Question

The absolute age of an igneous rock can be determined by

Answer 1

Igneous Rocks: The 'age' of an igneous rock refers to the time when the magma or lava from which it formed cooled below a certain temperature. A useful material for dating that time is the mineral zircon, a minor but common constituent of igneous rocks. As magma or lava solidifies, the elements zirconium (Zr), silicon (Si) and oxygen (O) link together to form zircon crystals. If uranium (U) atoms are in the vicinity, they may be incorporated into the zircon in place of Zr atoms. This substitution is possible because the size and charge of the U is similar to that of Zr. That is, the U can 'fit' in the sites normally occupied by Zr.

Any lead (Pb) in the vicinity cannot be incorporated in the zircon because it can't 'fit' in any of the sites. Assuming the zircon has not been affected by weathering or metamorphism, any Pb subsequently found in the zircon must have come from decay of the U; it was not there to start with. Determination of the Pb-207/U-235 ratio in the zircons gives their age and thus the age of the igneous rock in which they occur. (It is true that not all minerals that crystallize from a magma or lava form simultaneously, but except for extremely young igneous rocks, the time required for solidification is very short compared compared to the age of the rock.)