Effect of water in depleted mantle on post-spinel transition and implication for 660 km seismic discontinuity

We have determined the post-spinel transition boundary in anhydrous and hydrous Mg2SiO4 in a temperature range from 1173 to 2023 K at 19.3–25.4 GPa using synchrotron in situ X-ray diffraction measurements. The phase boundary in Mg2SiO4 is located at 22 GPa and 1800 K and 22.1 GPa and 1500 K, which is slightly lower (~0.3–0.5 GPa) than that determined in the previous in situ measurements using the same pressure scale [e.g. Katsura et al., 2003, Post-spinel transition in Mg2SiO4 determined by high P–T in situ X-ray diffractometry. Phys. Earth Planet. Inter. 136, 11–24]. The Clapeyron slope of Mg2SiO4 was found to be gentle i.e. between −0.4 and −0.7 MPa/K, which is also consistent with previous in situ measurements, but inconsistent with diamond anvil cell experiments and theoretical estimations. The phase boundary in Mg2SiO4+2 wt% H2O which is relevant to Fe free-depleted harzburgitic composition is located between 23.4 and 23.6 GPa and 1500 K, which shifts the hydrous boundary to the higher pressures relative to anhydrous Mg2SiO4 from 1.3 to 1.0 GPa. The result for hydrous Mg2SiO4 shows steeper Clapeyron slope between −3.2 and −3.1 MPa/K compared with anhydrous Mg2SiO4 and hydrous pyrolite system. The present data suggest that water has a strong influence on 660 km discontinuity and the depressions observed at this boundary in several regions, especially related to subduction zones, can be explained by the presence of water in depleted harzburgite component.