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## Über dieses Buch

This new volume on boron isotope geochemistry offers review chapters summarizing the cosmochemistry, high-temperature and low-temperature geochemistry, and marine chemistry of boron. It also covers theoretical aspects of B isotope fractionation, experiments and atomic modeling, as well as all aspects of boron isotope analyses in geologic materials using the full range of solutions and in-situ methods. The book provides guidance for researchers on the analytical and theoretical aspects, as well as introducing the various scientific applications and research fields in which boron isotopes currently play a major role. The last compendium to summarize the geochemistry of boron and address its isotope geochemistry was published over 20 years ago (Grew &Anovitz, 1996, MSA Review, Vol.33), and there have since been significant advances in analytical techniques, applications and scientific insights into the isotope geochemistry of boron. This volume in the “Advances in Isotope Geochemistry” series provides a valuable source for students and professionals alike, both as an introduction to a new field and as a reference in ongoing research.
Chapters 5 and 8 of this book are available open access under a CC BY 4.0 license at link.springer.com

## Inhaltsverzeichnis

### Chapter 1. Boron Isotopes in the Earth and Planetary Sciences—A Short History and Introduction

Abstract
This volume on boron isotope geochemistry contains chapters reviewing the low- and high-temperature geochemistry, marine chemistry, and cosmochemistry of boron isotopes. It covers theoretical aspects of B isotope fractionation, experiments and atomic modeling, as well as all aspects of boron isotope analyses in geologic materials by the full range of solution and in situ methods. The book provides guidance for researchers on the analytical and theoretical end, and introduces the various scientific applications and research fields in which boron isotopes play a growing role today. This chapter provides a brief history of boron isotope research and analytical development and provides an overview of the other chapters of the volume “Boron Isotopes—The Fifth Element” in the series Advancements in Isotope Geochemistry.
Horst R. Marschall, Gavin L. Foster

### Chapter 2. Boron Isotope Analysis of Geological Materials

Abstract
Over the last twenty years applications of the boron isotope system have expanded from the analysis of boron-rich phases (e.g., tourmaline, borates) to include other materials with low B concentrations (e.g., carbonates, basaltic glass). The accurate and precise determination of the boron isotopic composition of geological materials is however a difficult task, particularly for those where boron is present in low-concentration. For solution methods, this difficulty arises principally from the near ubiquitous level of boron contamination in most standard clean laboratories, the light mass of the element, the occurrence of only two stable isotopes, and the large mass difference between them. For in situ approaches, such as secondary-ion mass spectrometry, additional difficulties arise from the restricted availability of well-characterized reference materials, from surface contamination, from limited precision in low-concentration samples, and limitations in reproducibility in high-concentration samples that may partly arise from small-scale heterogeneities in the analyzed materials. Nevertheless, a variety of novel techniques, strategies and methodologies have been developed over the past two decades to meet these challenges. We describe here some of these developments and focus on those that we feel are going to play a major role in the growing use of the boron isotope system in the earth and planetary sciences in decades to come.
Gavin L Foster, Horst R Marschall, Martin R Palmer

### Chapter 3. Boron Isotope Fractionation Among Vapor–Liquids–Solids–Melts: Experiments and Atomistic Modeling

Abstract
A quantitative understanding of the principle factors that govern their geochemical behavior is required to employ boron and its isotopes as geochemical tracers of any vapor-, liquid- or melt-mediated process in the Earth’s interior. Feedback between experiments and computational predictions are required to gain insight into the processes driving isotope partitioning. This chapter comprises methods and results of selected experimental studies and first principles atomistic modeling techniques aimed at determining and predicting temperature-, pressure-, and pH-dependent B-isotope fractionation among B-bearing geomaterials.
Piotr M. Kowalski, Bernd Wunder

### Chapter 4. Boron Incorporation into Marine CaCO3

Abstract
The isotopic composition (δ11B) and abundance (B/Ca) of boron in the marine CaCO3 minerals calcite and aragonite are used as paleoceanographic tracers for past oceanic pH and carbon chemistry. These environmental proxies depend upon the ability of CaCO3 minerals to incorporate trace concentrations of B within their structure, and record the state of the pH-dependent equilibrium between B(OH)3 and $${{{\text{B}}\left( {\text{OH}} \right)_{4}}^{ - }}$$, and the relative abundance of B and C in seawater. To achieve this CaCO3 minerals must either incorporate a single species of aqueous B, or take up a predictable mixture of both species. Initial investigations found evidence to suggest the sole incorporation of aqueous $${{{\text{B}}\left( {\text{OH}} \right)_{4}}^{ - }}$$ into the anion site of CaCO3 minerals. These observations established the required link between aqueous B chemistry and CaCO3 hosted B, and provided the foundation for the development and application of the δ11B and B/Ca proxies. However, advances in our understanding of aqueous B chemistry, improvements in the accuracy of B isotopic measurements of carbonates, and new data from controlled precipitation experiments have since revealed more complex, structure-dependent mechanisms of B incorporation into CaCO3. Studies of aragonite appear to support a relatively straightforward substitution of $${{{\text{B}}\left( {\text{OH}} \right)_{4}}^{ - }}$$ into the mineral anion site. Conversely, a growing number of studies of calcite suggest either that both aqueous B(OH)3 and $${{{\text{B}}\left( {\text{OH}} \right)_{4}}^{ - }}$$ are taken up into the mineral, or that B is subject to a significant isotopic fractionation during incorporation. While a growing body of theoretical and experimental work are moving toward an understanding of B uptake in CaCO3, we currently lack a systematic description of this key process, particularly in calcite. As long as the mechanisms of B incorporation remain unknown, the relationships between δ11B and B/Ca and ocean chemistry must be treated as empirical, adding uncertainty to the paleoceanographic records derived from them. This chapter will explore our current understanding of B incorporation into marine CaCO3 minerals, in context of their structure and growth mechanisms. We will consider the broad question of ‘how does B get from seawater into calcite and aragonite?’
Oscar Branson

Open Access

### Chapter 5. Boron Isotopes in Foraminifera: Systematics, Biomineralisation, and CO2 Reconstruction

Abstract
The boron isotope composition of foraminifera provides a powerful tracer for CO2 change over geological time. This proxy is based on the equilibrium of boron and its isotopes in seawater, which is a function of pH. However while the chemical principles underlying this proxy are well understood, its reliability has previously been questioned, due to the difficulty of boron isotope (δ11B) analysis on foraminferal samples and questions regarding calibrations between δ11B and pH. This chapter reviews the current state of the δ11B-pH proxy in foraminfera, including the pioneering studies that established this proxy’s potential, and the recent work that has improved understanding of boron isotope systematics in foraminifera and applied this tracer to the geological record. The theoretical background of the δ11B-pH proxy is introduced, including an accurate formulation of the boron isotope mass balance equations. Sample preparation and analysis procedures are then reviewed, with discussion of sample cleaning, the potential influence of diagenesis, and the strengths and weaknesses of boron purification by column chromatography versus microsublimation, and analysis by NTIMS versus MC-ICPMS. The systematics of boron isotopes in foraminifera are discussed in detail, including results from benthic and planktic taxa, and models of boron incorporation, fractionation, and biomineralisation. Benthic taxa from the deep ocean have δ11B within error of borate ion at seawater pH. This is most easily explained by simple incorporation of borate ion at the pH of seawater. Planktic foraminifera have δ11B close to borate ion, but with minor offsets. These may be driven by physiological influences on the foraminiferal microenvironment; a novel explanation is also suggested for the reduced δ11B-pH sensitivities observed in culture, based on variable calcification rates. Biomineralisation influences on boron isotopes are then explored, addressing the apparently contradictory observations that foraminifera manipulate pH during chamber formation yet their δ11B appears to record the pH of ambient seawater. Potential solutions include the influences of magnesium-removal and carbon concentration, and the possibility that pH elevation is most pronounced during initial chamber formation under favourable environmental conditions. The steps required to reconstruct pH and pCO2 from δ11B are then reviewed, including the influence of seawater chemistry on boron equilibrium, the evolution of seawater δ11B, and the influence of second carbonate system parameters on δ11B-based reconstructions of pCO2. Applications of foraminiferal δ11B to the geological record are highlighted, including studies that trace CO2 storage and release during recent ice ages, and reconstructions of pCO2 over the Cenozoic. Relevant computer codes and data associated with this article are made available online.
James W. B. Rae

### Chapter 6. Boron Isotopic Systematics in Scleractinian Corals and the Role of pH Up-regulation

Abstract
The boron isotopic composition (δ11B) of scleractinian corals has been used to track changes in seawater pH and more recently as a probe into the processes controlling bio-calcification. For corals that precipitate aragonite skeletons, up-regulation of pH appears to be a general characteristic, typically being ~0.3 to ~0.6 pH units higher than ambient seawater. The relationship between the pH of the corals calcifying-fluid (pHcf) and seawater pHT (total scale) is shown to be dependent on both physiological as well environmental factors. In laboratory experiments conducted on symbiont-bearing (zooxanthellate) corals under conditions of constant temperature and seawater pH, changes in the δ11B derived calcifying fluid pHcf is typically 1/3 to 1/2 of that of ambient seawater. Similar linear relationships are found for cold water corals that live in relatively stable, cold, deep-water environments but at significantly elevated levels of pHcf (~0.5–1 pH units above seawater), a likely response to the lower pH of their deep-sea environments. In contrast, zooxanthellae-bearing corals living in shallow-water reef environments that experience significant natural variations in temperature, light, nutrients and seawater pH, show different types of responses. For example, over seasonal time-scales Porites corals from the Great Barrier Reef (GBR) have a large range in pHcf of ~8.3 to ~8.5, significantly greater (~×2 to ~×3) than that of reef-water (pHT ~8.01 to ~8.08), and an order of magnitude greater than that expected from ‘static’ laboratory experiments. Strong physiological controls, but of a different character, are found in corals grown in a Free Ocean Carbon Enrichment Experiment (FOCE) conducted in situ within the Heron Island lagoon (GBR). These corals exhibit near constant pHcf values regardless of external changes in temperature and seawater pH. This pattern of strong physiologically controlled ‘pH-homeostasis’, with elevated but constant pHcf has been found despite large natural seasonal variations in the pH (±0.15 pH units) of the lagoon waters, as well as the even larger super-imposed decreases in seawater pH (~0.25 pH units) designed to simulate year 2100 conditions. In natural reef environments we thus find that the processes influencing the up-regulation of pHcf in symbiont-bearing corals are subject to strong physiological controls, behaviour that is not well simulated in the current generation of aquaria-based experiments with fixed seawater pH and temperature. Conversely, cold-water corals that lack symbionts and inhabit the relatively stable deep-sea environments hold the best prospects for providing reliable reconstructions of seawater pH. Clearly, further studies utilising the δ11B-pHcf proxy combined with other DIC/carbonate-ion proxies (e.g. B/Ca), but conducted under realistic ‘natural’ conditions, are required to elucidate the processes controlling coral bio-calcification and to better understand the vulnerability of scleractinian corals to anthropogenic driven warming and ocean acidification.
Malcolm T. McCulloch, Juan P. D’Olivo, James Falter, Lucy Georgiou, Michael Holcomb, Paolo Montagna, Julie A. Trotter

### Chapter 7. Boron in the Weathering Environment

Abstract
This chapter reviews the state of art of the use of boron isotopes to understand water-rock interaction in the Critical Zone, the thin and reactive layer at the Earth’s surface. Because boron isotopes are largely fractionated by adsorption, coprecipitation and evaporation-condensation processes, boron isotopes are well adapted to trace the main processes that convert rocks into soils and sediments on terrestrial surfaces. The difference in affinity of boron isotopes between trigonal and tetrahedral species is the main cause of isotope fractionation of boron at the Earth’s surface. Due to the competition between the speciation of boron in solution and the speciation on boron onto or into solids or gas, large isotopic variations are predicted and observed. Measured boron isotopic composition in the weathering environment varies over a considerable range of about 70‰. Precipitation, rivers and biomass are usually enriched in 11B, while a complementary depletion in 11B (enrichment in 10B) is observed in clay minerals and on organic or inorganic surfaces. At the ecosystem scale, boron appears to behave as a micronutrient with a major flux of boron associated with biological recycling. The inputs of boron to ecosystems by chemical weathering or from the atmosphere are minor. When the residence time of water in the critical zone is high, such as in groundwater systems, boron contents increase and are much more dominated by a weathering signal. Boron is mainly added to the ocean by rivers, while the most important sink of boron is adsorption on clay minerals. This makes boron a particularly good tracer of the weathering/erosion balance of terrestrial surfaces in addition to its capacity for tracing the pH and ancient seawater. A lot remains to be done to better understand the behavior of boron and boron isotopes at the Earth’s surface and on the secular evolution of boron isotopes in the ocean but our review of the available literature shows that this tracer has a great potential at a local (ecosystem) and global (ocean) scale.
Jérôme Gaillardet, Damien Lemarchand

Open Access

### Chapter 8. Boron Isotopes in the Ocean Floor Realm and the Mantle

Abstract
This chapter reviews the boron isotopic composition of the ocean floor, including pristine igneous oceanic crust such as mid-ocean ridge basalts and ocean island basalts and their implications for the B isotopic composition of the mantle. The chapter further discusses the B isotopic effects of assimilation of altered crustal materials in mantle-derived magmas. The systematics of seawater alteration on oceanic rocks are discussed, including sediments, igneous crust and serpentinization of ultramafic rocks and the respective marine hydrothermal vent fluids. The chapter concludes with a discussion of the secular evolution of the B isotopic composition of seawater.
Horst R. Marschall

### Chapter 9. Boron Isotopes as a Tracer of Subduction Zone Processes

Abstract
This chapter reviews recycling of boron (B) and its isotopes in subduction zones. It discusses the profound changes in B concentrations and B isotope ratios of various materials involved in convergent margin evolution, in particular highlighting the fate and evolution of progressively dehydrating subducting slabs and the behavior of B during burial and subsequent metamorphism. We review various models used to parameterize these complex and often poorly understood processes and critically evaluate the available data from the literature. We proceed by reviewing B isotope data from mafic arc volcanic rocks and explore systematic variations with subduction zone geometry as well as familiar geochemical tracers of subduction processes. Finally, the role of serpentinisation in the mantle wedge is discussed in the light of new geochemical and petrological insights on the importance of serpentinites and subduction erosion. We provide recommendations for further research on B isotopes in subduction zones and directions where we think this exciting stable isotope tracer may make the biggest impact.
Jan C. M. De Hoog, Ivan P. Savov

### Chapter 10. Boron Isotopes in the Continental Crust: Granites, Pegmatites, Felsic Volcanic Rocks, and Related Ore Deposits

Abstract
Boron is an incompatible lithophile element that is readily transported by granitic melts and hydrous fluids and therefore is concentrated in the continental crust relative to the mantle. The isotopic composition of boron in crystalline rocks of the continental crust (e.g., metamorphic and igneous lithologies) varies over a wide range of −20 to +10‰, depending on the B-isotope composition of the protoliths and on fractionation effects caused by phase transitions (metamorphic devolatilization reactions, fluid exsolution from magmas). Studies of progressive metamorphism and anatexis show that the behavior of boron and its isotopes depends heavily on the presence or absence of B-retentive minerals like tourmaline. In general, boron is prone to loss during devolatilization reactions, and metamorphic fluid preferentially removes the heavier isotope, but growth of tourmaline can minimize or prevent these effects. A new compilation of over 250 B-isotope analyses from about 90 localities of felsic igneous rocks in the continental crust shows a first-order distinction in composition between I-type magmas (subduction-related having meta-igneous sources) and S-type magmas (derived from metasedimentary rocks). Boron in I-type magmas is isotopically heavy (mean δ11B = −2‰, s.d. = 5) relative to unaltered MORB (mean δ11B = −7‰, s.d. = 1), presumably because of a greater contribution by subducted oceanic crust and pelagic sediments. Boron in S-type granitic rocks has a much lighter isotopic signature (mean δ11B = −11‰, s.d. = 4). The latter corresponds to the commonly cited B-isotope value of −10‰ for continental crust, but because much of Earth’s crust is derived from I-type magmas, its average B-isotope value is probably higher than previously thought. The dichotomy of B-isotope compositions in I- and S-type granitoids is also observed in their genetically related magmatic-hydrothermal ore deposits, as we demonstrate in a review of data from porphyry and Iron Oxide-Copper-Gold (IOCG) systems (I-type) and from Sn-W veins and granitic pegmatites (S-type). However, it is important to note that in all of these systems, there are significant and locally complex effects of isotopic fractionation due to magmatic fluid exsolution and to mixing of boron sourced from externally derived fluids.
Robert B. Trumbull, John F. Slack

### Chapter 11. The Cosmochemistry of Boron Isotopes

Abstract
The boron elemental abundances and isotopic compositions in the universe and constituents (stars, interstellar medium and the Solar System material) within it have important implications for the astrophysical origins of this element. Astronomical observations and laboratory analysis have revealed that despite a significant difference in boron abundances among different objects, the 11B/10B ratio of 4 appears to be ubiquitous (within measurement uncertainties) across the Galaxy. Galactic Cosmic Ray (GCR) spallation, which yields 11B/10B = 2.5, cannot have been the sole source of B; another mechanism that favors the production of 11B over that of 10B must have operated over the Galactic timescale. However, how exactly the Galaxy, interstellar medium, and the Solar System acquired the 11B/10B ratio of 4 remains poorly understood.
Ming-Chang Liu, Marc Chaussidon
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