Oxidation state of iron in tektite glasses from the Cretaceous/Tertiary boundary

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Tektite glasses from Beloc, Haiti, formed by impact melting, show bimodal composition of dominantly silicic melt of crustal origin (black glass) and a minor CaO-rich melt of sedimentary origin (yellow glass). Low-velocity Mössbauer spectra of the black glass show the typical broad line doublet of amorphous silicates, and high velocity spectra at 78°K confirm the absence of crystalline iron oxides larger than about 10 nm. High wt%Fe(III)/Σ wt%Fe ratios are confirmed by Mössbauer spectroscopy and chemical analysis. Thus ferric iron represents 40% and 74% of total iron in the black and yellow glass, respectively. The corresponding calculated oxygen fugacities are the highest reported for tektite glasses, or log fO2 about-5 to-6 for the yellow glass and log fO2 about-7 for the black glass, as compared to log fO2 = -10.5 in typical australite glass. The composition of the trace of volatiles in the black glass, which is degassed at 1,000 to 1,250 °C, is 69 mol% H2O and 31 mol% CO2, indicating that the source material contained water and carbonate. Isotopic composition and anomalously high sul-fate and alkaline earths of the glasses indicate material input from marine sediments. The observed high oxidation state of iron in the K/T boundary glasses suggests mixing between sources of different intrinsic oxygen fugacities: a crustal source close to the quartz-magnetite-fayalite (FMQ) buffer and a volatile-bearing sedimentary source of high intrinsic oxygen fugacity. Incipient shock melting of volatile-bearing phases produced a congruent melt. The redox state of iron in the melted material adjusted to the overall oxygen fugacity of the sources during incipient melting. At still higher pressures, the volatiles escaped, leaving a dry melt. The two glass types represent melting of a stratified target, and they preserve the different overall intrinsic oxygen fugacities of their sources. The black glass represents crustal rocks containing few percent of volatile-bearing phases, whereas the yellow glass represents strata richer in marine sediments. During ejection the physically separated melts were only partially mixed. The glasses thus represent impact melts from a stratified sediment-bearing target in contrast to other tektites, which represent total melting of dry rocks with ferric/ferrous ratios controlled by the FMQ oxygen buffer at 870 to 1,120 °C.

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Special Paper of the Geological Society of America