The Noble Gases as Geochemical Tracers

The atmosphere is the primary terrestrial reservoir of the heavy noble gases (Ne, Ar, Kr, Xe) and precise knowledge of the isotopic composition of atmospheric noble gases is important for many—if not all—fields of noble gas geochemistry. Air noble gases, including helium, are very commonly used as a running laboratory standard for calibrating instrumental discrimination and sensitivity (see Chap.​ 1), hence any potential temporal or spatial heterogeneities in the atmospheric noble gas composition could have consequences for the reliability and comparability of noble gas data. Metrological measurements such as the determination of Avogadro’s constant and the gas constant also depend on accurate determination of the isotopic composition (and isotopic masses) of atmospheric noble gases. However, absolute isotopic measurements are not straightforward and this section reviews both how absolute isotopic determinations have been made and assesses the temporal and spatial variability of the atmosphere at the present and in the recent (<2 Ka) past.

Polar ice cores constitute excellent archives of past environmental conditions and provide us with glimpses into the Earth’s climatic history over hundreds of thousands of years. For the past two decades, noble gases, used as conservative tracers, have played a substantial role in extracting information from these archives. Noble gas analysis can be performed on two types of ice core samples. First, atmospheric air, trapped in bubbles in polar ice, can be extracted and analyzed for its noble gas composition. Variations in the isotopic and elemental composition of noble gases reveal changes in the composition of the paleoatmosphere or the impact of climate-related physical processes acting when the air was enclosed. For example, thermal fractionation allows for the creation of a ‘gas isotope thermometer’ in ice cores. Second, particles that were deposited on the surface snow and incorporated in the ice matrix can be analyzed for their noble gas isotope composition. These analyses reveal information about the input and origin of both terrestrial aerosols and extraterrestrial dust. The conservative nature of noble gases, ensuring that such fingerprints will be preserved over long periods of time, and the exceptionally good dating of polar ice cores make noble gas analysis of ice cores a versatile tool to study a wide spectrum of geochemical and paleoclimatic processes: from constraints on the magnitude of temperature changes during abrupt climate transitions in the Earth’s history, to determining the rate of Argon-40 degassing from the solid Earth over its history, or quantifying variations in the accretion of extraterrestrial dust on the Earth’s surface. Below, we present case studies to review some of the applications of noble gas analysis in the polar ice archive, both in trapped air bubbles and in particles incorporated in the ice matrix.

Noble gases are biologically and chemically inert, making them excellent tracers for physical processes. There are 5 stable noble gases: He, Ne, Ar, Kr, and Xe, with a range of physicochemical properties; the diffusivities of the noble gases in seawater differ by approximately a factor of 5 and the solubilities of the noble gases in seawater differ by approximately a factor of 10. This broad range in physicochemical characteristics leads to differing response to physical forcing. Thus, measurements of multiple noble gases made concurrently allow quantification of many physical processes. In seawater studies, noble gas measurements have been used to investigate air-sea gas exchange, allowing explicit separation of the bubble component from the diffusive gas exchange component, and to study equilibration during deep water formation. Argon has been used to quantify diapycnal mixing and the heavier noble gases could be useful in such studies as well. Helium, Ne, and Ar have yielded insights on ocean-cryospheric processes such as sea ice formation and basal melting of glaciers. The isotope³ He has been used extensively in studies of ocean circulation, and also for quantifying ocean-lithospheric interactions. Additionally, noble gases can be combined with biologically active gases, such as O₂ or N₂, in order to quantify rates of biological production and denitrification.

Concentrations of dissolved atmospheric noble gases in water constitute a thermometer, whose application to the groundwater archive provides a method of paleoclimate reconstruction. In addition, noble gases have found wide application as tracers in hydrogeology. This chapter reviews the historical development, the theoretical foundations, the sampling and analytical techniques, as well as the spectrum of applications of this important tool of tracer hydrology. A detailed account of currently available sampling techniques is given, as this information is of great practical importance but not fully available in the scientific literature. The analytical methods are better documented in the literature, although the many lab-specific details and constant development make it hard to provide an authoritative overview, so that this part is kept comparatively short. The focus of the chapter lies on the methods for data reduction and interpretation, which have undergone rapid and important development in the recent past. Nevertheless, in this respect still substantial research needs exist. Finally, this chapter provides an overview of applications of noble gases in groundwater hydrology, which range from the classical paleothermometry and the determination of other paleoclimate parameters such as humidity to various hydrological investigations, such as groundwater dating or the study of water origin and recharge conditions in hydrothermal, glaciated, alluvial, coastal, managed, and mountainous aquifer systems.

In well-studied aquatic systems such as surface waters and groundwater, noble gases are used extensively as natural tracers to reconstruct palaeoenvironmental conditions, to study transport and mixing, and to identify the geochemical origin of geogenic fluids. It has been suggested that less well-studied aquatic systems such as the porewaters of lacustrine and oceanic sediments and the fluid inclusions present in stalagmites might also be suitable as noble gas archives for environmental studies, but until recently the lack of adequate experimental techniques had hindered the development of noble gas geochemistry in these systems. This chapter reviews recent technical advances in this field and describes the scientific applications that these advances have made possible. The porewaters of lacustrine and oceanic sediments are now well established as noble gas archives in studies of temperature, salinity and mixing conditions that prevailed in the overlying water body in the past, as well as in studies of the transport and origin of solutes and pore fluids in the sediment. The geochemistry of noble gases in stalagmite fluid inclusions is still in the early stages of development. However, the results available to date suggest that stalagmite fluid inclusions have great potential as a noble gas archive in reconstructing palaeoclimatic conditions near caves with suitable stalagmites.

Most ³He in deep-sea sediments is derived from fine-grained extraterrestrial matter known as interplanetary dust particles (IDPs). These particles, typically <50 μm in diameter, are sufficiently small to retain solar wind-implanted He with high ³He/⁴He ratios during atmospheric entry heating. This extraterrestrial ³He (³He_ET) is retained in sediments for geologically long durations, having been detected in sedimentary rocks as old as 480 Ma. As a tracer of fine-grained extraterrestrial material, ³He_ET offers unique insights into solar system events associated with increased IDP fluxes, including asteroid break-up events and comet showers. Studies have used ³He_ET to identify IDP flux changes associated with a Miocene asteroid break-up event and a likely comet shower in the Eocene. During much of the Cenozoic, ³He_ET fluxes have remained relatively constant over million-year timescales, enabling ³He_ET to be used as a constant flux proxy for calculating sedimentary mass accumulation rates and constraining sedimentary age models. We review studies employing ³He_ET-based accumulation rates to estimate the duration of carbonate dissolution events associated with the K/Pg boundary and Paleocene-Eocene Thermal Maximum. Additionally, ³He_ET has been used to quantify sub-orbital variability in fluxes of paleoproductivity proxies and windblown dust. In order to better interpret existing records and guide the application of ³He_ET in novel settings, future work requires constraining the carrier phase(s) of ³He_ET responsible for long-term retention in sediments, better characterizing the He isotopic composition of the terrigenous end-member, and understanding why observed extraterrestrial ³He fluxes do not match the predicted variability of IDP accretion rate over orbital timescales.

Unequivocable evidence for warming of the climate system is a reality. An important factor for reducing this warming is mitigation of anthropogenic CO₂ in the atmosphere. This requires us to engineer technologies for capture of our carbon emissions and identify reservoirs for storing these captured emissions. This chapter reviews advances made in understanding multiphase interactions and processes operating in a variety of subsurface reservoirs using noble gases. We begin by discussing the types of reservoir available for carbon storage and the mechanisms of viable CO₂ storage, before summarising the physical chemistry involved in data interpretation and the sampling/sample storage techniques and requirements critical to successful sample collection. Theory of noble gas partitioning is interspersed with examples from a variety reservoirs to aid our knowledge of long term CO₂ storage in the subsurface. These include hydrocarbon reservoir and natural CO₂ reservoirs. In these examples we show how good progress has been made in using noble gases to explain the fate of CO₂ in the subsurface, to quantify the extent of groundwater interaction and to understand CO₂ behaviour after injection into oil fields for enhanced oil recovery. We also present recent work using noble gases for monitoring of subsurface CO₂ migration and leakage in CO₂ rich soils, CO₂ rich springs and groundwaters. Noble gases are chemically inert, persistent and environmentally safe and they have the potential to be extremely useful in tracing migration of CO₂. It is imperative that the many upcoming pilot CO₂ injection studies continue to investigate the behaviour of noble gases in the subsurface and develop suitable noble gas monitoring strategies.

Noble gas geochemistry consists of a series of inert natural tracers which allow quantitative geological modeling relative to the migration, trapping and extraction of hydrocarbon reserves. The noble gas data allow accurate description of complex natural and technological processes involved in identifying and extracting oil and gas discoveries. Among others, the origins of non hydrocarbon gas compounds such as CO₂ present in hydrocarbon accumulations, the physical interactions between hydrocarbon phases and water, the dynamics of oil and gas migration through porous sedimentary rocks, are some of the current applications of noble gases to hydrocarbon exploration in high risk areas. Unconventional hydrocarbon exploration and production, which may represent the future of our hydrocarbon consumption, raise new questions which may also be assessed with these unique tracers. Oil and gas shale production as well as gas hydrates, are future targets which may benefit from the specific properties of noble gases as tracers in hydrocarbon fluids.

This chapter describes the practice in the analysis and interpretation of noble gases in modern hydrothermal systems, including sample collection and analytical methods, implications of geographical distribution of helium isotopes in the large scale (100–1000 km) and the small scale (1–100 km), temporal variation of helium isotopes in some volcanoes, and the other noble gas isotope and abundance variations in hydrothermal systems. First, details of sampling method of volcanic and hot spring gases are discussed together with characteristics of two types containers, Giggenbach-type and lead-glass. Second, analytical techniques of noble gas abundances by an isotope dilution method using a QMS-based system, and neon interference on helium isotope measurements by a magnetic sector type mass spectrometer are written precisely. Third, helium isotope variations in three modern volcanic regions, such as hot spot, mid-ocean ridge, and subduction zone are compiled and discussed together with geo-tectonic settings and geophysical data. Fourth, across the island arc variations of helium isotopes are described against recent seismic tomography data in Northeast Japan, Southwest Japan, North Island of New Zealand, and Kamchatka peninsula of Russia. Then smaller size of the isotope variations around the independent volcano such as Mt Ontake and Mt. Nevado del Ruiz are discussed. Fifth, temporal variations of helium and neon isotopes in volcanic discharges are discussed with examples showing the effects of changes in volcanic activity on noble gas ratios. Sixth, the isotopic compositions of neon, argon, krypton and xenon isotopes in volcanic and hydrothermal systems is discussed and related to mantle and crustal degassing processes. The last section (seven) provides applications of noble gases to traces sources and crustal contamination processes of more abundant gases such as carbon dioxide, methane and nitrogen with examples from well studied hydrothermal systems in New Zealand, Italy, Central America and Greece. In summary noble gases have a wide range of applications in volcanic and hydrothermal systems and are key indicators of tectonic setting, mantle and magma degassing; they provide valuable information on the current activity of a volcano and in combination with major gases can provide insights to understanding other geologically important volatiles such as carbon dioxide, methane and nitrogen.

Fluid inclusions provide the only means possible for sampling fluids from the Earth's deep-interior and ancient past. Noble gas isotope analysis can provide quantitative information about the sources of volatile components in fluid inclusions (e.g. atmosphere, crust and mantle), whereas halogens provide complementary information about the fluids, acquisition of salinity and/or the presence of (I-rich) organic components. The aims of this chapter are to: (1) review methods for analysis of noble gases in fluid inclusions, and halogen analysis by the ‘noble gas method’ (extended ⁴⁰Ar–³⁹Ar methodology); and (2) summarise case studies of noble gases and halogens in fluid inclusions. The case studies include hydrothermal fluids involved in ore genesis in a range of geological environments encompassing mid-ocean ridge vents, sedimentary basins, near-pluton magmatic environments and metamorphic settings, as well as fluid inclusions in eclogite facies high-grade terranes relevant to subduction recycling processes. In contrast to modern ground waters, the fluid inclusion data suggest that most crustal fluids source some (additional) atmospheric noble gases within the crust (from sediments and hydrous minerals formed during seawater-alteration), and that low salinity fluids can acquire significant Br, as well as I, from organic-rich (meta-)sediments. Fluid–rock interactions are an important control on the composition of deep-crustal fluids; however, the orders of magnitude variation in noble gas isotope compositions and halogen abundances mean that they can preserve information about fluid sources that is overprinted in other stable and radiogenic isotope systems.

Noble gas geochemistry provides powerful tools for constraining mantle degassing through geological time. However, noble gas elemental and isotopic ratios are often disturbed by melting, magma degassing and atmospheric contamination. It is necessary to understand and quantify these shallow influences in order to obtain the noble gas elemental and isotopic ratios in the mantle. In this chapter, we present an overview of the key parameters that are necessary to derive mantle compositions. We discuss solubilities in silicate melts, crystal/melt partition coefficients during melting, and different models for vesiculation and degassing, along with the preferred method to correct for atmospheric contamination, using neon isotopic compositions. Using selected samples from mid-ocean ridge basalts (MORB) and ocean island basalts (OIB), we give the probable mantle contents and elemental and isotopic compositions of He, Ne, Ar and Xe in the depleted mantle and in the high ³He source. These estimates cannot be reconciled with ancient depletion models for unradiogenic noble gas isotopic compositions, found in some oceanic island basalts, which are best explained by relatively undegassed sources deep in the mantle.