Non-radiogenic argon is generally observed in terrestrial rocks and can be classified into two components : atmospheric and non-atmospheric argon. Atmospheric argon is trapped both in unretentive sites such as the surface of minerals, grain boundaries, interstitials, etc. and retentive sites such as crystal lattices. The amount of atmospheric argon trapped in minerals or rocks is not so small, and depends on mineral structures, grain sizes or degree of alteration. Hence, old volcanic or sedimentary rocks may contain atmospheric argon when the rocks were formed. Among the non-atmospheric argon, excess
40Ar occurs in such geological environments where high partial pressure of
40Ar exists or rapid cooling prevents the complete degassing of the pre-existed
40Ar in the rock. Excess
40Ar in deep-seated rocks might have been produced as dissolved argon. Further, in some deep-seated rocks,
36Ar has been observed to correlate with excess
40Ar. Such
36Ar might be brought from the earth's deep interior and may be primordial.
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