A team of researchers at the University of Toronto and the Universit P. et M. Curie in Paris has developed a method to directly date individual grains of a group of clay minerals, called glaucony, that commonly form within sediments while they are being deposited in water.
Using a method developed at U of T-- laser probe argon-argon dating-- the study analysed individual glaucony grains from three bulk samples previously used to construct the geologic time scale. The individual grains yielded ages scattered over millions of years and almost all of the ages were younger than the true age. Only the oldest glaucony grains gave the correct ages, which were known before by comparison to dates from igneous minerals.
Until now, scientists have calculated the ages of sedimentary rocks either by inferring from the ages of surrounding igneous rocks or using potassium-argon dating to obtain average ages on large glaucony samples within the sediment. The glaucony technique has been considered unreliable since the ages arrived at are often a few million years younger than those found in the surrounding igneous rocks, and for this reason some geologists have ignored glauconies in constructing their time scales.
"The ability to look in detail at a sample grain by grain is what proved to be crucial in discovering why glaucony dates come out too young," says Norman Evensen of the U of T's department of physics. "We hope other scientists will now reintegrate glauconies to revise dates and ultimately produce a better geologic time scale."
The study says the different states of evolution among grains in a
sample may also indicate variable sea level
Contact: Megan Easton
University of Toronto