Figure 4. Degassing of individual isotopes ⁴⁰Ar*, ³⁹Ar_K, and ³⁷Ar_Ca in millivolt per degree Celsius, vs. temperature, for plagioclases 83R77-82 (solid line) and 85R18-23 (dashed line) (1 millivolt = 7 × 10^-13 cm³ STP). Because similar quantities of sample were analyzed, the relative volumes of gas released can be compared.