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 ar