Neodymium (Nd) and Hafnium (Hf) Geochemistry Overview
Long-lived radiogenic isotopes – particularly neodymium (Nd), and hafnium (Hf) – have been widely used as proxies to track the physical and chemical erosion of earth’s surface. This is possible because source rocks possess a distinct radiogenic composition, and weathered products are highly conservative, not prone to precipitation, chemical interaction or biological uptake. This gives them broad applicability in the fields of solid earth, and earth system sciences where they are often tracked through the use of seawater isotope curves. Some applications include investigating 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.
Neodymium is a light, rare earth element with seven naturally occurring isotopes (142Nd, 143Nd, 144Nd, 145 Nd, 146Nd, 148Nd and 150Nd), two of these are radiogenic 142Nd (decaying from 146Sm with halflife of 68 millions years) and 143Nd (decaying from 147Sm with halflife of 106 billion years). Because of their similar masses stable Nd isotope ratios are functionally invariable, as such the two most commonly used in geological and environmental research are the stable 144Nd and radiogenic 143Nd. 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. Our understanding of the early differentiation of the earth’s crust/mantle is enabled by our understanding of Nd isotope systematics, with 147Sm/143Nd and 146Sm/142Nd used as fingerprints of mantle source regions. In nature, it’s typically found in monazite and bastnaesite deposits and is used for a variety of industrial applications including batteries, magnets, glass and wind turbines. Nd enters the ocean via river and wind deposition and leaves by the adsorption onto organic and Fe-Mn oxide crust particle coatings. The balance between the input and output is such that its residence time is less than ocean mixing, which makes it heterogeneously distributed and useful as a tracer.
Hafnium is a transition metal with more than 35 observed isotopes. The most common of which are 174Hf, 176Hf, 177Hf, 178Hf, 179Hf and 180Hf; two of these (176Hf, 177Hf) are commonly used in geological research applications. In nature, hafnium is found in combination with zirconium compounds including zircon. It is typically used in nuclear reactors and as a strong metal alloy for various applications. In a marine setting, Hafnium has similar addition, removal, and residence time to Neodymium; the decoupling of these two signals can thus be used to deconvolve the input sources as well as the Sm-Nd and Lu-Hf systematics of the ocean. In particular Hf excess is observed near hydrothermal vent sites, which are a source of Hf, but are associated with aggressive scavenging of Nd.
Multi-isotope Applications
Dust Source and Atmospheric Circulation
Strontium-Neodymium-Hafnium isotopes (Sr-Nd-Hf) are inherent in many geological settings, and thus have been used to track the origin of sediments and dust. By tracking the composition of dust based on these isotopes, it is possible to pinpoint the likely originating geological setting.
Mantle-crust Evolution
Strontium (Sr), neodymium (Nd), hafnium (Hf), and lead (Pb) are four isotopes that are inherent in many geological settings, and thus have been used to trace the evolution of metamorphic and igneous rocks, analyze weathering regimes and reconstruct past ocean circulation. Read more!
Geochemical Fingerprinting (Provenancing) & Source Tracking
The 87Sr/86Sr 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.
Geochemical fingerprinting of dust (Sharifi et al, 2018, EPSL).
Read our posts to learn more:
The use of isotopic analysis on bones: dating, environmental analysis & migration
Tracing the diet of herbivores and omnivores through isotopic analysis
Reference
Sharifi, A., Murphy, L.N., Pourmand, A., Clement, A.C., Canuel, E.A., Beni, A.N., Lahijani, H.A., Delanghe, D. and Ahmady-Birgani, H., (2018). Early-Holocene greening of the Afro-Asian dust belt changed sources of mineral dust in West Asia. Earth and Planetary Science Letters, 481, pp.30-40. DOI: 10.1016/j.epsl.2017.10.001