Geocheminar winter-2015

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.