Seismically deduced thermodynamics phase diagrams for the mantle transition zone

- We use seismic images of transition zone discontinuities from Ps receiver functions.
- We develop a method to estimate the Clapeyron slopes of mantle phase transitions.
- We build “seismic phase diagrams”, a proxy for thermodynamics phase diagrams.
- We suggest the presence of a few minor phase changes and estimate their Clapeyron slopes.

Seismic discontinuities at 410 and 660 km depth are usually attributed to solid phase changes within the olivine component of the mantle. The Clapeyron slopes γ410 and γ660, i.e. the thermal dependence of the depths of reactions, have been shown experimentally to be of opposite signs. Yet, their values are not well constrained by laboratory measurements. Seismological observations have not been more precise due to the difficulty to separate the competing effects of background wave-velocities and of temperature on the topography of discontinuities. In this study we use conversion imaging of interfaces under western US. We propose a new approach to derive a seismological estimate of the Clapeyron slopes with respect to γ410 for the major and minor phase changes of the transition zone. We obtain γ660≈−3 MPaK−1 for γ410≈+3 MPaK−1. We construct “seismic phase diagrams” of the transition zone that can be directly compared with experimental phase diagrams. We also apply a “Z-Γ” transform to better constrain the Clapeyron slopes γ of the minor phase changes. Although tenuous, signals in seismic phase diagrams suggest that minor phase transitions, both in the olivine and the non-olivine component of the mantle, have visible seismic expressions. They can tentatively be described as follows. The '410' is overlaid at low temperature by an interface corresponding to a decrease of velocity with depth and γ≈+3 MPaK−1. The '660' widens at high temperature and is preceded at low temperature by an interface, the '620', with γ≈+7 MPaK−1. A '520' is suggested with γ≈2-3 MPaK−1. These last two interfaces correspond to velocity increases with depth. At last, near 590 km depth, an interface may be associated with a velocity reduction showing a weak dependence on temperature (γ∼0 MPaK−1).