Geochronology and radioisotopes; isotope geology, especially to use naturally occurring radioactivity and its daughter products as a geologic clock (for example, to date the crystallization of a magma), and as a geochemical tracer (for example, to characterize the source rock whose melting produced the magma); applying the Rb-Sr isotopic age method to clay minerals, one possibility being to date when weathering had produced an ancient soil zone