Limited mobility of argon in a metamorphic terrain

Abstract

Excess 40Ar in biotite from some relatively anhydrous charnockitic rocks in the Appalachian Piedmont indicates limited mobility of argon. Biotite from the Arden pluton of the granulite-facies Wilmington Complex apparently formed as a retrograde product at the expense of pyroxene and K-feldspar. Rb-Sr ages of biotite from all rocks are approximately 365 Myr. The same micas have apparent K-Ar ages which range from about 365–590 Myr, six of which clearly exceed the Sr isotope whole-rock date of 500 Myr. They contain variable amounts of excess 40Ar incorporated during crystallization or recrystallization of biotite at about 365 Myr ago. None of the other minerals appears to contain significant amounts of excess argon. The K-Ar apparent ages show strong, positive correlation with whole-rock K concentrations. These relations yield a correlation between excess argon in the biotite phase and rock potassium. This suggests that excess 40Ar in biotite is of local derivation and is due to an imprint of the local argon activity. If the amount incorporated is roughly proportional to the prevailing argon partial pressure then substantial differences in p Ar existed. Argon did not have a uniform chemical potential over large rock volumes. Analysis of closely spaced samples suggests different argon activity over the scale of less than 10m. This implies restricted transport of Ar and is probably due to very low effective permeability of the anhydrous assemblages.