Shocker: xenotime can date impacts

Constraining precise ages for impact events is crucial in establishing Earth’s history, and several geochronometers have been developed to date impacts. We present electron backscatter diffraction (EBSD), sensitive high-resolution ion microprobe (SHRIMP) and atom probe tomography (APT) data from shocked xenotime [(Y,HREE)PO₄] collected from two impact sites to investigate the potential of xenotime as an impact geochronometer. A detrital xenotime grain from the Vredefort dome (South Africa) contains planar fractures, planar deformation bands and {112} twinning, the latter of which are diagnostic shock microstructures. However, APT analysis from the twin domains and also from the host yielded no evidence of Pb mobility at the nanometer scale during the impact. SHRIMP analysis (n=24) on the grain yielded a discordia with an upper intercept of 3136 ± 110 Ma and an imprecise lower intercept of 1793 ± 280 Ma. These correspond, respectively, to the bedrock age and a post-impact, cryptic terrane-wide fluid infiltration event. Three neoblastic grains from the Araguainha dome (Brazil) experienced partial to complete recrystallisation. The least recrystallised grain yields the oldest ²³⁸U/²⁰⁶Pb age of 479 ± 26 Ma, whereas a completely recrystallised neoblastic grain gave an age of 257 ± 11 Ma.  APT analysis on the latter grain showed different nanoscale features that shed light on Pb mobility during shock deformation and recrystallisation.  Based on observations of nanoscale Pb mobility and the correlation between recrystallisation and isotopic resetting, and prior published ages, we interpret 257 ± 11 Ma to date the impact event. These data confirm that recrystallised neoblastic xenotime is a useful impact geochronometer.