Seconds after impact: Insights into the thermal history of impact ejecta from diffusion between lechatelierite and host glass in tektites and experiments

Abstract

Tektites contain inclusions of lechatelierite, nearly pure SiO_2 glass formed by quenching of quartz grains melted during hypervelocity impacts. We report the discovery in a tektite of chemically zoned boundary layers (ca 20 μm) between lechatelierite and host felsic glass. These boundary layers in tektites formed by chemical diffusion between molten silicainclusions (quenched to lechatelierite on cooling) and surrounding felsic melt. We reproduced the details of these boundary layers via experiments on mixtures of powdered natural tektite plus quartz grains heated to 1800–2400 °C for 1–120 s using an aerodynamic levitation laser heating furnace. The results of these experiments were used to provide quantitative constraints on possible thermal histories of the natural sample. The experiments successfully reproduced all major aspects of the concentration profiles from the natural sample including diffusion length scale, strong asymmetry of the concentration profiles with respect to the Matano plane (due to the strong concentration dependence of the diffusivities of all oxides on SiO_2 content), similarities in lengths of the diffusive profiles (due to control by the diffusion of SiO_2 on the diffusivity of the other oxides), and differences in the shapes of the profiles among the oxides (including a maximum in the diffusion profile of K_2O due to uphill diffusion). The characteristic lengths of all non-alkali oxide profiles are proportional to t from which diffusivities and activation energies can be derived; these results are consistent with measurements in melts with lower SiO_2 contents and at lower temperatures reported in the literature. We also fit the experimental profiles of SiO_2 and Al_2O_3 using simple formulations of the dependence of their diffusivities on SiO_2 content and temperature, yielding results similar to those obtained from the t dependence of the characteristic profile lengths. The quantitative characterization of diffusion in boundary layers based on our experiments allow us to set limits on the thermal history of the natural tektite in which the boundary layers were discovered. If the interdiffusion between the silica and felsic melts occurred at constant temperature, the duration of heating experienced by the natural tektite we studied depends on temperature; possible solutions include heating at ∼2000 °C for ∼70 s, −2400 °C for ∼3 s. We also explored non-isothermal, asymptotic cooling histories; for a maximum temperature of 2400 °C, a characteristic cooling time scale of ∼50 s is implied, whereas, for 2000 °C, the time scale is ∼1400 s. Further, a maximum temperature of ∼2360 °C yields an effective diffusive time scale of ∼5 s, a cooling time scale of ∼90 s, and a cooling rate at the glass transition temperature of ∼5 °C/s; results that are consistent with independent estimates of cooling time scales for ∼1 cm clasts (Xu and Zhang, 2002), as well as cooling rates at the glass transition temperature (Wilding et al., 1996) – thus satisfying all currently available relevant data. More complex T-t paths are possible and can also be modeled using our experimental results and compared with and used as tests of the accuracy of physical models of tektite-forming impact events.

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