Thematic Articles

Life on Earth produces innumerable structurally diverse biomolecules. Biomarkers, a subset of these compounds, are sufficiently specific in the structure that they serve as tracers of organisms present in the environment or preserved in the geological record. Biomarkers can be used as proxies for organisms and the biogeochemical processes they mediate or to which they respond. They can help to document and understand processes that are other- wise difficult to study, and their fossil derivatives can be used to reconstruct past ecosystems, environmental conditions, and climate variations. Biomarker science interfaces with biology, chemistry, environmental, and Earth sciences, and provides valuable opportunities to learn more about how the Earth system has evolved over time.

The four stable halogens (F, Cl, Br, and I) are low-abundance elements that are widely distributed in nature. Two of the halogens, Cl and Br, each have two stable isotopes showing a range in natural isotope variation of up to a few parts per thousand. A variety of analytical techniques have been developed to determine the abundance and isotopic ratios of the halogens: these include in situ techniques for high spatial resolution studies and bulk determinations, and they have been applied to a range of materials, including whole rocks, minerals, glasses, and fluid inclusions. Here, we summarise some of the established methods for determining halogen abundances and isotopes and highlight key advances.

Halogens are volatile elements present in trace amounts in the Earth’s crust, mantle, and core. They show volatile behavior and tend to be incompatible except for fluorine, which makes them key tracers of fluid-mediated and/or melt-mediated chemical transport processes. Even small quantities of halogens can profoundly affect many physicochemical processes such as melt viscosity, the temperature stability of mineral phases, the behavior of trace elements in aqueous fluids, or the composition of the atmosphere through magma degassing. Experiments allow us to simulate deep-Earth conditions. A comparison of experimental results with natural rocks helps us to unravel the role and behavior of halogens in the Earth’s interior.

Understanding the atmospheric geochemical cycle of both natural and anthropogenic halogens is important because of the detrimental effect halogens have on the environment, notably on tropospheric and stratospheric ozone. Oceans are the primary natural source for atmospheric Cl, F, Br, and I, but anthropogenic emissions are still important, especially for Cl. While emissions of human-made halocarbons (e.g., chlorofluorocarbons or CFCs) are expected to continue to decrease allowing progressive stratospheric ozone recovery, volcanic activity (e.g., clusters of mid-scale explosive eruptions or large-scale explosive eruptions) might disturb this recovery over the next decades. This review provides a synthesis of natural halogen fluxes from oceanic, terrestrial, and volcanic sources, and discusses the role of natural halogen species on atmosphere chemistry and their environmental impact.

Halogens are important elements that participate in a variety of biogeochemical processes and influence the solubility of metals in subduction-zone fluids. Halogens are powerful tracers of subducted volatiles in the Earth’s mantle because they have high abundances in seawater, sediments, and altered oceanic lithosphere but low concentrations in the mantle. Additionally, Br/Cl and I/Cl ratios, as well as Cl-isotope ratios, have characteristic ranges in different surface reservoirs that are not easily fractionated in the mantle. Current data suggest that subduction of serpentinised lithosphere is a major source of halogens in the Earth’s mantle.

Halogens are mobile in geological fluids, making them excellent tracers of volatile activity. Halogen-bearing minerals in diverse planetary materials, coupled with chlorine isotope compositions of bulk samples and minerals, can be used to infer the presence of fluids on planetary surfaces, crusts, and interiors. Halogen element and isotopic evidence helps define the role that halogens play in diverse planetary environments (e.g., asteroids, the Moon, and Mars), which offers insights into fluid activity in the early Solar System and in the role such fluids have played in volatile transport, alteration processes, and habitability throughout geological history.

The halogen group elements (F, Cl, Br, and I) and the stable isotopes of
Cl and Br collectively are powerful tracers of terrestrial volatile cycling.
Individually, their distinct geochemical affinities inform on a variety of fluid-mediated and magmatic processes. They form a wide-range of halogenbearing minerals whose composition reflects the source fluids from which they evaporated or crystallized. Fluorine’s geochemical cycle is generally decoupled from that of the heavier Cl, Br, and I, which are concentrated into Earth’s surface reservoirs. Throughout history, the salt-forming halogens have been integral to human health and are key constituents of many industries. These common elements have an important role in tracing geochemical processes across many geologic environments – from the surface to the deep planetary interior.

The formation of continental crust via plate tectonics strongly influences the physical and chemical characteristics of Earth’s surface and may be the key to Earth’s long-term habitability. However, continental crust formation is difficult to observe directly and is even more difficult to trace through time. Nontraditional stable isotopes have yielded significant insights into this process, leading to a new view both of Earth’s earliest continental crust and of what controls modern crustal generation. The stable isotope systems of titanium (Ti), zirconium (Zr), molybdenum (Mo), and thallium (Tl) have proven invaluable. Processes such as fractional crystallization, partial melting, geodynamic setting of magma generation, and magma cooling histories are examples of processes illuminated by these isotope systems.

T he variability of iron isotopes among rocky bodies in the inner Solar System provides a window onto the diversity of materials and mechanisms from which they formed. The magnitude of isotopic variation in mantle-derived rocks within a given body is similar to that between different planetary bodies. Isotopic signatures arising from primordial events, namely, evaporation/condensation, core formation and melting/crystallization, may be progressively diluted, modified, and redistributed over time by global recycling processes such as plate tectonics. Here, we assess the relative influence of these primordial mechanisms on the iron isotope compositions of igneous rocks and their implications for the structure and accretion histories of rocky planets.

Evaporation of magma oceans exposed to space may have played a role in the chemical and isotopic compositions of rocky planets in our Solar System (e.g., Earth, Moon, Mars) and their protoplanetary antecedents. Chemical depletion of moderately volatile elements and the enrichment of these elements’ heavier isotopes in the Moon and Vesta relative to chondrites are clear examples. Evaporation is also thought to be an important process
in some exoplanetary systems. Identification of evaporation signatures among the rock-forming elements could elucidate important reactions between melts and vapors during planet formation in general, but the process is more complicated than is often assumed.

The detection of exoplanets and accretion disks around newborn stars has spawned new ideas and models of how our Solar System formed and evolved. Meteorites as probes of geologic deep time can provide ground truth to these models. In particular, stable isotope anomalies in meteorites have recently emerged as key tracers of material flow in the early Solar System, allowing cosmochemists to establish a “planetary isotopic genealogy”. Although not complete, this concept has substantially advanced our understanding of Solar System evolution, from the collapse of the Sun’s parental molecular cloud to the accretion of the planets.

Stable isotopes provide deep insights into processes across a wide range of scales, from micron- to cosmic-size systems. Here, we review how continued advances in mass-spectrometry have enabled the analysis of ever-smaller samples and brought the field of heavy stable isotope geochemistry to its next frontier: the single-crystal scale. Accessing this record can be as enlightening as it is challenging. Drawing on novel systematics at different stages of development (from well-established to nascent), we discuss how the isotopes of heavy elements, such as magnesium, iron, zirconium, or uranium, can be used at the single-crystal and subcrystal scales to reconstruct magma thermal histories, crystal growth timescales, or, possibly, magma redox conditions.