Mathematics – Logic
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27s.205b&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 205
Mathematics
Logic
1
Scientific paper
U-Pb isotopic results for individual zircon grains from the fire- ball ejecta layer of the Cretaceous/Tertiary (K/T) boundary claystone, Raton Basin, Colorado, lie on a Pb loss line between 550 +- 10 Ma and 65.5 +- 3 Ma. This implies that the target area is dominated by rocks of the older age, and that grains plotting near the younger end of the Pb-loss line record a high degree of shock-related isotopic resetting. Intersection of the Pb loss line with the younger end of the concordia curve at 65.5 +- 3 Ma defines the age of the impact event. Zircon grains that record increasing degrees of shock metamorphism in K/T ejecta can be classified into three textural types: 1) grains that show only shock lamellae or planar deformation features (PDF), 2) grains that show both PDF and granular (polycrystalline) texture, and 3) grains that show only well-developed granular texture. The degree of isotopic resetting correlates closely to the three textural types of zircons that represent increasing intensities of shock. PDF in zircons were originally discovered in K/T ejecta [1], and granular textures were also first observed in K/T zircons. Both granular textures and PDF have been reported recently in zircons from Onaping Fm. fall-back ejecta at the Sudbury impact structure [2]. Granular texture in zircons indicates exposure to a very high level of shock, resulting in diaplectic transformation and subsequent recrystallization--all below the fusion temperature. In our concordia diagram, an unshocked zircon grain showed the least discordant point (3%), while a second grain displaying PDF was displaced down the Pb loss line by 12%. A third grain with both PDF and granular texture plotted with a displacement intermediate to those with PDF only and the completely granular grains. Five completely polycrystalline zircon grains represent the greatest degree of isotopic resetting and show the largest displacements along the Pb loss line (49%, 58%, 62%, 82%, and 90%), approaching the ca. 65 Ma age of the resetting event. Twelve of the fourteen analyzed zircons plotted on or close to a discordant linear array that indicates an original age of ~550 Ma. However, two other grains gave primary ages of ~300 and ~350 Ma from poorer quality data, suggesting a possible bimodal source of zircons. The quality of data often varies from grain to grain, but not because of a high laboratory Pb background (ours is one of the lowest ever recorded). Instead, the variation in quality is due to the low radiogenic Pb content of these small (3 to <1 microgram) and geologically young zircons grains, often barely exceeding our detection limits. The refractory nature of zircon allows it to record shock-induced textural features without any annealing by later thermal events. A volcanic origin for the K/T ejecta is ruled out by the presence of PDF and polycrystalline textures in these zircons, as well as by their older isotopic ages. The analytical data show that the source of the fireball layer ejecta is dominated by target rock with zircons having a U-Pb isotopic age of about 550 +- 10 Ma. This adds another constraint to possible impact sites. References: [1] Bohor, B.F. et al. (1990) Meteoritics 25, 350; [2] Bohor, B.F. and Betterton, W.J. (1992) Inter. Conf. on Large Meteor. Impacts (Sudbury), in press.
Bohor Bruce F.
Kamo Sandra L.
Krogh Thomas E.
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