Occurrence of Excess 40Ar in Amphibole: Implications of 40Ar/39Ar Dating by Laser Stepwise Heating and in vacuo Crushing

The joint methods of 40Ar/39Ar laser stepwise heating and in vacuo crushing have been applied to date amphiboles from the North Qaidam ultra-high pressure metamorphic amphibolites. Two amphibole samples analyzed by laser heating yielded saddle-shaped age spectra with total gas ages of 574.5 +/- 2.5 and 562.5 +/- 2.5 Ma. These ages are much older than the reported zircon U-Pb ages (similar to 495 Ma) from Yuka eclogite, indicating the presence of excess 40Ar. In order to decipher the occurrence of excess 40Ar and constrain the age of amphibolite-facies retrogression, two duplicate amphibole samples were further employed for 40Ar/39Ar in vacuo crushing analyses. Both samples exhibit similar monotonically declining release spectra, which are characterized by rapid decline of anomalously old apparent ages in the early steps. The data of the late steps yielded concordant apparent ages with plateau ages of 460.9 +/- 1.2 and 459.6 +/- 1.8 Ma. We interpret that gases released in the early steps derive from the significant excess 40Ar containing secondary fluid inclusions (SFIs) due to their distribution characteristics along cracks leading to be easily extracted, whereas those released in the later steps represent the contribution of the small primary fluid inclusions (PFIs).