Diffusion creep in the upper mantle: An example from the Tanlu Fault, northeastern China

Ultramafic xenoliths in Neogene alkalic basalts emplaced within the Yitong graben associated with the Tanlu Fault, northeastern China, are mostly variably deformed Iherzolites, werhlites and minor pyroxenites. Glass, rich in SiO2, Al2O3, K2O and Na2O, is optically recognised as occurring in veins, triple-point grain junctions, and patches in werhlite and some Iherzolite samples. Textural observations also suggest that the glass (melt) was involved in the deformation, and was present prior to sampling by the host basalt. Electron microprobe scans for these glass elements indicate that glass is not only present at triple-point grain junctions and in veins, but also completely along some planar grain boundaries. A complex textural history is recognised. The earliest recognised deformation resulted in a reduction in grain size through granuloblastic and porphyroclastsic, with a subsequent relaxation event involving olivine grain growth. A final deformation event producing further grain-size reduction and a mylonitic mosaic texture. Geothermometry calculations from electron microprobe data indicate that the early event occurred at similar to 1000 degrees C, while the final event occurred between 800 and 750 degrees C; the latter event occurred near the glass transition temperatures calculated on the basis of thermodynamics and chemical composition(s) of the glass. Olivine rheology and textures suggest that the early event was accomplished by grain-size insensitive non-linear flow (dislocation creep), with the second event accomplished by grain-size sensitive linear flow (diffusion creep), These two events are believed to have occurred at different times, depths and temperatures within the upper mantle beneath Yitong. They are envisaged being related to changing displacements of the Pacific Plate during its subduction beneath the Asiatic Plate in eastern China during Tertiary times that resulted in the activation of the long-lived and historically active Tanlu Fault.