Constraining shock ages and Sr‐Os isotope geochemistry of brachinites and brachinite‐like achondrites

Brachinites and brachinite‐like achondrites record melting processes at more oxidizing conditions than many other achondrite meteorites, attested by relatively Fe‐rich silicate phases. Here, new major‐, trace‐, and highly siderophile‐element (Os, Ir, Ru, Pt, Pd, Re) abundances, as well as Re‐Os and Rb‐Sr isotope data are reported for the brachinite El Medano 395 and for the anomalously apatite‐rich (~1.26% modal) brachinite‐like achondrite Northwest Africa (NWA) 12,532. Both meteorites have Rb‐Sr and Re‐Os isotope systematics that have been significantly affected by terrestrial alteration, reflecting variable addition of Sr and variable loss of Rb and Re. Brachinite‐like achondrite NWA 12532 experienced late addition of metallic phases and apatite on its parent body, having nearly chondritic abundances of Os, Ir, Ru, Pt, and Pd. The apatite in NWA 12532 gives an age of 4491.3 ± 14.1 Ma, overlapping with apatite U‐Pb ages for ordinary chondrites, as well as for other brachinites and brachinite‐like achondrites. This age is significantly younger than available Mn‐Cr ages for brachinite silicate phases. The young U‐Pb ages of brachinites and brachinite‐like achondrites likely reflect significant impact shock resetting events at ~4.43 to 4.49 Ga across the solar system, also recorded in ordinary chondrites. These resetting events may be related to a significant dynamical reorganization at that time.