Yellow impact glass from the K/T boundary at Beloc (Haiti): XANES determination of the Fe oxidation state and implications for formation conditions

Details Yellow impact glass from the K/T boundary at Beloc (Haiti): XANES determination of the Fe oxidation state and implications for formation conditions Yellow impact glass from the K/T boundary at Beloc (Haiti): XANES determination of the Fe oxidation state and implications for formation conditions

Sep 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008m%26ps...43..981g&link_type=abstract

Meteoritics & Planetary Science, vol. 43, Issue 5, p.981-986

Computer Science

Cretaceous-Tertiary, Impact Glasses, Oxygen Fugacity, Xanes

Scientific paper

We determined the iron oxidation state and coordination number in five samples of yellow impact glass from the Cretaceous-Tertiary (K/T) boundary section at Beloc, Haiti, which formed as the result of impact melting during the Chicxulub impact event. The samples were analyzed by Fe K-edge XANES spectroscopy and the results were compared with published data on eight black impact glasses and one high Si-K impact spherule from the same impact layer. The pre-edge peak of our high-resolution XANES spectra displays evident variations indicative of significant changes in the Fe oxidation state, spanning a wide range from about 75 to 100 mole% Fe3+. Yellow K/T glasses have significantly higher Fe3+/(Fe2+ + Fe3+) ratios compared to black K/T impact glasses (from 20 to 75 mole% Fe3+) and high Si-K glass (20 mole% Fe3+). In particular, all the pre-edge peak data on these three types of impact glasses plot between two mixing lines joining a point calculated as the mean of a group of tektites studied so far (consisting of [4]Fe2+ and [5]Fe2+) to [4]Fe3+ and [5]Fe3+, respectively. Thus, the XANES spectra of the yellow K/T glasses can be interpreted as a mixture of [4]Fe2+, [5]Fe2+, [4]Fe3+, and [5]Fe3+. Our observations can be explained by a very large range of oxygen fugacity conditions during melt formation. Furthermore, there is a clear positive relationship between the Fe3+/(Fe2+ + Fe3+) ratio and the Ca content of these glasses, suggesting that the Fe oxidation state was influenced by the relative contribution of Ca-sulfate-and Ca-carbonate-bearing sedimentary rocks at the impact site.

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