Sr-Nd-Hf Isotope Geochemistry
Long-lived radiogenic isotopes – particularly strontium (Sr), neodymium (Nd), and hafnium (Hf) – have been widely used in the fields of solid earth and earth system sciences to investigate the evolution of igneous and metamorphic rocks, source tracking of sediments, continental weathering regimes, dust provenance, hydrothermal inputs to the oceans, and past and present ocean circulation patterns. These long-lived radiogenic isotopes are characterized by a slow decay of the parent isotope compared with the age of the Solar System.
Strontium-87 (87Sr) is produced by beta decay of 87Rb with a half-life of 4.88 billion years. In geologic and environmental investigations, the ratio 87Sr/86Sr is typically used. This ratio is – a function of the Rb concentration, geochemical origin, and age of the source rock. Because of the high mass of Sr, the ratio is essentially unchanged as the rock weathers and moves through the water cycle and food chain. Thus, the 87Sr/86Sr ratio is extensively used in geochemical fingerprinting, source tracking, contamination prediction, and migration/mobility studies.
In the samarium–neodymium (Sm–Nd) system, 147Sm decays to the radiogenic daughter 143Nd with a half-life of 106 billion years. Differentiation of Sm and Nd during melting and magma evolution together with ingrowth of 143Nd results in characteristic 143Nd/144Nd fingerprints in rocks, which vary as a function of age and lithology.
Radioactive decay of 176Lu to 176Hf with a half-life of 35.9 billion years has proven to be a very effective tool to trace the history of the Solar System and mantle-crust evolution. The Nd and Hf isotopic composition are typically expressed in the epsilon notation (εNd and εHf), which denotes the deviation of measured 143Nd/144Nd and 176Hf/177Hf ratios from the bulk chondritic uniform reservoir (CHUR) values of 0.512638 and 0.282785 respectively in parts per 10,000.
Hafnium isotopic ratios (176Hf/177Hf) provide strong constraints on various processes in the earth’s reservoirs. It has been demonstrated that the Hf isotope ratios correlate positively with Nd isotope ratios for magmatic rocks and can be used as a tracer to study the earth system processes.