Clues on the Australasian impact crater site inferred from detailed mineralogical study of a monazite inclusion in a Muong Nong tektite

Tektites are terrestrial impact-generated glasses distributed over regions of Earth’s surface with ejection distances up to 10,000 km. The Australasian tektite strewn field is the largest and the youngest discovered so far (788 ka). However, the location of the source crater remains unsolved. The present work is the first to investigate the only monazite ever found as an inclusion in a Muong Nong tektite (MNT) from Indochina. In-depth observations down to the nanoscale revealed that the monazite experienced very high temperature, with silicate melt injection sometimes trapped within porosity at the grain boundaries, followed by a recovery mechanism responsible for dislocation migration and subgrain formation. The absence of radiation damage confirms that this recovery episode occurred recently, in line with the age of the tektite. The preservation of a primary zonation (Th component) and the absence of detectable diffusion profiles indicate that the monazite did not reach the melting point (∼2050 °C) before initial rapid cooling (∼1000 °C/s). The U-Th−total Pb dates of the monazite thus remained unchanged during the impact: 73 ± 6 Ma in a Th-rich domain and 156 ± 15 Ma in a Th-poor domain. This allows the source of the MNT to be constrained. Comparison with a detailed database of monazite ages and Th/U ratios in SE Asia indicates that the Australasian crater should be sought for in the triangle made up of the Philippines, coastal south China, and northern Vietnam, though the latter appears less probable.