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

Abstract

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.