Major and trace element compositions of georgiaites: Clues to the source of North American tektites

Abstract

Electron microprobe and laser ablation, inductively-coupled plasma mass spectrometer analyses of 24 georgiaites show that these tektites are all Si-rich (79-83 wt% SiO2) glasses with variable major and trace element abundances (e.g., FeO varies from 2.1 to 3.7 wt%). Glass compositions are similar to but not identical with average upper continental crust. For example, georgiaites are light rare earth element enriched with small negative Eu anomalies (Eu/Eu* = 0.73-0.86) and La-Th-Sc systematics are intermediate between that of Archean and post-Archean continental crust. When the georgiaite data are placed in the context of data for all North American tektites, triangular arrays appear on some oxide-oxide plots (e.g., FeO-MgO). Large variations in refractory element abundances and ratios compared to the variation in SiO2 favors mixing over volatilization as a cause of the compositional variation. If all the tektites formed as a result of a single impact, then triangular arrays in oxide-oxide variation diagrams require at least three source components. These components include a Si-rich material, probably a quartz-rich sand that was predominant in the formation of georgiaites. Two relatively silica-poor and Fe-rich components have compositional characteristics similar to shales and greywackes. The La-Th-Sc systematics of the georgiaites and most other North American tektites are distinctive and could potentially be used to link the tektites to Eocene sediments at the Chesapeake Bay impact structure.