Nd and Sr isotopic evidence for the origin of tektite material from DSDP site 612 off the New Jersey coast

Abstract

Abstract-Late Eocene tektite material from DSDP site 612 is composed of angular to spherical tektites and microtektites containing abundant vesicles and a few unmelted to partially melted mineral inclusions. The major element compositions of the 6l2-tektites are generally comparable to those of North American tektites, but the physical features suggest that the DSDP-6l2 tektites were formed by less severe shock melting. The 8'Sr/ 8·Sr and 143Nd/ 144Nd compositions of 6l2-tektites: a) show much wider ranges than the tightly constrained group of North American tektites and microtektites, and b) are significantly different from those of other groups of tektites. The existence of large isotopic variations in tektites from DSDP site 612 requires that they were formed from a chemically and isotopically heterogeneous material in a regime that is distinctive from that of other groups of tektites. T~~uR and T~R model ages of the 6l2-tektites indicate that they were formed from a crustal source of late Precambrian mean age (800-1000 Ma) which in middle Palaeozoic time (""400 Ma) was further enriched in Rb/Sr during sedimentary processes. These source characteristics suggest that the impact which produced the 6l2-tektites occurred in rocks of the Appalachian orogeny or sediments derived from this orogenic belt. Potential source materials for both 6l2-tektites and North American tektites are present on the eastern and southeastern part of the North American continent and its adjacent shelf. The distinct isotopic differences between 6l2-tektites and North American tektites indicate that the two groups of tektites were either formed by the impact of more than one bolide in the same general area, or by a single impact event that sampled different layers.

Sr and Nd analyses of upper Eocene spherules and their implications for target rocks

“…3a Shaw and Wasserburg 1982;Stecher et al 1989;Whitehead et al 2000) for microtektites MT-9a and MT-9d are probably not meaningful since there is no significant Rb/Sr fractionation as indicated by their low f(Rb/Sr) values (∼2 and ∼4, respectively). That is why the two samples give much older T(UR)Sr model ages.…”

“…Several microtektites from hole 709C and hole 1090B were mounted on glass slides, ground down to expose an interior surface, polished, coated with carbon, then analyzed by scanning electron microscopy (SEM) with EDX for major element contents (see Glass and Koeberl 1999a for a detailed discussion of the major oxide analyses). Microtektites and cpx spherules that were selected for Sr and Nd isotopic studies were leached in dilute acids to remove potential localized, secondary ferromanganese, and iron sulfide coatings following the method described by Stecher et al (1989). The sample weights ranged from 0.4 to 4 mg.…”

“…b) The Sr and Nd model ages of upper Eocene microtektites and cpx spherules. The bigger circles are from this study; the smaller circles are from Shaw and Wasserburg (1982), Ngo et al (1985), Stecher et al (1989), Whitehead et al (2000), and Kettrup et al (2003) (TNdDM). The range of Nd ages (microtektites from the cpx spherule layer and cpx spherules) is wider than the range of Popigai impactites and target rocks.…”

“…a) The Nd and Sr isotopic compositions of upper Eocene microtektites and cpx spherules. Bigger circles are from this study; smaller circles and triangles are from Shaw and Wasserburg (1982), Ngo et al (1985), Stecher et al (1989), Glass et al (1998), Whitehead et al (2000), and Kettrup et al (2003). G, T, and Q stand for gneiss, tagamite, and quartzite, respectively.…”

Establishing the link between the Chesapeake Bay impact structure and the North American tektite strewn field: The Sr-Nd isotopic evidence

“…This second layer is commonly referred to as the clinopyroxene (cpx) spherule layer, as cpx-bearing microkrystites form the prominent characteristic of this ejecta deposit (e.g., Glass and Koeberl 1999). Although the microtektites of both layers are geochemically very similar (e.g., Glass et al 1998), isotopic data (Shaw and Wasserburg 1982;Ngo et al 1985;Stecher et al 1989) indicate that these older microtektites and microkrystites belong to an ejecta layer different to the one represented by the North American tektites (Fig. 2).…”