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 150 km, large strain deformation occurred at low stress (10 MPa) and modest temperature (1300 K), involving high water content (>1000 H/106Si in olivine). This was followed by low strain deformation at lower water content (200–1000 H/106Si) and modest stress (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.