Multielement geochemical investigations by SRXRF microprobe studies on tectite material: Evidence from the NE-Mexican Cretaceous/Tertiary record

The K/T boundary is long known as one of a few mass extinctions in earth history. The impact of a big meteorite at the Chicxulub on the northern Yucatan peninsula in Mexico is discussed to have triggered the faunal mass extinction and the rapid change of the palaeoenvironmental conditions near the K/T boundary. Tectite material, especially spherules are explained from many of the sections in correlation to the K/T-boundary event. This rare, glassy or alterated material is extremely variable in its major element chemistry, morphology and stratigraphic position in K/T transitions worldwide. For the first time, we perfom trace element analysis on tectites from the K/T boundary using synchrotron radiation XRF (SRXRF). Measurements were performed at the Hamburger Strahlungssynchrotronlabor HASYLAB at DESY (Hamburg, Germany) and at the ANKA (Karlsruhe, Germany) with polychromatic and monochromatic excitation, respectively collimating the beam to 15 æm by capillary optics. Based on results from SRXRF microprobe determinations, these structures are to be interpreted as mixing of several melts with different chemical composition. The different components may represent melts from different sediment layers and possibly of basement material excavated by the Chicxulub impact. Igneous rocks with andesitic composition in cores at Chicxulub are considered to be impact melt rocks and are correlated mainly by the composition of major elements with the glass spherules found in the surrounding. Our investigations show that it is possible to trace elements with high sensitivity and a high spatial resolution. Some of the samples show clearly zonation and alteration parts, as well as carbonate inclusions, triggered by the Chicxulub impact event. In general, the results from the SRXRF show that the tectite material have different trace element patterns, formed by mixing of melts with different chemical composition derived from different sediment layers and possibly of basement material excavated by the Chicxulub impact. There is no evidence at the moment that there is a homogeneous origin in the sample material or distribution in the investigated sections. The enrichment of Ce in spherules from the Mesa-Juan Perez section indicates a possible origin from the Yucatan carbonate platform generated by the Chicxulub impact event near the K/T-boundary. Area scans from tektite material of the Bochil section show a clearly zonation in the inner part, dominated by Ba and Sr as well as a alteration margin dominated by secondary CaCO3. Glassy material of the Beloc (Haiti) section is characterised by a homogeneous trace element distribution but shows characteristic differences between Ca-rich and Ca-poor glass. Moreover there is no similarity to material from other sections investigated. A clear differentiation between alteration rims, non-alterated material and mixing of different source materials can be shown by space resolved trace element determination in æm scale of schlieren structures and inclusions.(see also Schulte et al. this volume)