Other
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28..364h&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 364
Other
1
Carbon, Cretaceous-Tertiary Boundary, Isotopic Variation, Oxygen, Tektites
Scientific paper
Glass spherules thought to be tektites from Haiti have previously been analyzed for their mineralogy and chemical composition to identify their origin and mode of formation [1]. They contain bubbles and occur in various colors dependent upon the original target rock. To investigate these spherules and the nature of any gas phase, several dark brown glasses have been analyzed for their carbon content and isotope composition, using stepped combustion analysis and static mass spectrometry. Both brown and yellow spherules were analyzed for oxygen isotope composition using laser fluorination and conventional dynamic gas-source mass spectrometry. Some spherules were analyzed whole for carbon but one was broken into fragments for the purpose of replication. Individual fragments were initially analyzed and found to yield a total of 0.2 wt% carbon in two components of different isotopic composition. The first, released between 350-400 degrees C had a delta^13C of -22 per mil whereas the second, between 500-600 degrees C had a delta^13C of -6.3 per mil. As the lower temperature release was presumed to be contaminated, other spherule fragments were pre- treated with 0.1M chromic acid to remove organic and carbonate components. Analyses of cleaned fragments indicated a variable carbon content from 0.005 to 2.6 wt% carbon but still with two isotopically different components. The first with a delta^13C of -0.8 per mil and the second, a delta^13C of -19.0 per mil. The spherules are both variable and heterogeneous. The -19 per mil component is apparently present in most of the spherules and released by 600 degrees C. A component with a similar combustion temperature and delta^13C has been encountered in K/T residues containing nanodiamonds [2]. There is currently no information available confirming its identity, but it does not appear to be surficial or an oxidizable organic. Identification of these carbon components by future work may reveal a possible source and mode of formation for the spherules and will also clarify the effect of the internal bubbles upon the compositions. Dark brown spherules selected for oxygen isotope measurements were broken into fragments to allow repeat analyses on the same spherule. Due to the smaller size of the yellow spherules they were analyzed whole. The dark brown spherules yield a delta^17O of 4.97 to 3.65 per mil and a delta^18O between 9.47 to 7.15 per mil. The yellow spherules yield a delta^17O of 6.77 per mil and a delta^18O of 13.02 per mil. Both closely follow the terrestrial fractionation line for Delta^17O with only a slight deviation and the delta^18O values agree with those previously measured by [3]. Heterogeneity's seen in the carbon data for the dark brown spherules seem to be reflected in the oxygen data with variations between fragments of the same spherule and between whole spherules. The yellow spherules appear to be homogeneous in terms of their oxygen isotopic compositions. References: [1] Koeberl C. and Sigurdsson H. (1992) GCA, 56, 2113-2119. [2] Gilmour I. et al. (1992) Science, 258, 1624-1625. [3] Sigurdsson H. et al. (1991) Nature, 349, 482-486.
Franchi Ian A.
Gilmour Iain
Hough Robert M.
Pillinger Colin T.
Sigurdsson Haraldur
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