Comment on ‘Paleozoic ages and excess 40Ar in garnets from the Bixiling eclogite in Dabieshan, China: New insights from 40Ar/ 39Ar dating by stepwise crushing by Hua-Ning Qiu and J.R. Wijbrans’

Abstract

In a recent study, Qiu and Wijbrans (2006) [Qiu, H.-N. and Wijbrans, J. R. (2006). Paleozoic ages and excess 40Ar in garnets from the Bixiling eclogite in Dabieshan, China: new insights from 40Ar/ 39Ar dating by stepwise crushing. Geochimica et Cosmochimica Acta 70, 2354–2370.] analyzed Ar-isotopes extracted by crushing garnets from the Dabie Shan Bixiling eclogite and claimed to have constrained a pre-Triassic (∼450 Ma) episode of UHP metamorphism from primary fluid inclusions. However, in the absence of careful sample characterization and stepped heating analyses, the reported ages are more easily explained as experimental artifacts related to Ar extracted from either mineral inclusions or the interface sites between mineral inclusions and the garnet matrix: Dabie Shan garnets commonly contain mineral impurities such as K-rich omphacite and/or K-feldspar. If Dabie Shan UHP metamorphism is of the generally accepted Triassic age (210–240 Ma), the apparent age of a phengite sample in equilibrium with the garnet can be explained by the presence of an extraneous 40Ar ∗ component with mean 40Ar/ 36Ar value of ∼5000. This value is similar to the composition of extraneous 40Ar ∗ in other eclogite facies terrane.