We detail here the general concepts behind using noble gases as a tracer of crustal fluid processes and illustrate these concepts with examples applied to oil-gas-ground-water systems, mineralizing fluids, hydrothermal systems and ancient ground-waters. Many of the concepts and processes discussed here are also directly applicable to the study of young ground and surface-water systems (Kipfer et al. 2002, this volume).

Same As It Ever Was: Nitrogen constitutes approximately 78% by volume of Earth's atmosphere and is a key component in its chemical and physical characteristics. It is not clear whether N ₂ has been so abundant throughout Earth's geological history. Marty et al. (p. 101 , published online 19 September) analyzed the isotopic compositions of nitrogen and argon from fluid inclusions trapped in hydrothermal quartz formed 3 to 3.5 billion years ago. The partial pressure and isotopic composition of atmospheric N ₂ were similar to today's. Thus, other factors are needed to explain why liquid water existed on Earth's surface despite the Sun being 30% less luminous.