EVAPORATION INDUCED ISOTOPE FRACTIONATION IN TEKTITES: AN EXPERIMENTAL STUDY

Tektites are natural glasses formed as quenched impact melt ejecta. Because they experienced extreme heating while entrained in a hot impact vapor plume, tektites allow insight into these ephemeral events, which play a critical role in planetary accretion and evolution. The chemical and isotopic composition of tektite parent materials may be modified by vapor/liquid fractionation at high T in the plume due to preferential loss of light isotopes from the melt through evaporation. Trends from O isotope studies reveal a dichotomy: tektite δ¹⁸O values are ~4.0-4.5‰ lower than their protoliths[1], opposite in direction to a vaporization induced fractionation; yet increasing δ¹⁸O with decreasing SiO₂ in tektites[2] is consistent with vapor fractionation. Copper isotope studies show that tektite δ⁶⁵Cu values are 1.98-6.99‰ higher than protoliths[3], with increases in δ⁶⁵Cu showing a negative correlation with Cu concentration; trends consistent with evaporation induced fractionation.

To understand how volatilization fractionates O and Cu isotopes in tektites and what to expect in terms of differences between tektites and their protoliths, we conducted two series of evaporation experiments at high T in an aerodynamic levitation laser furnace. (1) O isotope fractionation experiments used powdered tektite fused into 2 mm spheres and heated to 2150-2200 ºC for 50-90 s while levitated in Ar. Mass losses were from 23 to 26%, reflecting evaporation of Si and O from the melt. The starting tektite had a δ¹⁸O value of 10.06‰ and the residues ranged from 13.14‰ to 14.30‰. (2) Cu isotope fractionation experiments used synthetic basaltic glass spheres doped with Cu₂O and heated to 2000-2150 ºC for 30 to 180 s while levitated in Ar. 99% of Cu is removed within 30 s, and >99.99% within 60 s. The starting material had a δ⁶⁵Cu of 3.69‰, and the residues ranged from 20.26‰ to 21.51‰ after 30 s. These results, though preliminary, provide valuable insights on how evaporation fractionates O and Cu isotopes at high temperatures on Earth and other planetary bodies. Future experiments will continue to investigate isotope fractionation at high T to help understand the physical conditions during tektite formation.

[1]Luft et al., Geochim Cosmochim Acta 51 (1987)

[2]Taylor & Epstein, J Geophys Res 74 (1969)

[3]Moynier et al., Geochim Cosmochim Acta 74 (2010)