Strontium presents four natural isotopes 88Sr (82.56%), 87Sr (7.02%), 86Sr (9.86%) and 84Sr (0.56%). The relative abundance of 87Sr varies due to the radiogenic decay of 87Rb. This variation depends on the amount of Rb present in the sample, the time elapsed since its formation and the Sr isotope composition initially present in the sample.
Since the half-life of 87Rb is 47,000 Ma, the amount of radiogenic 87Sr formed is very small in samples with low-Rb contents and/or young rocks (<50 million years).
The 87Sr/86Sr ratio allows us to obtain information about the origin and processes that affected all kind of materials. This type of study is based on the fact that the amount of 87Sr has been increasing since the origin of the Earth due to the contributions from the decay of 87Rb and the geochemical behavior of Rb and Sr. Materials with an origin in Earth"s mantle present low Rb/Sr and hence low 87Sr/86Sr. On the contrary, materials from the upper crust have higher Rb/Sr and hence higher 87Sr/86Sr. Unexpected values can be explained in different ways, such as assimilation and/or crustal contamination.
Sr isotopic analysis can be performed for studies in geochronology, petrology, marine-carbonate dating, diagenesis, hydrogeology, and archaeology.
Neodymium in nature has 7 isotopes 142Nd (27.09%), 143Nd (12.14%), 144Nd (23.83%), 145Nd (8.29%), 146Nd (17.26%), 148Nd (5.74%) and 150Nd (7.47 %). The relative abundance of 143Nd varies due to radioactive decay from 147Sm, with a half-life of 1.06 Ga, and this is sometimes convenient to date relatively old rocks.
The 144Nd/143Nd ratio displays extremely narrow variations, so the epsilon-Nd value is widely used to identify variations in rocks. Materials formed in the Earth"s mantle have high values, with a maximum near +15 today, while crustal materials can display negative values down to -10.
The combination of epsilon-Nd and Sr data is a powerful tool for isotope geochemistry studies in geological materials, reflecting their origin and sometimes enabling quantification of the proportion of different materials that have contributed to their genesis.
Nd isotope ratios, when suitably variable, are used in dating metamorphic and magmatic rocks by internal isochrons, based on cogenetic minerals, and can provide useful information in petrological and metal ores studies.
Lead has four natural isotopes with atomic masses 204, 206, 207 and 208. Only 204Pb is naturally stable, while the other three are the result of the radioactive decay from 238U, 235U and 232Th, respectively. The half-lives of these three parent isotopes are very different: that of 238U is comparable to the age of the Earth, that of 235U is much shorter, which means that almost all 235U has now disintegrated to 207Pb, and that of 232Th is comparable to the age of the Universe.
"Common lead" is lead in a sample with a zero or negligible content of uranium, so that the proportion of radiogenic lead added to the original composition is very small. Thus in low-U materials (minerals or total rocks), variations in the isotope composition (207Pb/204Pb and 206Pb/204Pb) can by ascribed to the different rate of decay of parent isotopes (235U and 238U) in the source of the materials and these ratios may be used to estimate the age of formation without knowing the need to measure the concentration of U.
The Common Lead method has been used, mainly in the field of geosciences, for determining the model ages in galenas and cerussite, as well as an isotopic tracer in feldspars. In some cases, in very old rocks (> 1000 Ma) it has been used to estimate the age by the isochron method.
Over the last decade, there has been a considerable development of common lead analysis, as it can be extremely important in environmental studies as a useful tracer for contamination processes.