Overview

The group of elements known as the rare, inert or noble gases possess unique properties that make them important as geodynamic tracers. The daughter isotopes fractionate readily from their parents, they are inert, and they differ from other geochemical tracers in being gases and diffusing relatively rapidly, at least at low pressure and high temperature. Nevertheless, they can be trapped in crystals for long periods of time. They give information about the degassing history of the mantle and magmas, the formation of the atmosphere, and about mixing relationships between different mantle components. Isotopes made during the Big Bang are called the ‘primordial’ isotopes. Some of the noble gas isotopes are cosmogenic, nucleogenic or radiogenic and these were made at later times. The presence of 3He in a rock or magma has often been taken as evidence for the existence of a primordial reservoir in the mantle, one that had not previously been melted or degassed. A primordial reservoir is different from a primordial component. Primordial and solar components are still raining down on Earth. The most primordial materials on Earth, in terms of noble gases, are on mountain tops, in the stratosphere and in deep-ocean sediments.

Evidence for recent additions of noble gases to the Earth comes from deep-sea sediments where high concentrations of He (written [He]), high 3He/4He ratios (written R) and ‘solar’ neon are found.