Strontium, Neodymium, Hafnium, and Lead Isotope Systematics in Earth Science
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, track the origin of sediments and dust, analyze weathering regimes and reconstruct past ocean circulation.
Magma Formation and Petrogenesis
Tectonic settings are home to significant heterogeneity in the composition of mantle material, which reflects a combination of ancient as well as recently crystallized mantle material. When a mantle undergoes subduction, the isotopic composition of the rock material concurrently changes, making it difficult to pinpoint the evolution of mantle activity based on isotope composition of one element alone.
Strontium (Sr), lead (Pb) and neodymium (Nd) are known to be compatible elements – meaning they become trapped in the solid phase relatively easily during mantle subduction, being retained in the crust material. As a result, it is difficult to use these elements to track such processes given their preference to solidify (Pearce & Peate 1995, Davidson 1996). Conversely, it has been shown that hafnium (Hf) is an incompatible element, meaning it avoids being trapped in the solid phase during crustal melts, allowing it to be more readily transported down the mantle wedge during subduction. Since it remains in the mobile phase for a longer period of time, Hf can be used as a fingerprint for mantle evolution more readily than other elements.
For example, given the strong compatibility of neodymium (Nd) and the incompatibility of hafnium (Hf) to alteration and solidification, the combined analysis of the two – Hf-Nd systematics (i.e. the covariance of Hf and Nd) – can provide detailed information on the mantle formation and petrogenesis by analyzing the delta between the two (Pearce et al. 1999).
Mineral Dust Sources
Dust weathered from continental crusts hold very specific isotopic signatures depending on their origin region. Once weathered, this dust is integrated into the atmosphere and moved around the planet depending on the atmospheric circulation at the time of deposition. The dynamics of dust movement is very useful in the analysis of ice core records as the dust gets trapped within the air bubbles of the individual snow accumulations, eventually making up annual layers of ice within core records.
The cycling of dust can impact many different elements of the earth system. For example, high concentrations of atmospheric dust (or aerosols) can increase cloud opacity – resulting in more extensive reflection of solar radiation and reduced precipitation (Lohmann and Feichter, 2005). This has the potential to significantly impact the earth’s radiative balance, resulting in either a cooling or warming of the surface. On the other hand, when the atmospheric dust eventually settles on the ocean surface, it provides a significant source of minerals, which serve as nutrients for primary production.
Given the important role dust can play in the earth’s systems, there has been significant research attention to understanding dust sources, composition and ultimately dust dynamics through the atmosphere. It is possible to reconstruct the movement of dust through space and time by analyzing its isotopic composition – notably, the Sr and Nd (Grousset and Biscaye, 2005) as well as Hf ratios (Pettke et al. 2002). In order to pinpoint the dust origin, the isotopic composition of dust is compared to known dust source regions (i.e. potential dust sources, PSAs).
For example, in a study of dust isotopes in a peat core from NW Iran, Sharifi et al. (2018) identified the likely dust sources during different periods in the early to mid Holocene (Figure 2). This research shed light on the atmospheric conditions dynamics (e.g. shifting position of the Westerly Jet Stream) as well as the greening of the Afro-Asian Dust Belt over this important climate interval.
Reconstructing Ocean Circulation
In addition to providing information on the circulation of the atmosphere through the analysis of dust variability, isotopes can be used to trace the circulation of the ocean through time. For example, neodymium (Nd) is integrated into the ocean through the erosion of continental crusts, and thus the Nd isotopic signature of ocean water corresponds to that of the local crustal rock (Figure 3), known as boundary exchange (Lacan and Jeandel 2005; Arsouze et al., 2009). Any anomalies or shifts in local Nd isotopes represent variations in ocean circulation.
Nd isotopes as an oceanic circulation tracer have some advantages over carbon isotopes as they are not significantly impacted by biological fractionation or exchange at the air-sea interface.
Lead (Pb) isotopes have also been utilized to reconstruct ocean circulation. Anthropogenic activities represent a significant source of Pb in the ocean, thus the analysis of Pb distribution and flux in the ocean can provide information on anthropogenic Pb sources and distribution over time. For example, Echegoyen et al. (2014) found a significant injection of Pb in the Indian Ocean as a result of accelerated industrialization in India and related release of leaded gasoline.
Strontium (Sr), neodymium (Nd), hafnium (Hf) and lead (Pb) isotope systems can be used to assess a variety of geological settings, including mantle systematics, dust sources and variability, as well as erosion of continental crusts and circulation of seawater.
Learn more about Sr-Nd-Hf Isotope Geochemistry.
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