O-isotope variations in a porphyroclastic meta-anorthosite: diffusion effects and false isotherms

A detailed O-isotope study of annealed porphyroclastic meta-anorthosite was performed to assess the degree of post-peak-metamorphic and postshearing reequilibration. Stable isotope and petrologic evidence indicate that the Whitestone anorthosite, Ontario, was infiltrated by an ¹⁸O- and CO₂-rich fluid subsequent to peak Grenville granulite facies metamorphism. Ductile shearing occurred during and following peak metamorphism. The theoretical δ¹⁸O values of all minerals were calculated on the basis of O-volume diffusional considerations using diffusion rate data for both hydrous and anhydrous conditions. The Whitestone data set is in far better agreement with the diffusion data obtained in hydrothermal experiments, indicating that cooling probably occurred in the presence of a hydrous fluid. -from Authors