With the development of the rubidium-strontium (parent-daughter) method
by L. T. Aldrich and his co-workers at the Carnegie Institution of
Washington, came the realization that mica was also very useful for this
analysis, for it usually contains ample rubidium and not much original
strontium that would mask the presence of the radiogenic strontium. As a
result, mica, especially black mica (the mineral biotite), has enjoyed
great popularity as a good and easy-to-find closed system.
[Illustration: _A scientist making adjustments on an “argon train”, a
maze of glass tubing in which argon is released from minerals and
purified for analysis._]
Everything has its limits, and mica is no exception: Even mica tends to
leak argon at elevated, but still relatively low (geologically
speaking), temperatures. These effects also depend on pressure and other
factors, not all of which are well known; these elevated temperatures,
pressures and other conditions of course act to some extent on all rocks
buried in the earth’s crust. It is known that at only about 300°C at
moderate pressures argon is leaked from mica faster than it is being
generated in it by the decay of radioactive potassium. The temperature
needed to cause the rapid loss of strontium from mica is not much
higher. Mica, especially biotite, will recrystallize and lose all its
radiogenic constituents (argon and strontium) at temperatures where many
other minerals show little or no change.
That means that we cannot always rely on mica to give the date of the
_original_ crystallization of a rock—the time when it cooled from a
molten state. Instead, mica will tell us when the rock _last_ cooled
from, say, several hundred degrees centigrade, regardless of what may
have happened to the rock before that. The mica may have been reheated
as a result of being buried under a few miles of sediment, for example.
The mica will show when the rock last cooled—in other words, when it
came up again.