Similar to the case of He discussed in the last section, terrestrial Ar is dominated by its radiogenic isotope 40Ar, produced by the radioactive decay of 40K23. Indeed 40Ar forms a significant fraction of the modern atmosphere at 0.934%, as unlike 4He, which escapes to space, 40Ar has accumulated over Earth history. The predominance of the radiogenic isotope in terrestrial Ar is illustrated by the reference atmospheric ratios of 40Ar/36Ar = 298.6 and 38Ar/36Ar = 0.18824. Figure 4.1 further shows the variation of these two...

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