Late Eocene impact ejecta: geochemical and isotopic connections with the Popigai impact structure

Late Eocene microtektites and crystal-bearing microkrystites extracted from DSDP and ODP cores from the Atlantic, Pacific, and Indian oceans have been analyzed to address their provenance. A new analysis of Nd and Sr isotopic compositions confirms previous work and the assignment of the uppermost mi crotektite layer to the North American tektites, which are associated with the 35.5 Ma, 85 km diameter Chesapeake impact structure of Virginia, USA. E xtensive major element and Nd and Sr isotopic analyses of the microkrystite s from the lowermost layer were obtained. The melanocratic microkrystites f rom Sites 216 and 462 in the Indian and Pacific oceans possess major elemen t chemistries, Sr and Nd isotopic signatures and Sm-Nd, T-CHUR, model ages similar to those of tagamite melt rocks in the Popigai impact structure. Th ey also possess Rb-Sr, TUR, model ages that are younger than the tagamite T -CHUR ages by up to similar to 1 Ga, which require a process, as yet undefi ned, of Rb/Sr enrichment. These melanocratic microkrystites are consistent with a provenance from the 35.7 Ma, 100 km diameter Popigai impact structur e of Siberia, Russia, while ruling out other contemporaneous structures as a source. Melanocratic microkrystites from other sites and leucocratic micr okrystites from all sites possess a wide range of isotopic compositions (ep silon(Nd-143) values of -16 to -27.7 and epsilon(Sr-87) values of 4.1-354.0 ), making the association with Popigai tagamites less clear. These microkry stites may have been derived by the melting of target rocks of mixed compo sition, which were ejected without homogenization. Dark glass and felsic in clusions extracted from Popigai tagamites possess epsilon(Nd-143) and epsil on(Sr-87) values of -26.7 to -27.8 and 374.7 and 432.4, respectively, and T -CHUR and TUR model ages of 1640-1870 Ma and 240-1830 Ma, respectively, whi ch require the preservation of initially present heterogeneity in the sourc e materials. The leucocratic microkrystites possess diverse isotopic compos itions that may reflect the melting of supra-basement sedimentary rocks fro m Popigai, or early basement melts that were ejected prior to homogenizatio n of the Popigai tagamites. The ejection of melt rocks with chemistries con sistent with a basement provenance, rather than the surface similar to 1 km of sedimentary cover rocks, atypically indicates a non-surficial source to some of the ejecta. Microkrystites from two adjacent biozones possess stat istically indistinguishable major element compositions, suggesting they hav e a single source. The occurrence of microkrystites derived from a single i mpact event, but in different biozones, can be explained by: (1) diachronou s biozone boundaries; (2) post-accumulation sedimentary reworking; or (3) e rroneous biozonation. (C) 2000 Elsevier Science B.V. All rights reserved.