The effect of chlorite interlayering on 40Ar-39Ar biotite dating: an 40Ar-39Ar laserprobe and TEM investigation of variably chloritised biotites.

This study examines the outcome of an 40Ar-39Ar investigation on variably chloritised biotites from Ordovician intrusive rocks. The IR laserprobe technique and different gas extraction methods were adopted. Laser step-heating data on bulk samples yielded hump-shaped age spectra with meaningless young and old age-steps. Both the extent of anomalous old age-steps and the degree of discordance of the spectra were much more pronounced in the more chloritised biotite. In contrast, in-situ data on rock chips and total-fusion ages on single biotite flakes yielded ages concordant with, or younger than, the inferred emplacement ages. TEM was used to texturally characterise samples at the nanometer scale and to document the complex decomposition-transformation process affecting interlayered biotite-chlorite during in-vacuo IR-laser heating (from ~600 to >1000°C). TEM results suggest that hump-shaped age profiles result from an interplay between 39Ar redistribution by recoil during sample irradiation and differential release of argon isotopes hosted in three main reservoirs (from least to most retentive): extended defects, chlorite and biotite. The final descending age segment is attributed to the progressive release of argon with increasing T from large biotite domains for which 39Ar recoil loss was less important. Results suggest that 40Ar-39Ar ages when recoil effects are minimised, provide minimum estimates that approach the true biotite age, when pristine domains are analysed.