Around eight hundred thousand years ago, an asteroid impacted the Earth, devastating Indochina and ejecting molten droplets from the Earth's surface (tektites, also known as australites), all the way to Australia and Antarctica. These tektites have been known since Darwin's time, but the crater has yet to be discovered.
A nano-scale study, conducted by CNRS Earth & Universe, on a 50-micron grain of monazite (a rare earth phosphate) recovered from a large layered Muong Nong-type tektite in Thailand (see photo), provides major clues to locate the crater.
The combination of advanced micro-characterization tools, such as microprobes and electron microscopes (scanning and transmission), reveals that the monazite is the remnant of a single detrital grain. This grain underwent extremely high temperatures, with the injection of molten silicate trapped in the porosity at the grain boundaries, followed by annealing that promoted the migration of dislocations and the formation of sub-boundaries.
The absence of radiation damage in the monazite confirms that this annealing event occurred recently, consistent with the age of the impact. The preservation of a primary zoning within the grain, and the absence of detectable diffusion profiles at these scales, indicate that the monazite did not reach its melting point (~3720 °F [~2050 °C]) before undergoing rapid cooling (~1800 °F/s [~1000 °C/s]).
The (U-Th-Pbtotal) dates obtained from the monazite remained unchanged during the impact: 73 ± 6 Ma in the Th-rich domain and 156 ± 15 Ma in the Th-poor domain. These ages correspond to the rocks present at the target site of the impact.
A farmer handling a layered tektite she found while plowing her field near Muong Nong, Laos.
© P. Rochette
Thus, a comparison of the results of the study with a detailed database containing ages of monazites and Th/U ratios from Southeast Asia indicates that the Australasian crater should be sought within the triangle formed by the Philippines, southern coastal China, and northern Vietnam.
This excludes a significant portion of previously proposed locations (southern Indochina, northern China) and provides new keys to finally solve the mystery of the lost crater!
References:
A.-M. Seydoux-Guillaume, P. Rochette, E. Gardés, P.-M. Zanetta, S. Sao-Joao, Ph. de Parseval, B.P. Glass;
Clues on the Australasian impact crater site inferred from detailed mineralogical study of a monazite inclusion in a Muong Nong tektite.
Geology 2024.