Volatile loss under a diffusion-limited regime in tektites: Evidence from tin stable isotopes

John B Creech,Frédéric Moynier,Christian Koeberl

doi:10.1016/j.chemgeo.2019.119279

Abstract

Tektites are glasses derived from near-surface continental crustal rocks that were molten and ejected from the Earth's surface during hypervelocity meteorite impacts. They are among the driest terrestrial samples, although the exact mechanism of water loss and the behaviour of other volatile species during these processes are debated. Based on the difference in magnitude of the Cu and Zn isotopic fractionations in tektites, and the difference of diffusivity between these elements, it was suggested that volatile loss was diffusion-limited. Tin is potentially well suited to testing this model, as it has a lower diffusivity in silicate melts than both Cu and Zn, but a similar volatility to Zn. Here, we analysed the Sn stable isotopic composition in a suite of seven tektites, representing three of the four known tektite strewn fields, and for which Zn and Cu isotopes were previously reported. Tin is enriched in the heavier isotopes (≥2.5‰ on the 122Sn/118Sn ratio) in tektites, correlated with the degree of Sn elemental depletion in their respective samples as well as with Cu and Zn isotope ratios, implying a common control. While the isotope fractionation of Sn, Cu and Zn is a result of volatility, the magnitude of isotope fractionation is strongly moderated by their relative rates of diffusion in the molten tektite droplets. An Australasian Muong Nong-type tektite analysed has the least Sn depletion and Sn isotope fractionation, consistent with these samples being more proximal to the source and experiencing a shorter time at high temperatures.

Highlights

Tektites are distal impact glasses, typically up to a few cm in size, formed by rapid melting of terrestrial upper crustal rocks during hypervelocity impacts on the Earth's surface (e.g., Shaw and Wasserburg, 1982; Koeberl, 1986, 1990, 1994; Koeberl et al, 1996; Melosh and Artemieva, 2004)
Tektites are strongly fractionated towards heavy Sn isotopic compositions relative to those of terrestrial upper crust samples (Table 1; Fig. 1)
The initial composition is an assumed terrestrial upper crust composition

Summary

Introduction

Tektites are distal impact glasses, typically up to a few cm in size, formed by rapid melting of terrestrial upper crustal rocks during hypervelocity impacts on the Earth's surface (e.g., Shaw and Wasserburg, 1982; Koeberl, 1986, 1990, 1994; Koeberl et al, 1996; Melosh and Artemieva, 2004). Tektites are generally chemically and isotopically similar to the upper crust, they are characterized by extremely low water contents (Beran and Koeberl, 2010; 0.002–0.02 wt.%; Koeberl, 1994), indicating a depletion of water (and potentially other volatiles) from their precursor rocks (typically sediments, with >1% water) while the tektites were molten. The mechanism for this depletion and, more broadly, the behaviour of volatile elements and molecules during tektite formation is not well understood. Even relatively refractory elements (e.g., Mg, Si, Ca, Ti) have been shown to have isotopic variations that are attributed to high-temperature volatility-related processes in the early Solar

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Discussion

Conclusion