. Klahr, Audrey Bouvier
Abstract
We compare 13 Tafassasset-related meteorites and propose that they form the first meteorite group of carbonaceous primitive achondrites. We name this new group the Tafassites, which form a continuum from equilibrated petrological type 6 chondrites (termed T6) to partially molten type 7 primitive achondrites (T7) and bear carbonaceous meteorite-like (C) mass-independent isotopic signatures. We use SIMS Pb-Pb Ca phosphate ages to model the Tafassite parent body (TPB) accretion at 1.1 +0.3 -0.4 Myr before rapid cooling to below 720 K within 9.0 5.0 Myr after CAI formation, respectively. This scenario is consistent with other primitive achondrites but incompatible with a commonly assumed CR chondrite parent body, which was constrained by Al-Mg, Hf-W, and Pb-Pb chondrule ages up to >3.7 Myr after CAIs. Given their carbonaceous-like affinity, Tafassites therefore constitute the first early accreted chondritic meteorite group from an outer region of the protoplanetary disc, presumably close to the further CR feeding zone. Our findings support that planetary formation in the outer protoplanetary disc evolved nearly coevally with the inner part of the disc, with limited admixing of inward material during planetesimal formation over 4 million years after CAIs.