Graham R. Thompson

An explanation for low radiometric ages from glauconite

Abstract K-Ar and Rb-Sr ages from glauconites are about ten to twenty per cent lower than the age of sedimentation. Previous studies have indicated that these low ages are not attributable to normal diffusion loss of Ar from glauconite crystallites. The possibility of argon loss from ‘open’ potassium sites, such as on crystal surfaces and from expanded layers, was investigated by acid dissolution techniques. These studies show that potassium is removed from glauconites with low expandabilities at three different rates. The highest dissolution rate corresponds to cation exchange and comprises five to ten per cent of the total potassium. About five per cent of the total potassium is removed at a much slower rate than that of cation exchange, but at an order of magnitude faster than the bulk of the potassium. Activation energies calculated from rate constants determined at 50° and 80°C, for one sample gave values of 19 kcal/mole for the lowest dissolution rate and 14 kcal/mole for the intermediate rate. It appears that low radiometric ages from glauconites can be largely explained by the presence of potassium in sites where argon is readily lost, although such factors as late epigenetic gain of potassium by glauconite may also contribute to their low radiometric ages. A method is described for making quantitative corrections for such daughter product loss in radiometric age determinations.

Land-based evidence for Tertiary climatic variations: Northern Rockies

Paleoclimatic interpretations of sedimentation patterns, paleosol mineralogy, and vertebrate faunal assemblages of extensive Tertiary continental clastic sediments of Montana and Idaho indicate four major changes in precipitation in this area during Tertiary time: wet to dry in late Eocene, dry to wet in late early Miocene, wet to dry in early middle Miocene, and dry to wet in late Miocene–Pliocene time. This climate pattern is synchronous with similar climatic changes inferred from deep-ocean sedimentation rates for the Indian, Pacific, and Atlantic Oceans, and to oxygen isotope paleo-temperature data for the North Pacific Ocean and New Zealand. Other paleoclimatic evidence supports the pattern. The evidence suggests widespread synchronous Tertiary climate changes that may provide useful time-stratigraphic markers.