Context matters – Ar–Ar results from in and around the Manicouagan Impact Structure, Canada: Implications for martian meteorite chronology

As an analog for interpretations of the ages of martian shergottite meteorites, we have conducted an argon isotopic study of plagioclase feldspars exhibiting varying levels of shock from in and around the Manicouagan impact structure, Canada. Plagioclase from the impact melt sheet at Manicouagan yields an age of 215.40 ± 0.16 Ma, which indicates the time of impact. Plagioclase from a clast within melt-bearing breccias of the melt sheet and a hornfels adjacent to the melt sheet yield ages of 216 ± 3 Ma and 218 ± 7 Ma, respectively, which are interpreted to have been reset by contact metamorphism from the impact melt. Country rocks that were unaffected by the impact gives ∼849 Ma ages, consistent with the known Grenvillian target rock history. Maskelynite (amorphous plagioclase, which has been transformed in the solid state) yields an age of 567 ± 6 Ma. This age is geologically meaningless because it is not consistent with the target age, the impact age, or regional metamorphic ages at Manicouagan. Our results show that maskelynite argon ages are not meaningful, and that context is critical for proper interpretation of impact-affected argon ages.