THE PERSISTENT CHALLENGE OF ACCURATELY AND PRECISELY DATING IMPACT EVENTS

In an era when geochronology and thermochronology are becoming increasingly accurate and precise for most applications, our confidence in the ages of impacts has lagged considerably. Only a small number of the impact craters on Earth have been accurately and precisely dated. This has significant implications for how well we can test possible cause-and-effect relationships among extinction and impact events in the geologic record. Unprotected by a thick atmosphere and unaffected by plate tectonics, the Moon has many more impact craters, but our understanding of their ages are even less precise and accurate than those of the well-dated craters on Earth. This is a major impediment to empirical tests of ideas regarding the early impact flux to the inner Solar System, such as the Late Heavy Bombardment hypothesis. We should aspire to do better.

 Thanks to the transient thermal effects of impact, not every “date” of an impact rock (impactite) can be interpreted unambiguously as the “age” of a specific impact event. Many impact breccias from the Moon and other meteorite parent bodies contain evidence of multiple generations of impact melt of different ages (Mercer at al., 2015, Science Advances). Even if such breccias are monogenetic, they still contain abundant relict minerals derived from target rocks in addition to “neoblastic” minerals that crystallized from the impact melt. Experimental isotopic diffusivity constraints, coupled with notional estimates of the duration and maximum temperatures achieved during impact events, suggest that variable and incomplete resetting of relict mineral isotopic chronometers should be very common (Young et al., 2013, Geophysical Research Letters). Even the most accurate and precise dating tool at our disposal, the U/Pb zircon chronometer, cannot always be trusted to provide accurate estimates of the timing of specific impact events. Other chronometers – ⁴⁰Ar/³⁹Ar and (U-Th)/He ­– may either overestimate the ages of impact events when relict phases are incompletely degassed of radiogenic Ar or He, or underestimate impact ages if their systematics were disturbed during post-impact thermal events.

Both accurate and precise impact dating requires the application of multiple chronometers to the same samples, and a demonstration that these techniques yield concordant results.