Mantle Phase Changes Detected From Stochastic Tomography

• Published in: Journal of geophysical research. Solid earth Vol. 128; no. 2
• Main Authors: Cormier, Vernon F., Lithgow‐Bertelloni, Carolina, Stixrude, Lars, Zheng, Yingcai
• Format: Journal Article
• Language: English
• Published: Washington Blackwell Publishing Ltd 01-02-2023
• Subjects: Amplitudes; Arrays; Basalt; Body temperature; Body waves; Collections; Dehydration; Depth; Earth mantle; Earthquakes; Geophysics; Heterogeneity; mantle composition; mantle heterogeneity; mantle structure; Mineral composition; Minerals; P-waves; Phase change; Phase changes; Plate tectonics; Plates (tectonics); Pressure; Rocks; Sediments; Seismic activity; Seismic arrays; seismic body waves; seismic scattering; Seismic stability; Seismic velocities; Seismic waves; Seismometers; Stochasticity; Subduction; Subduction (geology); Tectonics; Temperature; Tomography; Transition zone; Travel time; Velocity; North America
• ISSN: 2169-9313; 2169-9356
• DOI: 10.1029/2022JB025035

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.
Plain Language Summary
The speed of seismic waves in a rock depends on the chemistry and arrangement of the atoms making up the minerals in the rock. These atoms are arranged in different ways, depending on temperature and pressure. Increasing pressure with depth in Earth squeezes the atoms of minerals into denser arrangements. An abrupt change with depth in the arrangement of mineral atoms, termed a phase change, can speed up, slow down, and scatter earthquake waves in different directions, changing their arrival times and amplitudes. This study describes a way in which the fluctuations in the arrival times and amplitudes of seismic waves, recorded by dense collections of seismometers, can be used to locate, detect, and image the depth and thickness of mineral phase changes in Earth's mantle. These phase changes are found to control much of the observed scattering of seismic waves in Earth's mantle from distant earthquakes and agree with the history of plate tectonics of western North America.
Key Points
Stochastic tomography can capture higher order phase and chemical changes undetectable from reflection imaging
The heterogeneity spectrum of the upper mantle is marked by peaks associated with depth regions undergoing phase or compositional change
The upper mantle beneath the western US is consistent with a history of subduction and slab stagnation in the transition zone