[IGPP Everyone] Kyte Geocheminar

[John Wasson]

Geocheminar

Slichter 3843

12 noon, 17 Feb 2015

Frank Kyte

IGPP & EPSS

Popigai Crater Probably Formed in the Late Eocene

 

The 90 km diameter Popigai crater in Siberia is one of the five largest
impact craters known on Earth. For at least 20 years it has been generally
assumed that this large impact event was the source of the global impact
deposit containing clinopyroxene-bearing (cpx) spherules. This ejecta
deposit clearly formed in the late Eocene in magnetochron 16N. 40Ar-39Ar
ages of ~35 Ma for Popigai are consistent with this date, but Wielicki et
al. presented a 33.9 Ma age at the 2014 Goldschmidt, based on (U-Th)/He
analyses of zircons from the Popigai impact melt. They suggested that the
Popigai impact could be related to the Eocene/Oligocene boundary mass
extinctions. An obvious implication of this age is that the cpx-spherule
layer must be from some other large but unknown impact event.

The cpx-spherule ejecta layer is in many ways comparable to, if a bit
smaller than, the ejecta layer from the larger 150 km Chicxulub crater at
65 Ma. Additionally, isotopic and chemical data are all consistent with the
cpx deposit being derived from Popigai. Since there is clear evidence of an
exceptionally large continental impact in the late Eocene, and no such
event at a later date, it’s reasonable to suggest that Popigai formed in
the late Eocene. Further, the Popigai melt sheet was extensive, up to 600 m
thick, and has experienced significant hydrothermal alteration. It’s
entirely possible that these zircons took up to 1 m.y. to cool below the
~100˚C closure temperature for the He system; a model of hydrothermal
activity in Chicxulub infers a duration of 1.5 to 2.3 Ma depending on
permeability.

GeocheminrAnnounc.docx

 

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