Discussion of "Extracting K-Ar ages from shales: a theoretical test"

Illite age analysis (Pevear, 1992) and similar mixing methods demonstrate n ear-linear behaviour when corrected for smectite content in illite-smectite and substitutions for K+ in illite layers. Srodon (1999) found that highly curved mixing lines result from theoretical calculations and challenged th e validity of IAA. However, our theoretical calculations indicate near-line ar mixing lines result when corrected for the amount of smectite and known K+ substitutions (Na+ or NH4+). We conclude that perfect linearity is unatt ainable because IAA attempts to unmix a system linearly, but the system is based on an exponential function (K-Ar equation). In practice, we believe I AA can successfully determine the age range of illite diagenesis, even thou gh perfect linearity is not attainable because: (1) significant deviations from linearity occur only if >500 Ma separate the diagenetic and detrital a ges; (2) IAA analytical uncertainties result in diagenetic and detrital age s that span a range of ages.