Geochemistry of impact glasses from the K/T boundary in Haiti: Relation to smectites and a new type of glass

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We have individually analyzed twelve black and seven yellow glasses from the K/T boundary section at Beloc, Haiti, plus three smectite mantles around black glasses. The main chemical differences between black and yellow glasses are higher S, Ca, Mg, Zn, As, Br, Sb, and Au, but lower K, Na, and Si abundances in the yellow glasses. We have found high-CaO yellow glasses with low S contents (0.08, 0.17 wt% ), which may be explained by fusion of carbonate-rich sediments. Microprobe studies of individual glasses show that the black glasses are very homogeneous in their composition at the 10-100 μm level, while yellow glasses show much more variability and different intra-sample elemental correlations. One of the black glasses has higher SiO2 and K2O abundances, but lower concentrations of all other major elements. This suggests the existence of a third glass type, which we have named the high Si-K variety (HSi,K glass). The glass shows areas consisting of pure SiO2 (maybe lechatelierite) and some schlieren with lower SiO2 content. The chemical composition indicates that the HSi,K glass may have originated from melting of a sedimentary source rock, such as shale or argillite. Severe changes in major and trace element composition occur during alteration of the glass, as documented by analyses of adhering smectites. The REE patterns in the glass are similar to average upper crust, but the smectites exhibit much lower abundances and flat patterns, not unlike the patterns observed in claystones at numerous K/T boundary sites. The suggestion that the Chicxulub structure in Yucatan may be the source crater for the K/T impact glasses is not fully supported by the currently available data. Impact glasses and tektites are produced from the surface layers of their targets. At Chicxulub, the target stratigraphy comprised mainly carbonates and evaporites. Although we cannot exclude Chicxulub as a source, the present data do not provide any firm chemical evidence for a connection. We conclude, however, that the Haiti glasses have formed by impact and have later undergone alteration by low-temperature hydrothermal processes to produce clays. The boundary claystones at K/T boundary sites may very well be alteration products of these impact glasses. © 1992.

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Geochimica et Cosmochimica Acta