GSA Annual Meeting in Indianapolis, Indiana, USA - 2018

Paper No. 139-13
Presentation Time: 5:00 PM


MACRIS, Catherine A.1, NI, Peng2, DARLING, Emilee A.1, SHAHAR, Anat2 and YOUNG, Edward D.3, (1)Department of Earth Sciences, Indiana University - Purdue University Indianapolis, 723 W. Michigan St., SL118, Indianapolis, IN 46202, (2)Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Rd. NW, Washington, DC 20015, (3)Earth, Planetary, and Space Sciences, UCLA, 595 Charles Young Drive East, Los Angeles, CA 90095

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)