Glauconite is a potassium- and rubidium-bearing clay mineral that forms at the seawater-sediment interface in marine environments. As such, it should be useful in

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the determination of absolute ages of sedimentation of sedimentary zones.

K/Ar and Rb/Sr ages from glauconites are about 10-20% lower than the age of sedimentation where external controls on the ages are available. Previous studies have indicated that these low ages are not attributable to normal diffusion loss of Ar from glauconite crystallites.

The possibilities 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 3 different rates. The highest dissolution rate corresponds to cation exchange and comprises 5-10% of the total potassium. About 5% of the total potassium is removed at a much slower rate than that of cation exchange, but at an order of magnitude faster than most of the potassium.

The amount of potassium in "open" sites, interpreted to be subject to argon loss, was compared with the difference between radiometric age and stratigraphic age for some samples. It appears that low radiometric ages from glauconites can be explained largely by the presence of potassium in sites where argon is readily lost, although such factors as late epigenetic gain of potassium by glauconite may contribute to their low radiometric ages.

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