Geochemistry of Earth Surface Systems

A derivative of the Treatise on Geochemistry

Heinrich D Holland, Karl K. Turekian

• Publisher: Academic Press
• Year: 2010
• Print ISBN: 9780080967066
• eBook ISBN: 9780080967073
• Topic: Physical Sciences
• Subtopic: Geology & Earth Sciences

Geochemistry of Earth Surface Systems offers an interdisciplinary reference for scientists, researchers and upper undergraduate and graduate level geochemistry students a sampling of articles on earth surface processes from The Treatise on Geochemistry that is more affordable than the full Treatise. For professionals, this volume will provide an overview of the field as a whole. For students, it will provide more in-depth introductory content than is found in broad-based geochemistry textbooks. Articles were selected from chapters across all volumes of the full Treatise, and include: Volcanic Degassing, Hydrothermal Processes, The Contemporary Carbon Cycle, Global Occurrence of Major Elements in Rivers, Organic Matter in the Contemporary Ocean, The Biological Pump, and Evolution of Sedimentary Rocks.

• Comprehensive, interdisciplinary and authoritative content selected by leading subject experts
• Robust illustrations, figures and tables
• Affordably priced sampling of content from the full Treatise on Geochemistry

1

Volcanic Degassing

C. Oppenheimer University of Cambridge, UK

NOMENCLATURE 2

1.1 INTRODUCTION 2

1.1.1 Earth Outgassing, Atmospheric Evolution and Global Climate 2

1.1.2 Magma Evolution and Dynamics, and Volcanic Eruptions 4

1.1.3 Volcanic Hazards and Volcano Monitoring 4

1.2 ORIGIN, SPECIATION, AND ABUNDANCE OF VOLATILES 6

1.2.1 Sources and Abundances of Volatiles in Magmas 6

1.2.2 Solubility and Speciation of Volatiles 8

1.3 DEGASSING 9

1.3.1 Saturation 9

1.3.2 Supersaturation and Nucleation 10

1.3.3 Bubble Growth and Magma Ascent 10

1.3.4 Bubble Coalescence 10

1.3.5 Gas Separation 11

1.3.6 Fragmentation 12

1.3.7 Excess Degassing 12

1.4 EMISSIONS 13

1.4.1 Styles of Surface Emissions 13

1.4.1.1 Noneruptive emissions 14

1.4.1.2 Eruptive emissions 16

1.4.2 Chemical Composition of Volcanic Gases 16

1.4.3 Measurement of Volatiles 16

1.4.3.1 In situ sampling and analysis 17

1.4.3.2 Portable remote sensing systems 19

1.4.3.3 Satellite remote sensing 19

1.4.3.4 Petrological methods 19

1.4.3.5 Ice cores 20

1.4.3.6 Application of geochemical surveillance to volcano monitoring 21

1.5 FLUXES OF VOLCANIC VOLATILES TO THE ATMOSPHERE 23

1.5.1 Sulfur 24

1.5.2 Carbon and Water 26

1.5.3 Halogens 26

1.5.4 Trace Metals 27

1.6 IMPACTS 27

1.6.1 Stratospheric Chemistry and Radiative Impacts of Volcanic Plumes 28

1.6.1.1 Formation of stratospheric sulfate aerosol veil 28

1.6.1.2 Impacts on ozone chemistry 28

1.6.1.3 Optical and radiative effects 30

1.6.2 Climatic Impacts of Major Volcanic Eruptions 31

1.6.2.1 Intermediate to silicic eruptions 31

1.6.2.2 Mafic eruptions 33

1.6.3 Tropospheric Chemistry of Volcanic Plumes 33

1.6.4 Impacts of Volcanic Volatiles on Vegetation and Soils 34

1.6.5 Impacts of Volcanic Pollution on Animal and Human Health 35

1.6.5.1 Gaseous and particulate sulfur and air quality 35

1.6.5.2 Fluorine 35

1.6.5.3 Carbon dioxide 35

1.7 CONCLUSIONS AND FUTURE DIRECTIONS 36

ACKNOWLEDGMENTS 36

REFERENCES 37

NOMENCLATURE

r_eff effective aerosol radius

D diffusion coefficient of given volatile in melt

I solar irradiance at given level in atmosphere

I_o solar irradiance at top of the atmosphere

K_R constant in vesicularity equation (2)

N constant in solubility equation (1)

P pressure

Pe Peclet number

R bubble radius

S constant in solubility equation (1)

V_g/V₁ ratio between the volumes of gas and melt at given pressure

$X_{{\text{H}}_2\text{O}}$

water solubility in melt

$X_{{\text{H}}_2\text{O}}^r$

residual melt water content at given pressure

$X_{{\text{H}}_2\text{O}}^0$

initial melt water content at saturation pressure

η melt dynamic viscosity

τ aerosol optical depth

τ_d timescale of volatile diffusion

τ_η timescale of viscous relaxation

ΔP oversaturation pressure

Θ solar zenith angle

Nature only reveals her secrets if we ask the right questions and listen, and listening in geochemistry means sampling, analyzing, plotting. (Werner Giggenbach, 1992b)

1.1 INTRODUCTION

Humans have long marveled at the odorous and colorful manifestations of volcanic emissions, and, in some cases, have harnessed them for their economic value (Figure 1). Moreover, the degassing of magma that is responsible for them is one of the key processes influencing the timing and nature of volcanic eruptions, and the emissions of these volatiles to the atmosphere can have profound effects on the atmospheric and terrestrial environment, and climate, at timescales ranging from a few years to > 1 Myr, and spatial scales from local to global (Oppenheimer et al., 2003a). Even more fundamental are the relationships between the history of planetary outgassing, differentiation of the Earth’s interior, chemistry of the atmosphere and hydrosphere, and the origin and evolution of life (e.g., Kelley et al., 2002).

Figure 1 “The burning valley called Vulcan’s Cave near Naples” or Solfatara (Campi Flegrei), from Bankes’s New System of Geography (~1800).

This chapter focuses on the origins, composition and flux, and the environmental impacts of volcanic volatile emissions. This introductory section sets the scene by considering the general context and significance of volcanic degassing. Several chapters in this volume interface with this one on volcanic degassing, and in particular the reader is referred to the chapters on hydrothermal systems and ore formation.

1.1.1 Earth Outgassing, Atmospheric Evolution and Global Climate

Volcanic emissions have occurred throughout Earth history, and have provided the inventory of volatile elements that take part in the major geochemical cycles involving the lithosphere, hydrosphere, atmosphere, and biosphere (Sections 1.2 and 1.5; Holland, 1984; Arthur, 2000; see Chapter 5). The mantle is an important reservoir for volatiles, and its concentration of carbon, sulfur, hydrogen, oxygen, and halogens has changed through Earth history as a result of differentiation. Anhydrous minerals such as olivine, pyroxene, and garnet can hold structurally bound OH⁻, while molecular water is present in amphibole, phologopite, and apatite. Carbon is present in carbonate minerals or in elemental form (e.g., diamond and graphite), and sulfur in sulfide minerals. Volatiles probably also exist in the mantle in intergranular films. Since the volatile species are incompatible, they partition into the melt phase during partial melting of the mantle. In this way, magmagenesis plays a key role in transferring volatiles between the mantle and the crust. Magma evolution then partitions volatiles between the crust and the atmosphere/hydrosphere via degassing and eruption (with important feedbacks on magma differentiation), and plate recycling ensures a return flux of a proportion of the volatiles back to the mantle.

Major changes have occurred in atmospheric composition and in greenhouse gas forcing over Earth history, in part coupled to the evolution of life, and interacting with changes in solar flux and planetary albedo to control global climate. Over timescales exceeding 1 Myr, the carbon cycle operates as a climate thermostat on the Earth. The Archean atmosphere was anoxic, even after the onset of oxygenic ph...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright page
• Introduction
• Contributors
• 1: Volcanic Degassing
• 2: Hydrothermal Processes
• 3: The Contemporary Carbon Cycle
• 4: The Global Sulfur Cycle
• 5: The History of Planetary Degassing as Recorded by Noble Gases
• 6: Natural Weathering Rates of Silicate Minerals
• 7: Soil Formation
• 8: Global Occurrence of Major Elements in Rivers
• 9: Trace Elements in River Waters
• 10: The Geologic History of the Carbon Cycle
• 11: Organic Matter in the Contemporary Ocean
• 12: The Biological Pump
• 13: The Biological Pump in the Past
• 14: The Oceanic CaCO3 Cycle
• 15: The Global Oxygen Cycle
• 16: The Global Nitrogen Cycle
• 17: Evolution of Sedimentary Rocks
• 18: Generation of Mobile Components during Subduction of Oceanic Crust
• Appendix 1: Periodic Table of the Elements
• Appendix 2: Table of Isotopes
• Appendix 3: The Geologic Timescale
• Appendix 4: Useful Values
• Index