Seismically deduced thermodynamics phase diagrams for the mantle transition zone

Abstract

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 γ 410 and γ 660 , 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 γ 410 for the major and minor phase changes of the transition zone. We obtain γ 660 ≈ −3 MPa K −1 for γ 410 ≈ +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 MPa K −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 MPa K −1 ).

The upper-mantle transition zone beneath the Ibero-Maghrebian region as seen by teleseismic Pds phases

“…Nevertheless, the values in Fig. S5a and b before correction are very low compared to other studies (Chevrot et al, 1999;Gao and Liu, 2014;Tauzin and Ricard, 2014). These poor correlation values probably indicate that depth changes are not controlled primarily by temperature changes and/or that the velocity structure in the upper-mantle and the TZ are anti-correlated with anomalies of the same order but opposite sign.…”

“…Nevertheless, the mantle structure leads to large positive correlation of the observed depth values (and relative travel times), e.g., +0.94 in Chevrot et al (1999), +0.84 in Gao and Liu (2014), and +0.6 in Tauzin and Ricard (2014). Ideally, a negative or near zero correlation value is expected after correction, but in practice it is observed that velocity corrections reduce the correlation to smaller positive values (e.g., Gao and Liu, 2014;Tauzin, 2009).…”

“…This is expected in studies covering smaller areas. For example, Tauzin and Ricard (2014) studied an area beneath west USA which is contained in the study area of Gao and Liu (2014) and found a smaller correlation value than in Gao and Liu (2014).…”

The Mantle Transition Zone in Fennoscandia: Enigmatic High Topography Without Deep Mantle Thermal Anomaly

“…Assuming homogeneous composition, we expect that the MTZ is thicker than normal at low temperature and thinner than normal at high temperature. apparent depths of discontinuities and introduce a positive correlation that is opposite to the temperature effect (e.g., Tauzin & Ricard, 2014). However, this ideal picture may be complicated by several effects.…”

Topography of the 410 km and 660 km discontinuities beneath the Japan Sea and adjacent regions by analysis of multiple‐ScS waves

“…Using equations and and assuming a vertical incidence ( p = 0) of the ray, the “true” depth of the discontinuity d can be expressed as where dv ( z ) is the depth‐varying S wave velocity anomaly, which can be obtained from seismic tomography models. We adopted this simple 3‐D velocity correction to obtain the true depth of discontinuities, in the same manner as performed by some receiver function studies [ Gao and Liu , ; Tauzin and Ricard , ].…”