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Published September 30, 2000 | public
Journal Article

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 microtektite layer to the North American tektites, which are associated with the 35.5 Ma, 85 km diameter Chesapeake impact structure of Virginia, USA. Extensive major element and Nd and Sr isotopic analyses of the microkrystites from the lowermost layer were obtained. The melanocratic microkrystites from Sites 216 and 462 in the Indian and Pacific oceans possess major element 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. They also possess Rb–Sr, T_(UR), model ages that are younger than the tagamite T_(CHUR) ages by up to ∼1 Ga, which require a process, as yet undefined, of Rb/Sr enrichment. These melanocratic microkrystites are consistent with a provenance from the 35.7 Ma, 100 km diameter Popigai impact structure of Siberia, Russia, while ruling out other contemporaneous structures as a source. Melanocratic microkrystites from other sites and leucocratic microkrystites from all sites possess a wide range of isotopic compositions (ϵ(^(143)Nd) values of −16 to −27.7 and ϵ(^(87)Sr) values of 4.1–354.0), making the association with Popigai tagamites less clear. These microkrystites may have been derived by the melting of target rocks of mixed composition, which were ejected without homogenization. Dark glass and felsic inclusions extracted from Popigai tagamites possess ϵ(^(143)Nd) and ϵ(^(87)Sr) values of −26.7 to −27.8 and 374.7 and 432.4, respectively, and T_(CHUR) and T_(UR) model ages of 1640–1870 Ma and 240–1830 Ma, respectively, which require the preservation of initially present heterogeneity in the source materials. The leucocratic microkrystites possess diverse isotopic compositions that may reflect the melting of supra-basement sedimentary rocks from Popigai, or early basement melts that were ejected prior to homogenization of the Popigai tagamites. The ejection of melt rocks with chemistries consistent with a basement provenance, rather than the surface ∼1 km of sedimentary cover rocks, atypically indicates a non-surficial source to some of the ejecta. Microkrystites from two adjacent biozones possess statistically indistinguishable major element compositions, suggesting they have a single source. The occurrence of microkrystites derived from a single impact event, but in different biozones, can be explained by: (1) diachronous biozone boundaries; (2) post-accumulation sedimentary reworking; or (3) erroneous biozonation.

Additional Information

© 2000 Elsevier Science B.V. Received 2 May 2000; received in revised form 12 July 2000; accepted 12 July 2000. We thank Amanda Amey for characterizing some of the microkrystite populations and for assistance in the microtektite separation laboratory. Henry H. Ngo performed the isotope analyses at Caltech, with the usual high precision. Eric Marchand of UNB's Applied Statistics Centre gave critical advice concerning statistical testing of the compositional data. The ejecta samples were supplied by the Ocean Drilling Program, and the Popigai tagamites were generously provided by Victor Masaitis of the Karpinsky Institute, St-Petersburg, Russia. The manuscript benefited from the perceptive comments of Alexander Deutsch and an anonymous reviewer. Microprobe analyses were carried out at UNB's Electron Microscopy Unit. This work is funded by NSERC grants awarded to J.G.S. and R.A.F.G., and by NASA grant NAG5-8251 at Caltech. Caltech Division Contribution #8726(1057). Contribution from the Geological Survey of Canada number 200038.[RV]

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