³⁹Ar recoil artifacts in chloritized biotite

Biotite separates from the metamorphosed granitic rocks of eastern Taiwan yield disturbed ⁴⁰Ar ³⁹Ar age spectra. Their disturbed profiles become more pronounced with increasing degree of chloritization and are attributed to mixed release of argon from sub-micron scale biotite and chlorite intercalations. ³⁹Ar internal recoil best explains the discordance of the age spectra. Chlorite is randomly intercalated with biotite at scales ranging from 10 Å to several tens of microns. Both ⁴⁰Ar^* and K were evidently lost during the chloritization reaction. During the neutron irradiation, ³⁹Ar atoms recoil into the chlorite from the adjacent biotite intercalations. The recoil and chloritization combine to produce a variable ⁴⁰Ar ³⁹Ar spatial distribution in the intercalated biotite/chlorite. During ⁴⁰Ar ³⁹Ar step-heating analyses, argon in biotite is released in two pulses centered at ~650°C and 950-1050°C which represent two stages of dehydroxylation of the biotite. Chlorite out-gasses in two pulses centered at ~550°C and 700-900°C during dehydroxylation and then decomposes. This differential thermal release of argon from each mineral phase with variable ⁴⁰Ar ³⁹Ar ratios yields discordant incremental heating age spectra.