Rheological properties of the lower mantle have strong influence on the dynamics and evolution of Earth. By using the improved methods of quantitative deformation experiments at high pressures and temperatures, we deformed a mixture of bridgmanite and magnesiowüstite under the shallow lower mantle conditions. We conducted experiments up to about 100% strain at a strain rate of about 3 × 10(-5) second(-1). We found that bridgmanite is substantially stronger than magnesiowüstite and that magnesiowüstite largely accommodates the strain. Our results suggest that strain weakening and resultant shear localization likely occur in the lower mantle. This would explain the preservation of long-lived geochemical reservoirs and the lack of seismic anisotropy in the majority of the lower mantle except the boundary layers.

a b s t r a c tObservations of seismic anisotropy can offer relatively direct constraints on patterns of mantle deformation, but most studies have focused on the upper mantle. While much of the lower mantle is thought to be isotropic, several recent studies have found evidence for anisotropy in the transition zone and uppermost lower mantle (the mid-mantle), particularly in the vicinity of subducting slabs. Here we investigate anisotropy at mid-mantle depths in the Tonga-Kermadec, Sumatra, New Britain, New Hebrides, and Philippines subduction zones using the source-side shear wave splitting technique. We measure splitting of direct teleseismic S phases originating from deep events (>300 km) that have been corrected for the effect of upper mantle anisotropy beneath the seismic stations. We find evidence for considerable anisotropy at mid-mantle depths in all subduction systems studied, with delay times averaging 1.0-1.5 s. Several measurements originating from depths greater than 600 km exhibit delay times greater than 1 s, suggesting a significant contribution from anisotropy in the uppermost lower mantle. We combine our results with those documented in previous studies into a quasi-global set of source-side shear wave splitting measurements that reflect mid-mantle anisotropy. We document significant variability in the dominant fast directions both within and among individual subduction systems, suggesting different deformation geometries among different subduction systems. As further constraints on the elasticity and deformation of mid-mantle minerals become available, our dataset can be used to constrain patterns of mid-mantle flow associated with subduction.

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Key Points Anhydrous phase B and stishovite formed directly from olivine in experiments at 14 GPa and 1400 °CThe structure of anhydrous phase B is determined ab initio from precession electron diffraction tomography in transmission electron microscopyElastic and seismic properties of anhydrous phase B are calculated

Quantifying the water loss of Himalayan glaciers due to global warming from direct measurement is difficult, as some glaciers are advancing or stable in spite of an overall retreat. We use a novel approach to provide an alternative estimate of the amount of Himalayan ice melt. Because a major part of this melted ice debouches into the Bay of Bengal through the Ganga–Brahmaputra basin, it causes significant variations in the oxygen isotopic composition (δ18O) and salinity (S) of the sea surface water and their mutual linear relationship. We document the temporal change in the δ18O–S relation for the bay at three different times during the period from 1994 to 2006, and using a model, we infer that 2.4 × 1011 m3 water was lost by melting from the Ganga–Brahmaputra basin during this period.