U‐Pb dating of zircon and monazite from the uplifted Variscan crystalline basement of the Ries impact crater

Abstract

Impact crater central peaks and peak ring complexes are important exploration targets for future missions to other planetary bodies, because they provide access to material uplifted from lower crustal levels. Material exposed there could also provide chronological constraints on crater formation events. Therefore, it is essential to understand if uplifted peak material preserves the chronological records of igneous and metamorphic protolith crustal rocks, or if such records are reset during impact events. To investigate this issue, we collected shocked gneiss and granite samples from uplifted crystalline basement megablocks in the 24 km diameter Ries impact crater in Germany, which is dated at ~14.8 Ma. Petrographic observations, electron beam imaging, and Raman spectroscopy suggest that these samples record the peak pressures of ~10-15 GPa. In situ U-Pb dating shows that monazite U-Pb systematics have not been affected by the Ries impact, as gneisses and granites yielded monazite U-Pb dates of ~370 and 330 Ma, consistent with known Variscan metamorphic and magmatic events. The U-Pb systematics of some zircon grains yielded U-Pb dates of approximately 5-10 Ma, which is younger than the age of the Ries impact event. These young dates correspond to U-rich metamict domains and may reflect recent Pb loss and/or U-gain during postimpact hydrothermal alteration or weathering. These observations indicate that dating uplifted crystalline material in impact craters on other bodies might provide useful petrological and chronological constraints on the underlying target rocks rather than directly dating impact events, for which sampling impact melt and impact melt-bearing lithologies should remain the primary target.

Thermal and Structural History of Impact Ejecta Deposits, Ries Impact Structure, Germany

“…30 m in an abandoned quarry atop a hill marking a topographic high on the rim of the central ring (Figure 1a). The exact age of the granodiorite is unknown, but gneisses and granites elsewhere in the Ries crater yield monazite U‐Pb dates from 370 to 320 Ma, consistent with Variscan metamorphic and magmatic events (Horn et al., 1985; Tartèse et al., 2022). Expected Variscan paleolatitudes from the European apparent polar wander path place Ries at low (5–10°S) latitudes in the southern hemisphere (Torsvik et al., 2008; Van der Voo, 1993).…”

Using Lunar Granulites to Constrain Re‐Equilibration Timescales of Contact Thermal Metamorphism on the Moon

“…Our modeling assumes a country rock (upper crust) temperature reflecting the local selenotherm at the time of impact. However, it is possible that in the seconds to minutes following an impact, the target rock could be locally heated as it experiences pulses of exceptionally high pressures (e.g., Tartèse et al., 2022). Such shock‐induced temperatures have been modeled to be >1,000°C (Ivanov, 2005; Ivanov & Deutsch, 1999), above the temperatures required for granulite metamorphism.…”