Single-Grain 40Ar-39Ar Ages of Glauconies: Implications for the Geologic Time Scale and Global Sea Level Variations
Patrick E. Smith,
Norman M. Evensen,
Derek York,
Gilles S. Odin
The mineral series glaucony supplies 40% of the absolute-age
database for the geologic time scale of the last 250 million years.
However, glauconies have long been suspected of giving young
potassium-argon ages on bulk samples. Laser-probe argon-argon dating
shows that glaucony populations comprise grains with a wide range of
ages, suggesting a period of genesis several times longer (~5 million
years) than previously thought. An estimate of the age of their
enclosing sediments (and therefore of time scale boundaries) is given
by the oldest nonrelict grains in the glaucony populations, whereas the
formation times of the younger grains appear to be modulated by global
sea level.
P. E. Smith, N. M. Evensen, D. York, Department of
Physics, University of Toronto, 60 St. George Street, Toronto, Ontario
M5S 1A7, Canada.
G. S. Odin, Département de Géologie
Sédimentaire, Université P. et M. Curie, 4 Place Jussieu,
Case 119A, 75252 Paris Cedex 05, France.
