Large‐scale mantle discontinuity topography beneath Europe: Signature of akimotoite in subducting slabs

Abstract

The mantle transition zone is delineated by seismic discontinuities around 410 and 660 km, which are generally related to mineral phase transitions. Study of the topography of the discontinuities further constrains which phase transitions play a role and, combined with their Clapeyron slopes, what temperature variations occur. Here we use P to S converted seismic waves or receiver functions to study the topography of the mantle seismic discontinuities beneath Europe and the effect of subducting and ponding slabs beneath southern Europe on these features. We combine roughly 28,000 of the highest quality receiver functions into a common conversion point stack. In the topography of the discontinuity around 660 km, we find broadscale depressions of 30 km beneath central Europe and around the Mediterranean. These depressions do not correlate with any topography on the discontinuity around 410 km. Explaining these strong depressions by purely thermal effects on the dissociation of ringwoodite to bridgmanite and periclase requires unrealistically large temperature reductions. Presence of several wt % water in ringwoodite leads to a deeper phase transition, but complementary observations, such as elevated Vp/Vs ratio, attenuation, and electrical conductivity, are not observed beneath central Europe. Our preferred hypothesis is the dissociation of ringwoodite into akimotoite and periclase in cold downwelling slabs at the bottom of the transition zone. The strongly negative Clapeyron slope predicted for the subsequent transition of akimotoite to bridgmanite explains the depression with a temperature reduction of 200–300 K and provides a mechanism to pond slabs in the first place.

Thermodynamic and Elastic Properties of Grossular at High Pressures and High Temperatures: A First‐Principles Study

“…Therefore, one can find that the phase transition of garnet could produce much larger velocity and density contrasts than those caused by the postspinel transition (Figure ). Although the actual velocity and density jumps depend on the amount of garnet, the phase transition of garnet may account for the detected complicated multiple discontinuity around 660 km (Cottaar & Deuss, ; Deuss, ; Simmons & Gurrola, ). The Clapeyron slope might be helpful to distinguish one phase transition from others (e.g., ~0 MPa/K for grossular; ~ +2.1 MPa/K for pyrope; ~1.2 MPa/K for majorite; and −2.9 MPa/K for ringwoodite; e.g., Akaogi et al, ; Hirose et al, ; Wang & Wu, ; Yu et al, ; Yu et al, ), but more researches are needed to clarify their phase boundaries, especially the one for grossular.…”

Mantle Structure in the Central Mediterranean Region From P and S Receiver Functions

“…In the central Mediterranean area a number of RF studies have focused on the crust (Amato et al, ; Piana Agostinetti & Amato, ; van der Meijde et al, ) and the lithosphere‐asthenosphere boundary (Belinić et al, ; Bianchi et al, ; Geissler et al, ; Miller & Piana Agostinetti, ; Miller & Piana Agostinetti, ). Several PRF studies investigated the upper mantle down to the transition zone (Cottaar & Deuss, ; Liu et al, ; Lombardi et al, ; van der Meijde et al, ; van der Meijde et al, ) and a very recent SRF work focused on the sublithospheric discontinuities (midlithospheric discontinuity and sublithospheric discontinuity) below central Europe (Kind et al, ). All these studies highlight a strong top‐to‐bottom and lateral heterogeneity in this area, where depth, topography of the discontinuities, and the character of the converted phase pulses appear to correlate with the different tectonic regions and the complexity of the various subduction zones.…”

“…Thickening of the MTZ by 10 to 40 km, mainly due to depression of the 660‐km discontinuity with no correlated anomalous topography of the 410, has been observed beneath the Pannonian Basin‐central Europe (Cottaar & Deuss, ; Hetényi et al, ), south of the Alps and western Alps (Cottaar & Deuss, ; Lombardi et al, ), in regions affected by past subductions. This increase in MTZ thickness has been considered of thermal origin, and unanimously related to the accumulation of cold subducted lithosphere presently ponding above 660‐km depth, although compositional effects could also be involved (Cottaar & Deuss, ). In southern Italy (Calabria) van der Meijde, Van Der Lee, and Giardini () observed an uplifted 410 and a thickened MTZ, and explained it as caused by the ongoing subduction of the Calabrian slab.…”

Stochastic Inversion of P‐to‐S Converted Waves for Mantle Composition and Thermal Structure: Methodology and Application

“…Constraints on absolute velocities are typically gained by adding a priori information from a global/regional seismic model (e.g., Ammon et ., ) or by combining RF waveforms with other geophysical observables such as surface wave group and phase velocities (e.g., Calò et al, ; Moorkamp et al, ; Shen et al, ). More commonly, RF waveforms are used to identify discontinuities in seismic velocities such as the Moho (e.g., Calò et al, ; Kind & Vinnik, ; Langston & Hammer, ; Lodge & Helffrich, ; Lombardi et al, ), the lithosphere‐asthenosphere boundary (Heit et al, ; Kind et al, ; Kumar et al, ; Sodoudi et al, ), and the 410‐ and 660‐km discontinuities (Cottaar & Deuss, ; Lawrence & Shearer, ; Tauzin et al, ; Vinnik et al, ). The latter discontinuities are the most seismically visible features of the mantle transition zone (TZ).…”