isotope geochemistry The study of the abundance ratios of isotopes (both stable and radioactive) of major and trace elements in rocks (e.g. Rb/Sr, Pb/U, etc.), to elucidate a number of geologic problems and processes. These include the age relationships of rocks, and the age of the Earth itself (see GEOCHRONOLOGY; ISOTOPIC DATING; and RADIOMETRIC DATING); palaeotemperatures and geothermometry (see OXYGEN-ISOTOPE ANALYSIS; and GEOTHERMOMETER); and the provenance of natural waters, ore-forming fluids, and magmas (see ISOTOPE FRACTIONATION; SMOW; D/H RATIO; OXYGEN-ISOTOPE RATIO; and STABLE-ISOTOPE STUDIES).

mantle array A term used in isotope geochemistry to describe a graphical plot of ¹⁴⁴Nd: ¹⁴³Nd against ⁸⁷Sr: ⁸⁶Sr for igneous rocks. Rocks which have been derived from the mantle tend to plot on a straight line; those that show evidence of crustal contamination tend to fall off the line, often in systematic ways.