SINGLE-GRAIN AR-40-AR-39 AGES OF GLAUCONIES - IMPLICATIONS FOR THE GEOLOGIC TIME-SCALE AND GLOBAL SEA-LEVEL VARIATIONS

The mineral series glaucony supplies 40% of the absolute-age database for the geologic time scale of the last 250 million years. However, gl auconies have long Been suspected of giving young potassium-argon ages on bulk samples. Laser-probe argon-argon dating shows that glaucony p opulations comprise grains with a wide range of ages, suggesting a per iod of genesis several times longer (similar to 5 million years) than previously thought. An estimate of the age of their enclosing sediment s (and therefore of time scale boundaries) is given by the oldest nonr elict grains in the glaucony populations, whereas the formation times of the younger grains appear to be modulated by global sea level.