Coesite discovered in Australasian microtektites

Abstract

Microtektites are microscopic glass spherules produced by large impacts on Earth. Whether they formed as impact melt droplets or as condensates from a target-dominated vapor plume is debated. Combining optical, scanning, and transmission electron microscopy, we studied microscopic silica-rich inclusions in four Australasian microtektites to search for high-pressure phases produced by shock metamorphism in the precursor materials. Three microtektites are from deep-sea sediment cores close to the putative impact location in Southeast Asia, and one is from the Transantarctic Mountains at the extreme reaches of the strewn field. Inclusions in the oceanic microtektites consist of a few partially resorbed microscopic quartz and coesite grains set in a silica-rich glass matrix; the latter hosts a multitude of individual nanoscopic coesite relicts. The inclusion in the Antarctic microtektite consists of featureless silica-rich glass, is devoid of coesite, and shows diffusive boundaries. Coesite grains in the deep-sea microtektites are interpreted as impact-melted relicts of larger crystals originally formed during shock metamorphism in a quartz-rich target precursor. The presence of coesite in deep-sea microtektites strengthens the argument that Australasian microtektites found closest to the impact location originated as impact melt spherules upon compression-decompression melting during impact cratering and not as target vapor plume condensates. The high degree of digestion of the inclusion in the Antarctic microtektite is in line with the view that the most distal Australasian microtektites experienced the highest thermal regimes.