High-resolution seismograms reveal details of mysterious irregularities in Earth’s lower mantle

(ORDO NEWS) — High-resolution seismic tomography revealed the inhomogeneity of the structure of the regions at the boundary between the core and the mantle with a reduced seismic wave velocity.

The lowest layer of the studied area slows down the waves even more than the rest of the ultra-low speed area. This may be due to the “dissolution” of the Earth’s core at the mantle boundary.

The Earth’s mantle is an interesting and little explored shell of the Earth, lying between the crust and the metallic core.

The idea that only a uniform rock mass is hidden under the earth’s crust is far from the truth: the mantle consists of several layers, each of which has its own characteristics and heterogeneities.

Initially, the mantle was composed of ultramafic silicate and aluminosilicate rocks such as olivine, pyroxene, plagioclase and their high pressure transformation products.

The mantle is in a state of viscoplastic convection : huge masses of rocks are mixed on time scales of tens of millions of years.

Convective flows carry heat from the hot core to the surface, pass through all layers of the mantle, undergoing changes in composition and properties, and interact with all structures encountered “along the way”, such as the remains of lithospheric plates that have sunk into it during subduction.

Everything we know about the structure of the mantle comes from seismic data. Seismic waves are refracted and reflected by the structures of the mantle, and the sequence of waves recorded far from the earthquake source carries information about all the structures through which the waves have passed.

High resolution seismograms reveal details of mysterious irregularities in Earths lower mantle 1
Scheme of propagation of various seismic waves in the Earth. Focus of earthquake – the point at which the earthquake occurred. The main processes are refraction (bending) during propagation in a medium with varying density, and reflection at the boundaries of heterogeneous layers

In the lower part of the mantle, seismologists have found unusual structures the so-called large low-shear velocity provinces and ultra-low velocity zones.

The exact nature of the anomalies is unknown, but the causes of their occurrence can be both internal and external.

These areas may be the bases of mantle plumes or “graveyards” of lithospheric plates that have sunk to the very bottom of the mantle, or they may be the product of the interaction of the Earth’s core with the mantle, or even fragments of the mantle of the protoplanet Theia , the collision with which gave rise to the Moon.

The high density of rocks in areas of reduced velocity speaks against the first two versions and in favor of the last two.

In a new study, scientists from the University of Cambridge, led by Zhi Li (Lithuanian Li) ” reviewed ” areas of ultra-low speed with a record spatial resolution of up to tens of kilometers.

To do this, they recorded seismograms of earthquakes, the waves of which passed through the areas of reduced speed “along the tangential” and experienced the greatest deviation, after which they performed computer simulations of the propagation of the initial waves in the bowels of the Earth. The original article describing the study is in the public domain.

Scientists “adjusted” the shape and parameters of the heterogeneity in the mantle so that the simulated seismograms coincided with the measured ones, and thus reconstructed the structure of the heterogeneity and its properties.

High resolution seismograms reveal details of mysterious irregularities in Earths lower mantle 4
Schematic of the propagation of seismic waves used for the study. Dashed circles – layer boundaries at depths of 410 and 660 km, the color reflects the relative anomaly of the shear wave propagation velocity (red – deceleration, blue – acceleration)

It turned out that flat “pancakes” a few hundred kilometers wide and tens of kilometers thick correspond best to the models. The velocity of low-frequency waves, which reflect the structure on large scales, decreases in these zones by 20%, and that of high-frequency waves, which are sensitive to small inhomogeneities, by 40%.

In this case, the waves are shifted by refraction towards the center of the Earth, and high-frequency waves are shifted more strongly than low-frequency ones.

This corresponds to a layered structure: apparently, in the zone of ultra-low speed, where seismic waves slow down anyway, there is a lower layer several kilometers thick: there the wave speed drops even more.

High resolution seismograms reveal details of mysterious irregularities in Earths lower mantle 5
Schematic representation of the ultra-low velocity region and the propagation of seismic waves in it. Pacific LLVP on the left is the low shear Pacific region, ULVZ is the ultra-low shear region, CMB is the core-mantle boundary

The authors of the work do not draw unambiguous conclusions about the nature of the studied areas, but tend to favor internal processes. They emphasize that the high density of their rocks must be due to the increased content of iron (which replaces the much lighter magnesium in the rocks).

The mantle of Theia, a smaller and more differentiated celestial body than the Earth, could be enriched in iron, but it is difficult to explain the sharp decrease in wave speed in the lowest and very thin layer of heterogeneity with its fragments.

On the contrary, the “dissolution” of the earth’s core is more suitable: metallic iron can be oxidized at its boundary, entering into the composition of rocks, and then the lowest layer of rocks will be the most enriched.

This can also be confirmed by the measured isotope anomalies of lava from Hawaiian volcanoes located 2900 kilometers above the studied zone, but connected with it by a mantle plume.


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