CONNECTING IMPACT AGES WITH STRUCTURES: OBSERVATIONS FROM EARTH, IMPLICATIONS FOR OTHER PLANETS (Invited Presentation)

Much impact research is currently centered on two main areas: i) identifying and describing unique microstructures which can be used for confirming the impact origin of given samples, and ii) determining the age of impact events and/or their deposits. These subfields have converged recently with advances in the studies of zircon microstructures, which can in some cases yield a U-Pb age of the impact event.

However, connecting microstructural and geochemical observations to impact structures poses the question: how do you know which impact event you are dating. On the Moon and Mars, there is often a likelihood of multiple impacts affecting the same area, and samples that come from any regolith could potentially contain material from multiple impacts. Therefore, directly tying impact structures to ages of shocked phases is challenging. On Earth, superimposed impacts are less frequently encountered but could still be a reality.

In 2018, Kjær et al. identified an approximately 31-km diameter depression underneath the Hiawatha Glacier of northwestern Greenland. They suggested an impact origin, but direct sampling is not possible because the entire structure is located under up to 1 km of ice. Planar deformation features (PDFs) in quartz and feldspars, and shocked microstructures in zircon have been identified in clasts and boulders from fluvioglacial sediment immediately adjacent to the Hiawatha Glacier. These samples contain shocked zircons which yield an age of 57.99 ± 0.54 Ma, which has been interpreted as the age of the Hiawatha impact event.

Interestingly, one texturally distinct pebble collected from an immediately adjacent, drainage system contained shocked zircons in a melt-matrix that yielded a U-Pb zircon age of 1039 ± 16 Ma. New ⁴⁰Ar/³⁹Ar analysis of crystallized feldspar from the melt matrix also yielded an age of 1062 ± 10 Ma, consistent with previous zircon studies. Hyde et al. (2024) has interpreted this to indicate a second older impact event in terrane, likely within the drainage network of these fluvioglacial sediments in Northern Greenland. This work highlights the challenges involved in connecting geochemical age determinations to impacts based on proximity to observed topographic features and serves as a cautionary tale for other cases, such as meteorites or breccias, where the context may be even more challenging due to higher rates of impact in the same region.