USING DIFFUSION IN TEKTITES AND EXPERIMENTS TO INVESTIGATE IMPACT PLUME DYNAMICS

Impact events are a fundamental and ubiquitous process in the Solar System, playing a role in planetary accretion, differentiation, and surface modification. A large impact event creates an impact plume, a turbulent mixture of vaporized, molten and solid rock fragments that expands rapidly away from the site of impact. To learn more about this ephemeral process, this study probes conditions in an impact plume by examining material once entrained inside it: tektites.

Tektites are natural glasses formed as quenched impact melt ejecta that commonly contain microscopic inclusions of nearly pure silica glass, lechatelierite. We measured SiO₂ diffusion profiles across the lechatelierite/host glass contact in tektites from the Australasian strewn field by EPMA. Comparison of the SiO₂ diffusion profile lengths from these samples with the results of experiments on mixtures of tektite glass plus silica at known T-t conditions using an aerodynamic levitation laser furnace (Macris et al., 2015), allowed for relative estimates of the tektites’ thermal histories. The data fall into two statistically distinct groups: (1) Muong Nong tektites (most proximal to the crater) have SiO₂ profile lengths that are ~1/3 the lengths of (2) splashform tektites collected from the Philippines and Australia (farther from the impact site).

One interpretation is that Muong Nong tektites were only briefly exposed to the plume as it travelled away from the impact site, while the splashform samples remained in the hot plume for longer. After the plume dissipated, the molten material quenched to glass and followed different ballistic trajectories leading to their final geographic dispersal in the Philippines and Australia. This agrees with data from stable isotope studies by Moynier et al. (2009 & 2010), which found that Muong Nong tektites have lower δ⁶⁶Zn and δ⁶⁵Cu than tektites collected from more distal regions, indicating that Muong Nong types did not experience significant vapor fractionation induced by high temperatures in the impact plume. Using the profile length method of the current study on a larger sample set may lead to a map of the thermal history of the Australasian impact plume.

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Moynier et al., Geochim Cosmochim Ac 74, 799-807 (2010)