Multicomponent Diffusion between Felsic and Silicic Melts: Insights from Tektites and Experiments

Abstract:

Tektites (natural glasses formed as quenched impact melt ejecta) commonly contain inclusions of nearly pure silica glass (“lechatelierite”) thought to be quenched from molten silica produced by melting of quartz grains. We investigated chemical diffusion between lechatelierite (~100% SiO₂) and surrounding felsic glass (~73% SiO₂) in a natural indochinite tektite and in experimental analogues. We discovered concentration profiles of major elements across lechatelierite-felsic glass contacts that reflect diffusion between the two melts at high T as the they followed a ballistic trajectory prior to quenching. The profiles provide information on multicomponent diffusion at the high-silica end of composition space and on the thermal histories of tektites.

To reproduce the profiles, we undertook a series of high T melting experiments using an aerodynamic levitation laser furnace. A starting mixture of powdered natural tektite plus quartz grains was exposed to temperatures of 1800-2400˚C for 1-120 s. Direct comparison of concentration profiles between the indochinite and experiments reveals a best match at 2200˚C and 50 s. This experiment successfully reproduced all major aspects of the concentration profiles observed in the natural sample including diffusion length scale, asymmetry, order of steepness of major element profiles, and uphill diffusion of K₂O.

Not enough information is available to model the full multicomponent diffusion problem, but SiO₂ and Al₂O₃ concentration profiles from lechatelierite to surrounding felsic glass in the experiments can be fit well as a diffusion couple between silica melt and adjacent felsic melt using an effective binary diffusion approach and assuming that the effective binary SiO₂ and Al₂O₃ diffusivities depend exponentially on SiO₂ concentration. Further quantification of the diffusion behavior should allow constraints on the thermal histories of tektites.