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The lower mantle, historically also known as the mesosphere, represents approximately 56% of Earth's total volume, and is the region from 660 to 2900 km below
Earth's surface; between the
transition zone and the
outer core.
The
preliminary reference Earth model
The preliminary reference Earth model (PREM) plots the average of Earth's properties by depth. It includes a table of Earth properties, including elastic properties, attenuation, density, pressure, and gravity.
PREM has been widely used as the ...
(PREM) separates the lower mantle into three sections, the uppermost (660–770 km), mid-lower mantle (770–2700 km), and the D layer (2700–2900 km).
Pressure and temperature in the lower mantle range from 24–127 GPa
and 1900–2600
K.
It has been proposed that the composition of the lower mantle is
pyrolitic,
containing three major phases of
bridgmanite,
ferropericlase
Ferropericlase or magnesiowüstite is a magnesium/iron oxide with the chemical formula that is interpreted to be one of the main constituents of the Earth's lower mantle together with the silicate perovskite (), a magnesium/iron silicate with a pe ...
, and calcium-silicate perovskite. The high pressure in the lower mantle has been shown to induce a spin transition of iron-bearing bridgmanite and ferropericlase,
which may affect both
mantle plume dynamics and lower mantle chemistry.
The upper boundary is defined by the sharp increase in
seismic wave
A seismic wave is a wave of acoustic energy that travels through the Earth. It can result from an earthquake, volcanic eruption, magma movement, a large landslide, and a large man-made explosion that produces low-frequency acoustic energy. S ...
velocities and
density
Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematical ...
at a depth of .
At a depth of 660 km,
ringwoodite () decomposes into
Mg-Si perovskite and
magnesiowüstite
Ferropericlase or magnesiowüstite is a magnesium/iron oxide with the chemical formula that is interpreted to be one of the main constituents of the Earth's lower mantle together with the silicate perovskite (), a magnesium/iron silicate with a ...
.
[ This reaction marks the boundary between the upper mantle and ]lower mantle
The lower mantle, historically also known as the mesosphere, represents approximately 56% of Earth's total volume, and is the region from 660 to 2900 km below Earth's surface; between the transition zone and the outer core. The preliminar ...
. This measurement is estimated from seismic data and high-pressure laboratory experiments. The base of the mesosphere includes the D″ zone which lies just above the mantle–core boundary at approximately . The base of the lower mantle is about 2700 km.[
]
Physical properties
The lower mantle was initially labelled as the D-layer in Bullen's spherically symmetric model of the Earth. The PREM seismic model of the Earth's interior separated the D-layer into three distinctive layers defined by the discontinuity in seismic wave
A seismic wave is a wave of acoustic energy that travels through the Earth. It can result from an earthquake, volcanic eruption, magma movement, a large landslide, and a large man-made explosion that produces low-frequency acoustic energy. S ...
velocities:
* 660–770 km: A discontinuity in compression wave velocity (6–11%) followed by a steep gradient is indicative of the transformation of the mineral ringwoodite to bridgmanite and ferropericlase and the transition between the transition zone layer to the lower mantle.
* 770–2700 km: A gradual increase in velocity indicative of the adiabatic compression of the mineral phases in the lower mantle.
* 2700–2900 km: The D-layer is considered the transition from the lower mantle to the outer core.
The temperature of the lower mantle ranges from at the topmost layer to at a depth of . Models of the temperature of the lower mantle approximate convection
Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the convec ...
as the primary heat transport contribution, while conduction and radiative heat transfer are considered negligible. As a result, the lower mantle's temperature gradient as a function of depth is approximately adiabatic. Calculation of the geothermal gradient observed a decrease from at the uppermost lower mantle to at .
Composition
The lower mantle is mainly composed of three components, bridgmanite, ferropericlase, and calcium-silicate perovskite (CaSiO3-perovskite). The proportion of each component has been a subject of discussion historically where the bulk composition is suggested to be,
* Pyrolitic: derived from petrological composition trends from upper mantle peridotite
Peridotite ( ) is a dense, coarse-grained igneous rock consisting mostly of the silicate minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium (Mg2+), reflecting the high prop ...
suggesting homogeneity between the upper and lower mantle with a Mg/Si ratio of 1.27. This model implies that the lower mantle is composed of 75% bridgmanite, 17% ferropericlase, and 8% CaSiO3-perovskite by volume.
* Chondritic: suggests that the Earth's lower mantle was accreted from the composition of chondritic meteorite
A chondrite is a stony (non-Metallicity, metallic) meteorite that has not been modified, by either melting or planetary differentiation, differentiation of the parent body. They are formed when various types of dust and small grains in the ea ...
suggesting a Mg/Si ratio of approximately 1. This infers that bridgmanite and CaSiO3-perovskites are major components.
Laboratory multi-anvil compression experiments of pyrolite Pyrolite is a term used to characterize a model composition of the Earth's mantle. This model is based on that a pyrolite source can produce the Mid-Ocean Ridge Basalt by partial melting. It was first proposed by Ted Ringwood (1962) as being 1 part ...
simulated conditions of the adiabatic geotherm and measured the density using ''in situ'' X-ray diffraction
X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...
. It was shown that the density profile along the geotherm is in agreement with the PREM model. The first principle calculation of the density and velocity profile across the lower mantle geotherm of varying bridgmanite and ferropericlase proportion observed a match to the PREM model at an 8:2 proportion. This proportion is consistent with the pyrolitic bulk composition at the lower mantle. Furthermore, shear wave velocity calculations of pyrolitic lower mantle compositions considering minor elements resulted in a match with the PREM shear velocity profile within 1%. On the other hand, Brillouin spectroscopic studies at relevant pressures and temperatures revealed that a lower mantle composed of greater than 93% bridgmanite phase has corresponding shear-wave velocities to measured seismic velocities. The suggested composition is consistent with a chondritic lower mantle. Thus, the bulk composition of the lower mantle is currently a subject of discussion.
Spin transition zone
The electronic environment of two iron-bearing minerals in the lower mantle (bridgmanite, ferropericlase) transitions from a high-spin (HS) to a low-spin (LS) state. Fe2+ in ferropericlase undergoes the transition between 50–90 GPa. Bridgmanite contains both Fe3+ and Fe2+ in the structure, the Fe2+ occupy the A-site and transition to a LS state at 120 GPa. While Fe3+ occupies both A- and B-sites, the B-site Fe3+ undergoes HS to LS transition at 30–70 GPa while the A-site Fe3+ exchanges with the B-site Al3+ cation and becomes LS. This spin transition of the iron cation results in the increase in partition coefficient between ferropericlase and bridgmanite to 10–14 depleting bridgmanite and enriching ferropericlase of Fe2+. The HS to LS transition are reported to affect the physical properties of the iron bearing minerals. For example, the density and incompressibility was reported to increase from HS to LS state in ferropericlase. The effects of the spin transition on the transport properties and rheology
Rheology (; ) is the study of the flow of matter, primarily in a fluid ( liquid or gas) state, but also as "soft solids" or solids under conditions in which they respond with plastic flow rather than deforming elastically in response to an appl ...
of the lower mantle is currently being investigated and discussed using numerical simulations.
History
Mesosphere (not to be confused with mesosphere
The mesosphere (; ) is the third layer of the atmosphere, directly above the stratosphere and directly below the thermosphere. In the mesosphere, temperature decreases as altitude increases. This characteristic is used to define its limits: it ...
, a layer of the atmosphere
An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A s ...
) is derived from "mesospheric shell", coined by Reginald Aldworth Daly
Reginald Aldworth Daly (March 18, 1871 – September 19, 1957) was a Canadian geologist.
Biography
Reginald Daly was educated at the University of Toronto, where geologist A.P. Coleman persuaded him away from teaching mathematics and into Earth S ...
, a Harvard University
Harvard University is a private Ivy League research university in Cambridge, Massachusetts. Founded in 1636 as Harvard College and named for its first benefactor, the Puritan clergyman John Harvard, it is the oldest institution of higher le ...
geology
Geology () is a branch of natural science concerned with Earth and other astronomical objects, the features or rocks of which it is composed, and the processes by which they change over time. Modern geology significantly overlaps all other Ear ...
professor. In the pre-plate tectonics
Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large ...
era, Daly (1940) inferred that the outer Earth consisted of three spherical layers: lithosphere (including the crust), asthenosphere, and mesospheric shell. Daly's hypothetical depths to the lithosphere-asthenosphere boundary ranged from , and the top of the mesospheric shell (base of the asthenosphere) were from . Thus, Daly's asthenosphere was inferred to be thick. According to Daly, the base of the solid Earth mesosphere could extend to the base of the mantle (and, thus, to the top of the core).
A derivative term, mesoplates
The term "mesoplates" has been applied in two different contexts within geology and geophysics. The first is applicable to much of the Earth's mantle, and the second to distinct layering within the Earth's crust.
Mantle
Rheological model
In ...
, was introduced as a heuristic
A heuristic (; ), or heuristic technique, is any approach to problem solving or self-discovery that employs a practical method that is not guaranteed to be optimal, perfect, or rational, but is nevertheless sufficient for reaching an immediate, ...
, based on a combination of "mesosphere" and "plate", for postulated reference frames in which mantle hotspots
Hotspot, Hot Spot or Hot spot may refer to:
Places
* Hot Spot, Kentucky, a community in the United States
Arts, entertainment, and media Fictional entities
* Hot Spot (comics), a name for the DC Comics character Isaiah Crockett
* Hot Spot (Tra ...
exist.
See also
* Large low-shear-velocity provinces
References
{{Earthsinterior
Earth's mantle
Structure of the Earth