GEOCHEMICAL CHARACTERISATION OF CHICXULUB-IMPACTEJECTA - NEW CONSTRAINS FROM THE GULF OF MEXICO AND THE CARIBBEAN

Glass spherule ejecta deposits in various sections from NE-Mexico,Texas, Guatemala, Belize, and Haiti have been investigated by electron microprobe, backscatter-electron-, scanning electron-, and transmission electron microscopy, in order to characterize the geochemistry of the ejecta, strewn-field mixing, alteration, fractionation within the strewn field, and distribution mechanisms. Earlier investigations include main and trace elements (Harting, 2004), isotope analyses (Kettrup, 2002) and litho- and biostratigraphic Investigations (Keller et al., 2003, 2004). In NE-Mexico and Texas, multiple ejecta layers are exposed up to 10 m below the K/T-boundary, with the oldest and original ejecta deposit predating the K/T boundary by about 300,000 years. In Texas, preliminary analyses suggests that the original ejecta, now a higly altered smectite layer, is also interbedded in late Maastrichtian clay. In Guatemala, Belize and Haiti the spherule layers are generally found above the K/T boundary in the early Danian and apparently eroded and reworked from the older original deposit. Geochemical investigations at all these localities reveal Chicxulub impact glass as the origin for all glass spherule ejecta layers. Although some spherules are altered, most remain glass particles are unaltered. Both geochemical and petrological features are well preserved and permit correlation to Chicxulub basement rocks, e.g. granites, gneisses, amphibolites and impact meltrocks. Several distinct silicic phases (Al-Fe-rich glasses) are observed. The range of geochemical compositions for NE-Mexico ejecta is: SiO2: 45-61, TiO: 0.1-0.6, Al2O3: 11-22, FeO: 2-14, MnO: 0-0.07, MgO: 2-11, CaO: 1-9, Na2O: 0-3, K2O: 0.2-4. The mean geochemical composition for Texas impact glass falls well within this range with SiO2: 50.81, TiO2: 0.33, Al2O3: 18.34, FeO: 4.99, MnO: 0.02, MgO: 3.42, CaO: 3.03, Na2O: 0.11 and for K2O: 1.31. These characteristics strongly imply an ejecta origin from mafic and felsic rocks of the Chicxulub-basement. However, the occurrence of different glass phases in the upper (reworked) ejecta layers of NE Mexico strongly suggests post-sedimentary mixing and/or fractionation of the ejecta. In contrast, the stratigraphically oldest ejecta layer (300,000 yrs pre-K/T) is geochemically more uniform, which reflects the absence of erosion, mixing and transport.