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Helium is formed as a product of radioactive decay of trace elements in several rocks and minerals. It is being produced continually in the earth's crust by the disintegration of uranium, thorium, and other elements which are alpha-particle emitters.
Helium-generating potential of a material is a measure of its alpha emission activity. One gram of uranium may generate 1.16 × 10-7 ml of helium in 1 year.
Nearly all igneous rocks contain trace amounts of the uranium series. The acidic types are usually significantly more radioactive than those of the basic types. The helium-generating potential of sedimentary and metamorphic rocks appears to be dependent upon the rock type and its history rather than on the age or location of the sample. Comparative Specific Radiation Activity (SRA) values for the three classes of rocks are presented, with the calculated percentages of uranium in each sample.
The helium-generating potential of sedimentary rocks is a particular study: as a uranium-bearing granite is emplaced, cooled, uplifted, and eroded, for example, alpha particles are emitted at a constant rate. During transportation of the sands, silts, and shales, and deposition as sediments, these clastic grains continue to give off alpha particles which become helium atoms that are trapped in the rocks.
Helium cannot be trapped permanently in a geologic trap. Rather, the helium will diffuse out, or migrate out, over a period of geologic time. Helium must either be generated or migrate into a trap, be detained by the geometry of the rock and then either move out by migration or by diffusion. The helium that is produced is "in transit" through the rocks in the trap. A given trap has the ability to hold no more than 2% helium depending on the geometry of the rock, the pressures, and the helium holders associated with the trap.
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