The Little Cation that Could

DOI: 10.2138/gselements.16.4.233

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Roberta L. Rudnick studies the origin and evolution of the continents, combining petrology, geochemistry, and geophysical data to interpret the nature of the lower continental crust and how (or if) crust composition has evolved over Earth’s history. A first-generation college student, she received her BS at Portland State University (Oregon, USA), her MS at Sul Ross State University (Texas, USA) and her PhD at the Australian National University. After holding research positions in Germany and Australia, she returned to the USA and has served on the faculty at Harvard University (Massachusetts) and the University of Maryland (USA). She is currently a professor at the University of California, Santa Barbara (USA).

Introduction

Lithium, the first element in the alkali metals group of the periodic table, may seem a strange choice as a subject for which to devote a whole issue of Elements. Why should you care about lithium? What can lithium do for you? Well, lots, as it turns out. Not only is lithium a critical metal for modern society, it is also becoming an increasingly important tool for Earth scientists.

Discovered more than two centuries ago, lithium (Z = 3) is a rather peculiar element. Its cosmic abundance, like that of Be and B, is quite low, which reflects the fact that it is by-passed by most stellar nucleosynthetic processes. In fact, lithium is consumed by fusion reactions in the interiors of proto-stars. Its two isotopes, 6Li and 7Li, have among the lowest binding energies per nucleon of any stable nuclide; lithium almost didn’t make it as a naturally occurring element. It has a very low density, 0.543 g cm−3, meaning that metallic lithium will float on water, as all the alkali metals do (see https://www.youtube.com/watch?v=m55kgyApYrY, and do not try this at home!), and even on oil (Fig. 1). Lithium has the highest specific heat capacity of all solids at 3.58 kJKg−1K−1, making it a useful addition to coolants. The peculiarities of lithium give it important properties that make it useful in many ways to humans, including Earth scientists.

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Figure 1. Lithium metal can float in paraffin oil (density 0.8 g cm−3). Source: Wikimedia Commons

The Foundation Stone of Modern Civilization

The name “lithium” derives from the Greek word for “stone” (lithos), because it was originally isolated from the mineral kingdom, in contrast to then known alkalis of sodium and potassium (first isolated by electrolysis of chemical melts). The appellation “foundation stone of modern society” is entirely appropriate for Li, given the many usages it has found in modern society, such as its use in emerging alternative energies. Chances are that you are reading this on an electronic device (laptop, tablet, smart phone) that is powered by a lithium-ion battery. If you drive an electric or newer-model hybrid vehicle, you are carting around with you several tens of kilograms of lithium every time you drive. Lithium is an important lubricant and is used widely in the manufacture of glass and ceramics, because it acts as a powerful flux to reduce the melting temperature of materials. In terms of human health, lithium is also commonly employed to treat bi-polar disorder.

There are other uses of lithium that could be viewed in a less rosy light. For example, both isotopes of Li play an important role in the nuclear realm. Lithium-6 has a large neutron capture cross-section (that is, it readily absorbs neutrons), and so it is used to manufacture tritium (3H) for use in boosted fission weapons and two-stage thermonuclear weapons. By contrast, 7Li has a very low neutron capture cross-section, so it is used in the cooling systems of nuclear power reactors. Lithium-7 hydroxide is added to coolant waters in pressurized water reactors to maintain a neutral pH, counteracting the corrosiveness of boric acid which is used as a neutron absorber. Lithium-7 fluoride can also be used in molten salt reactors.

Lithium Isotopes Illuminate Earth processes

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Figure 2. Dr. Lui-Heung Chan in her thermal ionization mass spectrometer laboratory at Louisiana State University (USA) during the 1970s. Photo courtesy of Clara Chans

One of the burgeoning areas of lithium research related to Earth ­sciences is the development of lithium isotope geochemistry. Although the first measurements of the relative abundances of lithium isotopes in natural samples were made back in the 1960s, the method did not really take off until near the turn of the twenty-first century. Prof. Lui-Heung Chan, at Louisiana State University (Fig. 2), reported some of the first high-precision (± 1‰–2‰, 2σ) data for lithium isotopes, which she analyzed using thermal ionization mass spectrometry (TIMS).

These were challenging measurements to make, because lithium has only two stable isotopes, making it difficult to correct for mass fractionation during thermal ionization in the mass spectrometer. Nevertheless, Prof. Chan reported the first lithium isotope compositions for a variety of natural samples, including seawater, fresh- and altered oceanic crust, mid-ocean-ridge hydrothermal fluids, and marine sediments and their pore waters (e.g., Chan and Edmond, 1988). This work demonstrated that large fractionation of Li isotopes occurs at the Earth’s surface, thus setting up lithium isotopes as a potential tracer of surface processes and crustal recycling. Since that pioneering work, and with the development of multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) for the analysis of lithium isotopes (Tomascak et al. 1999), the field of lithium isotopes has taken off (Fig. 3).

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Figure 3. The number of papers published on lithium isotopes in the Earth sciences over the past 35 years. Data Source: ISI Web of Knowledge

Some of the initial excitement surrounding the newly minted lithium isotope tool was to use it as a tracer of crustal recycling. Subduction zones were the logical place to apply the tool, because lithium in altered oceanic crust becomes significantly heavier than lithium in the mantle due to the uptake of seawater lithium during alteration. Moreover, because lithium is highly soluble, it was expected that it should be ­carried from the slab into the source region of arc basalts; thus, arc magmas might show this heavy isotopic signature. Alas, this is (generally) not the case. The vast majority of arc magmas have d7Li within the range observed in mid-ocean-ridge basalts, even though lithium is enriched in these magmas relative to elements with similar geochemical behavior. So, what’s going on? It may be that the lithium leaving the slab is a mixture of heavy seawater lithium and light lithium in subducted ­terrigenous sediments, which, when mixed together, generate a lithium signature similar to the mantle (Marschall and Tang 2020). Although elusive, the signature of subducting sediments has been documented in lavas of the Lesser Antilles island arc, where thick piles of isotopically light sediment derived from the heavily weathered Guiana Shield of South America are subducted beneath the arc (Tang et al. 2014).

There were other surprises in store for early lithium isotope practitioners. It was expected that there would be limited lithium isotope fractionation in high-temperature igneous and metamorphic rocks. This is, indeed, true for equilibrium isotope fractionation. However, because lithium is a small cation, it diffuses relatively rapidly, and as it diffuses 6Li diffuses significantly faster than 7Li (Richter et al. 2003). This generates very large kinetic isotope fractionation effects, which, if not recognized as such, may lead to erroneous interpretations (see Marschall and Tang 2020 this issue). This distinctive feature of lithium isotopes is turning out to be a significant strength: documenting diffusion profiles, coupled with experimentally derived diffusion coefficients, are providing powerful new applications for lithium isotopes to be used as geospeedometers for igneous and metamorphic processes (Marschall and Tang 2020 this issue).

Finally, another important application of lithium isotopes is in the near-surface environment, where equilibrium fractionation is large and there is significant isotope fractionation accompanying chemical weathering and the formation of secondary minerals. This fractionation leads to heavy-Li water and to light-Li weathered regolith. Demonstration that the lithium isotope composition of seawater has changed significantly throughout the Cenozoic (Misra and Froelich 2012) has raised the possibility that lithium isotopes may be a useful tracer of chemical weathering over Earth’s history.

Future studies will likely focus on developing lithium isotopes as a quantitative geospeedometer and as a means to track the weathering history of the continents. This issue of Elements provides a glimpse into the fascinating world of lithium.

References

Chan L-H, Edmond JM (1988) Variation of lithium isotope composition in the marine environment: a preliminary report. Geochimica et Cosmochimica Acta 52: 1711-1717

Marschall H, Tang M (2020) High-temperature processes: is it time for lithium isotopes? Elements 16: 247-252

Misra S, Froelich PN (2012) Lithium isotope history of Cenozoic seawater: changes in silicate weathering and reverse weathering. Science 335: 818-823

Richter FM, Davis AM, DePaolo DJ, Watson EB (2003) Isotope fractionation by chemical diffusion between molten basalt and rhyolite. Geochimica et Cosmochimica Acta 67: 3905-3923

Tomascak PB, Carlson RW, Shirey SB (1999) Accurate and precise determination of Li isotopic compositions by multi-collector sector ICP-MS. Chemical Geology 158: 145-154

Tang M, Rudnick RL, Chauvel C (2014) Sedimentary input to the source of Lesser Antilles lavas: a Li perspective. Geochimica et Cosmochimica Acta 144: 43-58

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