T116 Impact Cratering: From the Lab to the Field; from the Earth to the Planets (Posters)

Monday, 1 November 2010
8:00 AM - 6:00 PM
Hall D

NI AND CO IN PYRITE MARK THE K/PG BOUNDARY IN CROSSWICKS CREEK SECTION, NEW JERSEY COASTAL PLAIN

Paper number: T116-4
Program: Technical Sessions
Time: 8:00 AM – 6:00 PM

Authors

Abstract

On the New Jersey coastal plain, the Cretaceous/Paleogene (K-Pg) boundary was deposited in gently dipping offshore environments now recrystallized to glauconite-rich, fossiliferous, poorly consolidated layers. Several sections have been analyzed by the AMNH group (e.g., Landman N.H. et al. 2007, AMNH Bull. 303). At the ammonite-free Crosswicks Creek section (~50m paleodepth), the white layer between the New Egypt (Maastrichtian) and Hornerstown (Danian) Fms. was investigated for geochemical signatures of meteorite impact plume condensates (Ebel & Grossman 2005, Geology). Oriented blocks totaling 7.5 cm of outcrop thickness including the ~8mm thick white layer were dewatered and impregnated with epoxy. Bright grains in thick sections were analyzed by reflected light, BSE, EMP and SEM. Siderophile-rich opaque grains were analyzed by laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) at Lamont-Doherty Observatory for Fe, Ni, Co, Ir, and Si. Pyrite (FeS2) framboids above, in, and below the formational boundary show significant textural and abundance differences in any particular slice. Ilmenite (Fe-Ti oxide) grains were also analyzed, but none were found in the white layer.

Pyrite framboids (10 – 100 micron diameter) are well developed throughout the section. Pyrites within the boundary average 965 ppm and 1623 ppm Ni (n=14). Above the boundary (white) layer pyrite framboids average 7 ppm Co and 46 ppm Ni (n=6). Below the boundary, they average 13 ppm Co and 97 ppm Ni (n=8). Ir concentrations were too low to analyze in any framboids.

The high abundances of Ni and Co in pyrite within the very narrow formational boundary, and their sharp compositional contrast with framboids above and below the boundary, suggest that these pyrites inherited their siderophile elements during highly localized diagenetic reactions in a reducing environment, in which siderophile elements were not mobile in the sediment column. Ubiquitous recrystallization does not necessarily require re-mobilization of trace elements, in a non-humic setting. The sharp spikes in siderophile elements indicate minimal remobilization at the scale of 1-2 cm. This has implications for the ammonite-bearing Agony Creek section (Bigolski et al, AGU 2010). Ir concentrations at Crosswicks Creek are being obtained by INAA analysis.