• Impact ejecta
• Australasian
• microtektite
• impact scenario

The nature of the target rock hit by the km-sized impactor that generated the Australasian tektite/microtektite strewn field ~0.8 Ma ago is still poorly constrained, although most authors suggest a sedimentary source rock (e.g. greywacke, loess). Its identification is of crucial importance to address the outstanding issue of the yet unknown crater location of the largest tektite/microtektite strewn field so far discovered on Earth (~15% of Earth's surface). Many authors suggest, however, that it should be located somewhere in Indochina and that its diameter should exceed 30 km.
In order to better constrain the nature of the target rock, I focused on the petrographic and geochemical characterization of hundreds of impact ejecta particles (namely, rock fragments and partly glassy particles up to some hundreds of micrometers in size) from the Australasian microtektite layer in two deep-sea sediment cores in the Indochina area (ODP 1144A, and SO95-17957-2).
Data gathered by means of µRaman Spectroscopy, FE-SEM, EMPA and LA-ICP-MS confirm the occurrence of highly shocked target rock material in the microtektite layer consisting of unmelted to totally melted particles. Unmelted and partly melted particles show evidence of shock metamorphism including coesite, high-pressure TiO2 polymorph with an α-PbO2 structure (i.e., TiO2 II) and shocked quartz with PDFs. The glass in the partly melted to totally melted particles is vesicular with major element bulk composition similar to that of the Australasian tektites/microtektites, thereby documenting their petrogenetic association. The mineral assemblage include mainly fine-grained quartz plus less abundant K-feldspar, plagioclase, mica phase and as accessory garnet, rutile, zircon, ilmenite, titanite, epidote, and apatite. Remarkably, some partly to totally melted particles bear evidence for dust accretion (mainly SiO2 grains) during ejection. Additional evidence for late accretion are high Fe-Ni microcrystals, on the external surface of some particles, which may carry a signature of projectile contamination.
This study documents the potential for the impact ejecta materials found in the Australasian tektite/microtektite layer to yield further insights not only on the nature of the target rock but also on the impact scenario.