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Chapter 10 : The analysis and interpretation of noble gases in modern hydrothermal systems

by Y. Sano and T. Fischer

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 on large (100-1000 km) and small scales (1-100 km), temporal variation of helium isotopes in some volcanoes, and the heavy noble gas isotope and abundance variations in hydrothermal systems. 1. Details of sampling method of volcanic and hot spring gases are discussed together with the characteristics of two types of containers, Giggenbach-type and lead-glass. 2. Analytical techniques of noble gas abundances by isotope dilution methods (on quadrupole-based systems), and by peak height comparison using magnetic sector type mass spectrometers including the effects of neon interference on helium isotope measurements are comprehensively covered. 3. Helium isotope variations in modern volcanic regions, subdivided into hot spots, mid-ocean ridges, and subduction zones are compiled and discussed together with their geo-tectonic settings and geophysical data. 4. Across arc variations of helium isotopes are described in relation to recent seismic tomography data from Northeast Japan, Southwest Japan, North Island of New Zealand, and Kamchatka peninsula of Russia. Then, on a smaller spatial scale, the isotope variations around independent volcanos such as Mt Ontake and Mt. Nevado del Ruiz are discussed. 5. 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. 6. The isotopic compositions of neon, argon, krypton and xenon isotopes in volcanic and hydrothermal systems are discussed in relation to mantle and crustal degassing processes. 7. The final section provides applications of noble gases for tracing 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, the 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.


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Chapter 10

12 juin 2012
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30.7 Mo