Evidence of high water content in the deep upper mantle inferred from deformation microstructures

摘要

Deep upper-mantle rocks from the Norwegian Caledonides show evidence for large strain deformation in both olivine and garnet under varying water contents. Using microstructural observations, including lattice-preferred orientation of olivine and subgrain boundaries of majoritic garnet, we infer the following deformation history. At depths exceeding similar to 150 km, large strain deformation occurred at low stress (similar to 10 MPa) and modest temperature (similar to 1300 K), involving high water content (> 1000 H/10(6)Si in olivine). This was followed by low strain deformation at lower water content (similar to 200-1000 H/10(6)Si) and modest stress (similar to 40 MPa) in the shallower parts. These observations show that the deep upper mantle in this region had a considerably higher water content than the upper mantle near mid-ocean ridges.