ABSTRACT

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.