Geochemistry of aerodynamically distorted Australasian microtektites: Implications for ejecta on Mars and Venus

Abstract

We report microtektites recovered from a large area of the deep seafloor (Central Indian Ocean) that appear to have undergone aerodynamic distortion during re-entry into the atmosphere. Considering the geographic locations, stratigraphic position and chemical compositions these glassy forms belong to the Australasian tektite strewn field. The microtektites are elongated to lengths of cms, sometimes flattened, bent, folded and fused at both ends suggesting that this side could have been the Earth-facing side during the re-entry. The presence of flow lines and distortional features are indicative of high atmospheric pressures experienced by the microtektites. The location where these microtektites were recovered constitute distal ejecta, and the shape distortion, that occurred during re-entry of the ejecta, seems to have affected only a few amongst the extensive collection of microtektites. Most of the specimens contain lechatelierite inclusions and higher volatile oxides, which are indicative of incomplete homogenization after melting and lower temperatures of formation vis-�-vis other specimens at the same location. The element distribution patterns in aerodynamically distorted microtektites suggest that ablation was similar to normal spherical tektites in which volatile elements are preserved. In contrast, aerodynamically ablated forms of Australasian ejecta show skin melting where thin layers of the anterior portions of samples flow back giving rise to the familiar button shapes. Our observation of delicate, elongated, flattened, and viscously deformed specimens is perhaps the first to imply that at the distal end of ejecta, each spot in the specimens has undergone different levels of trajectories, heating and ablation. These investigations could have implications to understand ejecta emplacement characteristics on planetary surfaces that contain appreciable atmospheres such as Mars and Venus