In spite of early disappointments with potash feldspar for argon dating,
some of it is useful for rubidium-strontium procedures. It all depends
on how much original strontium the potash feldspar contains. Most
feldspars, unfortunately, contain far too much, but rapid screening by
X-ray fluorescence or flame photometry methods can weed these out and
identify specimens low enough in original strontium to be useful.
Otherwise, feldspar is an excellent closed system for rubidium and
strontium; it remains closed even at temperatures high enough to melt
many other minerals. It is not affected at all by the same degree of
heating that will drive argon out of biotite. The rubidium-strontium age
of feldspar usually comes close to the time of original crystallization
of the rock.
Obviously, here is a geologically important tool. If we find feldspar
and biotite in one rock, and if feldspar, tested by the
rubidium-strontium method gives the same age as biotite tested by
potassium-argon decay, then we can say with confidence that the rock has
not been reheated since shortly after it crystallized. Conversely, if
the biotite comes out much younger than the feldspar, we can be sure
that something _has_ happened to this rock long after it first
crystallized. Such information is not only valuable to pure science—it
can also be useful in locating areas favorable for ore prospecting and
in other practical ways.