Undetection of Australasian microtektites in three boreholes from the Beijing Plain

The geographic boundary of the largest and youngest Cenozoic strewn field, the Australasian strewn field (AASF) of tektites and microtektites, is important for identifying its source crater. Potential microtektites were reported in the Beijing Plain, but their identity is unconfirmed. If Australasian microtektites were deposited there, the boundary of AASF would extend further north than currently recognized, possibly indicating a different location for the source crater. We analyzed samples from three boreholes drilled from the Beijing Plain to search for potential Australasian microtektites, yielding three microspherules that resembled microtektites. Detailed inspections of their morphology, internal structures, and major element compositions revealed that they are a fluorapatite and two iron-rich cosmic spherules, respectively, and no AASF microtektites were positively identified. We found no evidence for a northward extension of the AASF geographic boundary, confirming the source impact was oblique and towards the south, with minor ejecta deposited in the uprange direction.