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Potassium isotope abundances in Australasian tektites and microtektites
Author(s) -
HERZOG G. F.,
ALEXANDER C. M. O'D.,
BERGER E. L.,
DELANEY J. S.,
GLASS B. P.
Publication year - 2008
Publication title -
meteoritics and planetary science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.09
H-Index - 100
eISSN - 1945-5100
pISSN - 1086-9379
DOI - 10.1111/j.1945-5100.2008.tb00634.x
Subject(s) - potassium , isotopes of potassium , chemistry , electron microprobe , analytical chemistry (journal) , mineralogy , microprobe , seawater , isotope , geology , environmental chemistry , oceanography , physics , organic chemistry , quantum mechanics
— We report electron microprobe determinations of the elemental compositions of 11 Australasian layered tektites and 28 Australasian microtektites; and ion microprobe determinations of the 41 K/ 39 K ratios of all 11 tektites and 13 of the microtektites. The elemental compositions agree well with literature values, although the average potassium concentrations measured here for microtektites, 1.1‐1.6 wt%, are lower than published average values, 1.9‐2.9 wt%. The potassium isotope abundances of the Australasian layered tektites vary little. The average value of δ41 K, 0.02 ± 0.12% 0 (1 s̀ mean), is indistinguishable from the terrestrial value (= 0 by definition) as represented by our standard, thereby confirming four earlier tektite analyses of Humayun and Koeberl (2004). In agreement with those authors, we conclude that evaporation has significantly altered neither the isotopic nor the elemental composition of Australasian layered tektites for elements less volatile than potassium. Although the average 41 K/ 39 K ratio of the microtektites, 1.1 ± 1.7% 0 (1 s̀ mean), is also statistically indistinguishable from the value for the standard, the individual ratios vary over a very large range, from −10.6 ± 1.4% 0 to +13.8 ± 1.5% 0 and at least three of them are significantly different from zero. We interpret these larger variations in terms of the evaporation of isotopically light potassium; condensation of potassium in the vapor plume; partial or complete stirring and quenching of the melts; and the possible uptake of potassium from seawater. That the average 41 K/ 39 K ratio of the microtektites equals the terrestrial value suggests that the microtektite‐forming system was compositionally closed with respect to potassium and less volatile elements. The possibility remains open that 41 K/ 39 K ratios of microtektites vary systematically with location in the strewn field.