Dynamics of a phase change at the base of the mantle consistent with seismological observations

Abstract

Abstract. The phase change model for the origin of the D" seismic discontinuity is tested by comparing the results of convection modeling with seismic observations. We compute a number of global dynamic models that incorporate a phase change at the base of the mantle with different characteristics and transform the resulting temperature field and the distribution of phases to seismic velocities. Over 900 two-dimensional synthetic waveforms are computed for each of the models from which $, $cS, and $cd phases are picked. The distribution of the relative amplitudes and differential travel time residuals for these phases are statistically compared with the distribution of data from four well studied regions (northern Siberia, Alaska, India, and Central America) in a search for the characteristics of a phase transition that best match these seismic observations. We find that the best fit among the models considered is obtained for phase transitions characterized by aClapeyron slope of • 6 MPa K -• and an elevation above the core-mantle boundary of -• 150 km under adiabatic temperature or 127 GPa and 2650 K on a (P,T)diagram. Dynamic models demonstrate that the value of Clapeyron slope and the density difference between the phases can have significant influence on the dynamics of plumes but probably only a minor influence on the dynamics of subducted slabs. We find that the thermal structure of subducted slabs can be important in giving rise to the seismic triplication; the strongest $cd arrivals in our models are observed in the area of subduction. The folding of the slab at the base of the mantle leads to patterns in differential travel time distributions consistent with seismic observations and suggests that the largest heterogeneity occurs at the top of the D" layer or just above it. Analysis of the spatial autocorrelation functions of the differential travel time residuals suggests that their characteristic peaks reflect the patterns of slab folding and may provide constraints on the rheology of slabs at the base of the mantle.

Effects of a perovskite‐post perovskite phase change near core‐mantle boundary in compressible mantle convection

“…A deep phase transition has been considered in only one Earth-related modeling study [Sidorin et al, 1999], which focused on the seismic signature associated with such a phase transition, and concluded that it offered the best explanation of observed seismic structures such as the ''Lay discontinuity'', but did not investigate the dynamical effects of such a transition. The only studies on the dynamical effects of deep phase transitions were modeling Mars, which, due to its smaller size and lower gravity, has the equivalent of Earth's transition zone in its' deep mantle.…”

Post-perovskite phase in selected sesquioxides from density-functional calculations

“…2,4 This transition to a post-perovskite phase answered questions related to the formation of the DЉ layer and offered reasonable explanations to most seismic observations. [4][5][6][7][8] The experimental difficulties related to obtaining the postperovskite phase in MgSiO 3 , due to the large transition pressures and temperatures and to the transparency of the pure material to laser, determined scientists to search for analogous materials, where a similar phase transition sequence is observed, but at lower pressures and/or temperatures. These analogous materials ideally present similar physical properties as the phase of interest, and are more easily observed.…”

Stirring in 3-d spherical models of convection in the Earth's mantle

“…Equations (6) and (7) are discretised by finite elements with linear basis functions and solved simultaneously (modified after Ramage and Walthan, 1992) using a multigrid method. The energy equation is discretised by the MPDAT algorithm (Smolarkiewicz, 1984) and integrated with a Runge-Kutta scheme using explicit time stepping. Domain decomposition is used for the parallelisation (Bunge & Baumgardner, 1995) with MPI.…”

“…The existence of geochemically distinct reservoirs in the Earth's mantle is inferred from the observation of worldwide rather homogeneous mid-ocean ridge basalts (MORB) on the one hand and heterogeneous ocean island basalts (OIB) on the other (Zindler and Hart, 1986, Silver et al, 1988, Hart and Zindler, 1989, Allègre and Lewin, 1995, Hofmann, 1997. Of course, what we observe today is the result of the interplay between chemical differentiation and convective stirring that started with the formation of the Earth and is still going on.…”