HOW TO INTERPRET THE ARGON AGES OF MATIAN SHERGOTTITES: LESSONS FROM THE MANICOUAGAN IMPACT CRATER, CANADA

One longstanding question regarding martian shergottites is their age. In particular, there has been debate as to whether the unexpectedly young ages (between ~150 and ~450 Ma, see [1-2] and refs therein) reflect original igneous crystallization or subsequent impact metamorphism. Using samples from the Manicouagan Impact Structure as analog for martian material, we evaluated the potential for shock resetting of ⁴⁰Ar/³⁹Ar ages by comparing the measured ages of maskelynite with ages of the target rocks and with impact melt.

Maskelynite grains appear to be partially reset. Although a plateau can be formally defined (>50% of the Ar-39 and >3 steps), their age is between that of the impact age of 215.6 Ma, and the Mesoproterozoic target age. Importantly, the apparent plateau also suggests that using step-heating experiments and the presence or absence of plateaus may not be a valid method for determing if the argon system has been reset during impact events. Additionally, Ar-Ar measurements of maskelynite are not reliable for chronology, and thus interpreting the timing of martian geologic events from Ar-Ar or K-Ar ages in shocked rocks such as the shergottites may be more complicated than previously recognized.

References: [1] Nyquist et al. (2009). GCA 73, 4288- 4309. [2] Bouvier et al. (2009). EPSL 280, 285-295.