Dating of Lunar Impactites

Reconstructing the early lunar impact chronology relies on the absolute dating of lunar impactites, especially those connected to large, pre-Imbrium events. However, lunar samples often pose several challenges that need to be overcome to successfully determine the age of a sample. These include disturbance by later impact events, neutron capture effects, small mineral grain sizes, and the effects of brecciation, such as the mixing of different rock types, shock melting, and infiltration of external melts. 

We are using a multi-chronometry approach for dating lunar impactites, in which we analyse several radiogenic isotopic systems (Lu-Hf, Sm-Nd, Rb-Sr, and Pb-Pb) for a single sample split. For samples that did not undergo heavy reworking after their formation, this approach allows us to constrain the age of a sample with several independent decay systems. In the case of reworked impactites, the use of several isotopic systems increases the chance of successfully dating the sample. Because the reworking (e.g., shock-induced reheating) will affect each system differently, one system might still retain the original age information whereas others are disturbed. In some cases, the multi-chronometer approach even allows us to put not only constraints on the formation event, but also on the reworking event.

Two of the isotopic systems we use (Lu-Hf and Sm-Nd) are affected by neutron capture on the lunar surface. If a sample has been exposed on the surface for tens of millions of years or longer, the isotopic change in those systems will lead to incorrect age determinations. For this reason, we routinely correct our Lu-Hf and Sm-Nd data for neutron capture effects. 

All our dating efforts require a proper mineral separation before chemical analysis. For many lunar impactites, this separation has to be carried out on fractions smaller than 63 µm to obtain sufficiently monomineralic grains. At such fine grain sizes, conventional mineral separation techniques, such as magnetic separation, do not work because the grains adhere to each other and to the separation track, owing to electrostatic effects. To overcome this challenge, we successfully developed an enclosed track filled with ethanol that allows us to separate minerals from a ≤20-μm size fractions.

The multi-disciplinary approach of the TRR170 allows us to work closely with researchers from other fields. Together, we are working, for example, on combining sample ages with crater size-frequency distribution model ages, which is crucial for placing better constraints on the lunar cratering chronology. We are also actively working on improving our understanding of adjacent research areas, for example by collectively organizing interdisciplinary conference sessions.