Finally there is still another way of obtaining a closed system by using
the whole rock, not just a crystal of a single mineral within it. A
large body of granite or similar rock may contain a number of minerals,
some or none of which may be closed systems. Yet as long as this body of
rock remains impermeable to solutions (which in nature means mostly to
water), no substance will be able to move very far in it because
diffusion in solids is so slow. Consequently it will remain a closed
system, as a whole, regardless of what happens to the individual mineral
grains.
If we take a piece from near the middle of this body of rock and if this
piece is much larger than the largest constituent grain in it, then we
have a fair sample of a closed system—the whole rock. The only
difficulty arises from the fact that few rocks are sufficiently
impermeable to solutions to retain argon, and many rocks contain so much
common strontium that rubidium-strontium analysis is impractical. Still,
we can use the rapid survey methods as for feldspar, selecting the few
rocks that would be useful. This work has been done frequently, and the
results have been fruitful for rubidium-strontium analysis. The
whole-rock rubidium-strontium age dates the time when the rock became
impermeable.