Origin of the Ca-phosphate inclusions in Ivory Coast and Australasian Muong-Nong-type tektites

Abstract

Tektites are reduced (Fe2+) glasses formed by the quenching of molten material ejected from Earth’s surface as a result of a hypervelocity impact. The vast majority of tektites are usually homogeneous glasses, but rare samples containing mineral inclusions can provide insights about the source material, sample thermal history, and tektite formation process. Tektites from two distinct strewn fields presenting Ca-phosphate inclusions detected from anomalous magnetic susceptibility were studied: one sample from the Ivory Coast tektite (ICT) field ejected at 1.07 Ma from the Bosumtwi crater (10.5 km in size) in Ghana and two Muong Nong type samples from the Australasian tektite field (MN-AAT) ejected at 0.79 Ma from a crater possibly situated in southeast Asia. In ICT, Ca-phosphate inclusions are systematically embedded in lechatelierite (SiO2 glass). In MN-AAT Ca-phosphate are either embedded in lechatelierite or in Fe-rich glass forming schlieren. Multiscale petrographic characterization using correlative microscopy associating scanning electron microscopy, microprobe and, transmission electron microscopy reveals that rounded inclusions in ivoirite are composed of acicular Ca-phosphates (merrillite) embedded in an amorphous P-rich glass. In MN-AAT, inclusions consist mostly of single droplets of Fe-Mg rich Ca-phosphate (structurally related to apatite), but few droplets often forming an emulsion texture show a complex assemblage of apatite, magnetite, pyroxene, and spinel growing from a Pt-rich nucleus. Diffusion profile around lechatelierite domains reveals maximum temperatures greater than 2200–2400 °C in the impact plume of the Australasian tektite and the Ivory coast tektite. Heating time is of the order of seconds-tens of seconds rather than minutes as previously suggested (20 s for MN-AAT and 5 s for ICT). The number, the density, and the fact that inclusions are entirely crystallized in MN-AAT support relatively slow cooling rates (<200 °C/h), in comparison with the faster cooling rates (>2000 °C/h) indicated by the precipitation of amorphous P-rich glass in ICT. In both impact events, ejecta that had been heated to high temperatures did not remain in the vapor plume for an extended period of time and landed rapidly (within tens of seconds) at a relatively high temperature (>1000 °C) on the Earth’s surface.