Ar-40-Ar-39 step heating ages of North American tektites and of impact melt rock samples from the Chesapeake Bay impact structure

摘要

This study presents Ar-40-Ar-39 step heating ages of four North American tektites (three bediasites and one georgiaite) and of two groundmass samples extracted at different depths from clast-rich impact melt rocks (CB-W61 and CB-W84) recovered by the USGS-ICDP Eyreville B drill-core about 9 km from the centre of the Chesapeake Bay impact structure. Radiometric age determination on both North American tektites and impact melt rocks from within Chesapeake Bay crater offers the first possibility to confirm the origin of these tektites. For this aim, argon isotopic data from 13 samples/aliquots of tektite rims, cores and bulk, and 4 samples/aliquots from two impact melt rocks were obtained over 15 to 26 step heating extractions. Age spectra of all tektite samples show plateaux comprising 62-98% of the Ar-39 release over consecutive intermediate and high temperature heating steps. Few low temperature extractions indicate excess Ar-40. Inverse isochron Ar-40/Ar-36 intercepts of tektite samples are indistinguishable from air (295.5). However, impact melt rock spectra presented complex Ar-release affecting primarily the low temperature heating-steps. Inverse isochrones indicate excess argon from which the Ar-40/Ar-36 intercept was used to correct the age calculation. CB-W61 and CB-W61-2 Ar-40/Ar-36 intercepts are 354.5 +/- 2.5 and 327.2 +/- 6.3, respectively, and those for CB-W84 and CB-W84-2 are 332.0 +/- 7.3 and 329.6 +/- 5.6, respectively. The inverse isochron weighted mean age (according to currently suggested K-decay constants revisions by Schwarz et al. (2011) and Renne et al. (2011)) for all four tektites is 34.86 +/- 0.25 Ma (MSWD = 0.96, P = 0.41; n = 4) and for the two impact melt rocks is 37.16 +/- 3.65 Ma (MSWD = 0.83, P = 0.36). The combined tektite and impact melt rocks isochron mean age of 34.86 +/- 0.23 (0.32) Ma (MSWD = 0.87, P = 0.43) is slightly - though not significantly - higher than the plateau mean age of 34.55 +/- 0.27 (0.36) Ma (MSWD = 0.66, P = 0.6