TRACING FLUID ORIGIN, TRANSPORT AND INTERACTION IN THE CRUST

Abstract

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). Noble gases in the Earth are broadly derived from two sources; noble gases trapped during the accretionary process (often called ‘primitive’, ‘juvenile’ or ‘primordial’ noble gases), and those generated by radioactive processes (e.g., Ballentine and Burnard 2002, this volume). Differentiation of the Earth into mantle and continental crust, degassing and early processes of atmosphere loss has resulted in the formation of reservoirs in which the abundance pattern and isotopic compositions of primitive noble gases have been variably altered. Combined with their different radioelement concentrations (U, Th, K) producing radiogenic noble gases, the mantle, crust and atmosphere are now distinct in both their noble gas isotopic composition and relative elemental abundance pattern. Fluids that originate from these different sources will contain noble gases that are therefore isotopically distinct and resolvable (Fig. 1⇓). Because the noble gases are chemically inert even if these fluids are lost through reaction or masked by addition of similar species from different sources, a conservative record of their presence and origin is preserved by the noble gases. Once resolved, the noble gas abundance patterns from the respective sources are particularly important, as these are sensitive to physical processes of fractionation. For example, from the distinct fractionation patterns it is possible to distinguish between for example, diffusive or advective transport processes. Similarly the abundance patterns enable the interaction of different phases to be identified and quantified. In a system that has fluids sourced from …