HEAT AND HELIUM IN GEOTHERMAL SYSTEMS

Abstract

The bulk of the Earth's heat budget and all of the 4He is produced by natural uranium and thorium radioactivity, thus uniquely coupling heat and helium in geothermal systems. In the relatively simple geothermal systems associated with mid-ocean ridges, the helium isotopic composition and heat/3He ratios are similar to the theoretically predicted values, confirming the coherence between heat and helium. These systems also reveal that magma degassing and aging can fractionate helium from heat.

In the more complicated continental geothermal systems, heat-helium coherence provides a technique for identifying and calculating the proportion of heat derived from each source (crust vs. mantle). Processes in addition to magma degassing and aging, such as adiabatic cooling, fluid mixing, and conductive heat loss, act on these systems fractionating helium from heat. However, detailed studies of individual fields can uniquely identify the heat-helium fractionation processes, which can then be used to constrain reservoir models.