LOCATION OF EXTRANEOUS ARGON IN GRANULITIC-FACIES MINERALS - A PAIRED MICROPROBE LASER PROBE 40AR/39AR ANALYSIS

We have investigated minerals from the In Ouzzal granulitic unit (Sahara, Algeria). As excess 40Ar frequently occurs in these rocks, we have applied the 40Ar 39Ar laser probe technique in an attempt to determine the location of the excess component in the minerals. Combining both electron microprobe and laser probe analyses, the excess 40Ar component was found to be located on the rim of the biotite and in the microcracks of the pyroxene and garnet which are filled by symplectic secondary minerals such as cordierite, secondary pyroxene, spinel and sapphirine. The cores of the biotites yield geologically meaningful ages as indicated by the lack of excess 40Ar in this part of the crystal. In plagioclase, the location of the excess component could not be pinpointed, indicating distribution throughout the mineral. The excess 40Ar is irregularly distributed throughout the perthitic feldspars. However, for the perthitic and plagioclase phases, conclusions are limited by the spatial resolution of the laser beam and size of the lasered areas. These results are compared with age spectra obtained by incremental heating applied to pure mineral separates. The interpretation in terms of energy site occupancy is enlightened by laser probe results. The incorporation of the excess 40Ar in biotite appears to have originated from the outgassing of 1.8-Ga-old perthitic phases during the Pan-African orogenic (600 Ma) which had resulted in an increase of Ar partial pressure in the granulitic rocks. Incorporation of excess 40Ar in garnet and pyroxene was contemporaneous with the formation of symplectites ∼2 Ga ago. © 1990.