Mantle Phase Changes Detected From Stochastic Tomography

Abstract

Introduction Observed and Predicted Phase ChangesThe best-known solid phase changes in Earth's silicate mantle are commonly detected from the reflection and conversion of body waves interacting with sharp, near discontinuous, changes in velocity and density occurring at or near 410 and 660 km depth (e.g., Niu et al., 2005;Shearer, 1991). These phase changes also produce multi-pathed body waveforms between 15° and 35° great circle range (e.g., Burdick & Helmberger, 1978;Chu et al., 2012). The steepness of the inferred P and S velocity gradients between 410 and 660 km in reference Earth models, however, cannot be explained by plausible silicate mantle chemistry without the existence of at least one more phase change between 410 and 660 km (Anderson, 1989). When the tools of mineral physics are applied to a constrained composition of the Earth's mantle, up to two additional phase changes are predicted to occur between the 410 and 660 km. Several other phase changes are also predicted above and below this zone, each having a unique behavior in their Clapeyron slope and the widths in depth over which each occurs (Figure 1). The widths of more than half of these are on the order of 15-60 km (e.g., Stixrude & Lithgow-Bertelloni, 2007, 2011, some expressed primarily by changes in velocity gradient, making them difficult to detect from reflected and converted body waves having similar or shorter wavelengths. As demonstrated in Figure 1, the parameterization Abstract Stochastic tomography, made possible by dense deployments of seismic sensors, is used to identify phase changes in Earth's mantle that occur over depth intervals. This technique inverts spatial coherences of amplitudes and travel times of body waves to determine the depth and dependence of the spatial spectrum of seismic velocity. This spectrum is interpreted using the predicted thermodynamic stability of mineral composition and phase as a function of temperature and pressure, in which the metamorphic temperature derivative of seismic velocities is used as a proxy for the effects of heterogeneity induced in a region undergoing a phase change. Peaks in the temperature derivative of seismic velocity closely match those found from applying stochastic tomography to elements of Earthscope and Flex arrays. Within ±12 km, peaks in the fluctuation of P velocity at 425, 500, and 600 km depth beneath the western US agree with those predicted by a mechanical mixture of harzburgite and basalt, 180 K cooler than a 1600 K adiabat in the mantle transition zone. A broad peak at 250 km depth may be associated with chemical heterogeneity induced by dehydration of subducted oceanic sediments, and a peak at 775 km depth with a phase change in subducted basalt. Non-detection of predicted phase changes less than 10 km in width is consistent with the resolution possible with the seismic arrays used in the inversion, including the sharp endothermic phase change near 660 km. These interpretations are consistent with the known history of plate subduction beneath North America.

How P‐Wave Scattering Throughout the Entire Mantle Mimics the High‐Frequency Pdiff and Its Coda

“…It originates from 3D structure at scales below the resolution limits of current imaging techniques. This small‐scale structure is of distinct importance for the investigation of geodynamic processes as it carries information about chemical heterogeneities (Cormier et al., 2023, e.g. phase changes) that can exist over longer times than thermal heterogeneity even on small scales.…”