Age determination of impact structures via the zircon U—Pb system remains challenging and often ambiguous due to highly variable effects of shock metamorphism on U-Pb geochronology. It is, therefore, crucial to link the observed zircon microtextures, including their temperature and pressure conditions associated with their formation, directly to the U—Pb ages preserved. Here, we analyzed three recrystallized zircon grains and one plastically deformed zircon crystal from the medium-sized Rochechouart impact structure in the northwestern Massif Central of France. For the Rochechouart impact structure the impact age Zircon microstructures were studied using scanning electron imaging, cathodoluminescence imaging, and electron backscatter diffraction EBSD mapping. The U—Pb depth profiling data demonstrate that plastically strained grains are incompletely reset and preserve rim and interior age domains reflecting typical pre-impact pre-Variscan and Variscan regional tectonic ages.
U-Pb dating by zircon dissolution method using chemical abrasion AND KOJI KAWASHITA
- Uranium/lead dating provides most accurate date yet for Earth's largest extinction
U and Th are found on the extremely heavy end of the Periodic Table of Elements. Furthermore, the half life of the parent isotope is much longer than any of the intermediary daughter isotopes, thus fulfilling the requirements for secular equilibrium Section 2. We can therefore assume that the Pb is directly formed by the U, the Pb from the U and the Pb from the Th. The ingrowth equations for the three radiogenic Pb isotopes are given by: 5. The corresponding age equations are: 5. This assumption cannot be made for other minerals, young ages, and high precision geochronology. The corresponding age equations then become: 5.
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