Absence of extraterrestrial He-3 in Permian-Triassic age sedimentary rocks

摘要

Helium concentration and isotopic composition were measured in a suite of samples across the Permian-Triassic boundary at Opal Creek, Canada, to determine whether high extraterrestrial helium concentrations are associated with a possible extinction-inducing impact event at this time. No extraterrestrial He-3 was detected, implying that neither fullerene-hosted nor IDP-hosted He is present at or near the boundary. This observation is consistent with similar studies of some Permian-Triassic sections, but contrasts sharply with reports of both fullerene- and IDP-hosted extraterrestrial He-3 at other sections. Step-heat experiments indicate rapid diffusion of extraterrestrial helium from sediments heated to temperatures above similar to 70 degrees C. Given the likelihood of burial and associated heating in Permian-Triassic age rocks, the initially unexpected absence of IDP-hosted He-3 likely indicates thermally induced diffusive loss. Indeed one of the key sections (Graphite Peak, Antarctica) from which extraterrestrial He-3 has been reported at and near the Permian-Triassic, boundary has been sufficiently heated that the reported preservation of extraterrestrial helium, in both IDPs and fullerenes, is inexplicable. Recent contamination provides a plausible explanation for extraterrestrial He-3 in these samples. While no extraterrestrial He-3 was detected at Opal Creek, there is a sharp increase in nucleogenic He-3 very close to or at the Permian-Triassic boundary. This presumably arises from the major lithologic change at this time, from cherts in the Permian to shales and siltstones in the Triassic, Increased nucleogenic He-3 is associated with increases in both lithium and organic carbon content into the Triassic. Either the production rate or the retention of this He-3 is higher in the shales and siltstones than in the cherts. Care must be taken to eliminate such artifacts before interpreting changes in He-3 concentration in terms of fluctuations in the delivery of He-3 from space.