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.
All Science Journal Classification (ASJC) codes
- Geochemistry and Petrology