Undetection of Australasian microtektites in the Chinese Loess Plateau

Australasian strewn field of tektites and microtektites, formed by a large impact event at ~0.788 Ma, is the youngest and largest known strewn field on Earth. The location of its source crater is still unconfirmed after nearly 60 years of exploration. The geographic boundary and the spatial density of tektites and microtektites within this strewn field are the most important information for the search of the potential parent crater. It has been noticed that the northern boundary of this strewn field may be extended further than currently sketched, as possible microtektites have been reported, but not confirmed in the Chinese Loess Plateau (i.e., CLP). The potential microtektites reported in CLP were used as a stratigraphic marker for Quaternary Science, and their existence may indicate a larger impact event that formed this strewn field, redirecting the search area for the source crater. In the past four years, we systematically searched for Australasian microtektites in the Chinese Loess Plateau, and this study is a final report for this project. Referring to the magnetic susceptibility and local deposition rates of loess materials, we sampled nineteen loess sections that cover deposition ages of ~0.788 Ma. The sections are distributed at eight different locations across CLP, and potential Australasian microtektites were reported in some of these sections. At each section, we performed continuous sampling with depths, and each sample was ~1500 g in weight. The total section area of the sampled sections is much larger than that of deep ocean drill cores where Australasian microtektites were searched. We processed over four hundred kilograms of samples and discovered twenty glass microspherules that have spherical and rotational shapes. These microspherules are scattered at different depths of the sections, and their water content is as low as that of typical microtektites. However, both the content of major elements and detailed surface morphology of the microspherules are different from those of Australasian microtektites, but consistent with those of manmade soda-lime glasses and coal fly ashes. This project concludes no discovery of Australasian microtektites, suggesting that microtektites are not widespread stratigraphic markers in CLP. The result further concurs that microtektites in this strewn field were preferentially deposited at the downrange of the impactor trajectory. While microtektites may have temporarily escaped the atmosphere during this impact event, subsequent atmospheric reentry and lateral transportation in the upper atmosphere did not deposit microtektites towards the uprange.