Ancient and recent collisions revealed by phosphate minerals in the Chelyabinsk meteorite

TL;DR

This study uses phosphate U-Pb dating and microtexture analysis of the Chelyabinsk meteorite to reveal its complex collision history, showing ancient impact events and recent collisional modifications.

Contribution

It introduces a combined microtexture and U-Pb dating approach to better interpret collision histories in meteorites, improving age constraints.

Findings

Phosphate U-Pb ages are independent of macro-scale thermal history.

Pristine phosphate domains show concordant isotopic ages.

Fracture-damaged domains display discordance and Pb-loss.

Abstract

The collision history of asteroids is an important archive of inner Solar System evolution. Evidence for these collisions is brought to Earth by meteorites, which can preserve impact-reset radioisotope mineral ages. However, as meteorites often preserve numerous mineral ages, their interpretation is controversial. Here, we combine analysis of phosphate U-Pb ages and mineral microtextures to construct a collision history for the highly shocked Chelyabinsk meteorite. We show that phosphate U-Pb ages in the meteorite are independent of thermal history at macro-to-microscales, correlating instead with phosphate microtexture. Isotopic data from pristine phosphate domains is largely concordant, whereas fracture-damaged domains universally display discordance. Combining both populations best constrains upper (4,473 +/- 11 Ma) and lower intercept (-9 +/- 55 Ma, i.e., within error of the present day) U-Pb ages for Chelyabinsk phosphates. We conclude that all phosphate U-Pb ages were completely reset during an ancient high energy collision. Fracture-damaged phosphate domains experienced further Pb-loss during mild collisional heating in the geologically recent past, and must be targeted to properly constrain a lower intercept age. Targeting textural sub-populations of phosphate grains can significantly improve the calculation and interpretation of U-Pb ages, permitting more robust reconstruction of both ancient and recent asteroidal collision histories.