Isotopes are atoms of the same element whose nuclei contain the same number of protons, but a different number of neutrons. Stable isotopes do not decay over time. Radiogenic isotopes decay into new atoms over time by emitting radiogenic particles. The original type of atom is called the parent and the element that is produced after radiogenic decay is called the daughter. The rate at which one atom decays into another is unique to each isotope system and can be quantified by the system’s half-life. Thus, radiogenic isotopes can be used to find the age of a material, in this case, a magma source.
Another critical factor that comes into play in determining the age of a magma source is the incompatibility of elements involved in the isotopic system. An element is incompatible if an atom’s size or charge is different from that of the atom it is replacing in the mantle minerals' crystal lattice. Elements that are commonly found in the mantle are Mg, Ca and Fe. These elements have a charge of 2+, making elements with another charge incompatible. As melting occurs in the mantle, if an element does not fit into spaces in the crystal lattice, it will leave the mantle, preferentially partitioning into the melt. Examples of incompatible elements are the Rare Earth Elements (REE) such as Sm and Nd, the Large Ion Lithophile Elements (LILE) such as Rb and Sr, and the High Field Strength Elements (HFSE). Over time, the mantle becomes depleted in incompatible elements and the crust becomes enriched. However, radiogenic decay of elements can affect these proportions. Thus, a combination of the initial parent/daughter ratios and the decay time of a radiogenic isotopic system can determine the age of separation of a rock body from the convecting mantle.
(from Dickin, 2002 and White, 2005)
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