Magnetite in Muong Nong‐Type Australasian Tektites From South China

Abstract

South China belongs to the northern geographic branch of the Australasian strewn field (AASF) of tektites and microtektites, and this area is assumed to be part of the uprange region of the putative impactor trajectory that formed the yet undiscovered source crater. Ferromagnetic minerals in impact glass may record the magnetization process and thermal history of impact melt, but the possible identity of ferromagnetic minerals in AASF tektites from South China is elusive. Here, we perform a rock magnetism and crystallography study of iron‐sulfur spherules in Muong Nong‐type tektites from South China, revealing abundant single‐domain and pseudo single‐domain nano‐grains of magnetite within the spherules. This result is consistent with the detection of magnetic signals caused by single‐domain and pseudo single‐domain magnetite in these samples. Compared to the large value ranges of magnetic properties of tektites in the entire strewn field, individual specimens of both Muong Nong‐type and single splash‐form tektites have relatively homogeneous magnetic properties in terms of magnetic susceptibility, natural remanent magnetization (NRM), saturation isothermal remanent magnetization (SIRM) and NRM/SIRM ratio. The results indicate that the impact melt that formed each tektite specimen experienced similar thermal history, and most AASF tektites from South China were cooled in an ambient magnetic field that is the same order of magnitude as the Earth's magnetic field.

Iron‐Rich Grain‐Decorated Depressions on Surfaces of Lunar Impact Glasses

“…They were not formed by deposition of Fe‐S rich impact ejecta in degassing pits either, considering that iron‐rich particles with comparable sizes and compositions are not visible elsewhere on surfaces of impact glasses in the Chang'E‐5 regolith (Yan et al., 2022a) or reported earlier in Apollo samples. On the other hand, chemical compositions and crystallography of the interior substances suggest that these features are most consistent with being formed due to immiscibility between Fe‐S rich melt and silicate melt during cooling (Hamann et al., 2018; McKay et al., 1970; Pan et al., 2023). The circular rimless depressions are consistent with being margins of solidified troilite, as volume shrinkage during cooling and the different cooling rates between Fe‐S melt and the surrounding silicate melt yielded partially detached margins.…”