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{{Short description|The region from 660 to 2900 km below Earth's surface}}(File:Earth poster.svg|thumb|upright=1.4|Structure of Earth. The mesosphere is labeled as Stiffer mantle in this diagram.)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.BOOK, The Earth's lower mantle: composition and structure, Kaminsky, Felix V., 2017, Springer, 9783319556840, Cham, 988167555, The preliminary reference Earth model (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).JOURNAL, Dziewonski, Adam M., Anderson, Don L., 1981, Preliminary reference Earth model, Physics of the Earth and Planetary Interiors, 25, 4, 297–356, 10.1016/0031-9201(81)90046-7, 1981PEPI...25..297D, 0031-9201, 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,BOOK, Ringwood, Alfred E.,weblink Composition and petrology of the earth's mantle, 1976, McGraw-Hill, 0070529329, 16375050, registration, containing three major phases of bridgmanite, ferropericlase, 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,JOURNAL, Badro, J., 2003-04-03, Iron Partitioning in Earth's Mantle: Toward a Deep Lower Mantle Discontinuity, Science, 300, 5620, 789–791, 10.1126/science.1081311, 12677070, 2003Sci...300..789B, 12208090, 0036-8075, free, which may affect both mantle plume dynamicsJOURNAL, Shahnas, M.H., Pysklywec, R.N., Justo, J.F., Yuen, D.A., 2017-05-09, Spin transition-induced anomalies in the lower mantle: implications for mid-mantle partial layering, Geophysical Journal International, 210, 2, 765–773, 10.1093/gji/ggx198, 0956-540X, JOURNAL, Bower, Dan J., Gurnis, Michael, Jackson, Jennifer M., Sturhahn, Wolfgang, 2009-05-28, Enhanced convection and fast plumes in the lower mantle induced by the spin transition in ferropericlase, Geophysical Research Letters, 36, 10, 10.1029/2009GL037706, 2009GeoRL..3610306B, 0094-8276,weblink free, and lower mantle chemistry.The upper boundary is defined by the sharp increase in seismic wave velocities and density at a depth of {{convert|660|km|mi|0|round=5|sp=us}}.BOOK, Condie, Kent C., 2001, 'Mantle Plumes and Their Record in Earth History, Cambridge University Press, 3–10, 0-521-01472-7, At a depth of 660 km, ringwoodite ({{chem|γ-(Mg,Fe)|2|SiO|4}}) decomposes into Mg-Si perovskite and magnesiowüstite. This reaction marks the boundary between the upper mantle and lower mantle. 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 {{convert|abbr=on|2700|to|2890|km|mi|0|round=5}}. 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.JOURNAL, Bullen, K.E., 1942, The density variation of the earth's central core,weblink Bulletin of the Seismological Society of America, 32, 1, 19–29, 10.1785/BSSA0320010019, 1942BuSSA..32...19B, The PREM seismic model of the Earth's interior separated the D-layer into three distinctive layers defined by the discontinuity in seismic wave 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 {{convert|1960|K}} at the topmost layer to {{convert|2630|K}} at a depth of {{convert|2700|km}}.JOURNAL, Katsura, Tomoo, Yoneda, Akira, Yamazaki, Daisuke, Yoshino, Takashi, Ito, Eiji, 2010, Adiabatic temperature profile in the mantle, Physics of the Earth and Planetary Interiors, 183, 1–2, 212–218, 10.1016/j.pepi.2010.07.001, 2010PEPI..183..212K, 0031-9201, Models of the temperature of the lower mantle approximate convection 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 {{convert|0.47|K/km|°C/km °F/mi}} at the uppermost lower mantle to {{convert|0.24|K/km|°C/km °F/mi}} at {{convert|2600|km}}.

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 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 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 simulated conditions of the adiabatic geotherm and measured the density using in situ X-ray diffraction. It was shown that the density profile along the geotherm is in agreement with the PREM model.JOURNAL, Irifune, T., Shinmei, T., McCammon, C. A., Miyajima, N., Rubie, D. C., Frost, D. J., Daniel Frost (earth scientist), 2010-01-08, Iron Partitioning and Density Changes of Pyrolite in Earth's Lower Mantle, Science, 327, 5962, 193–195, 10.1126/science.1181443, 19965719, 2010Sci...327..193I, 19243930, 0036-8075, 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.JOURNAL, Wang, Xianlong, Tsuchiya, Taku, Hase, Atsushi, 2015, Computational support for a pyrolitic lower mantle containing ferric iron, Nature Geoscience, 8, 7, 556–559, 10.1038/ngeo2458, 2015NatGe...8..556W, 1752-0894, 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%.JOURNAL, Hyung, Eugenia, Huang, Shichun, Petaev, Michail I., Jacobsen, Stein B., 2016, Is the mantle chemically stratified? Insights from sound velocity modeling and isotope evolution of an early magma ocean, Earth and Planetary Science Letters, 440, 158–168, 10.1016/j.epsl.2016.02.001, 2016E&PSL.440..158H, free, 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.JOURNAL, Murakami, Motohiko, Ohishi, Yasuo, Hirao, Naohisa, Hirose, Kei, May 2012, A perovskitic lower mantle inferred from high-pressure, high-temperature sound velocity data, Nature, 485, 7396, 90–94, 10.1038/nature11004, 22552097, 2012Natur.485...90M, 4387193, 0028-0836, 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.JOURNAL, Badro, James, 2014-05-30, Spin Transitions in Mantle Minerals, Annual Review of Earth and Planetary Sciences, 42, 1, 231–248, 10.1146/annurev-earth-042711-105304, 2014AREPS..42..231B, 0084-6597, 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.JOURNAL, Lin, Jung-Fu, Speziale, Sergio, Mao, Zhu, Marquardt, Hauke, April 2013, Reviews of Geophysics, 51, 2, 244–275, 10.1002/rog.20010, Effects of the Electronic Spin Transitions of Iron in Lower Mantle Minerals: Implications for Deep Mantle Geophysics and Geochemistry, 2013RvGeo..51..244L, 21661449, free, The effects of the spin transition on the transport properties and rheology of the lower mantle is currently being investigated and discussed using numerical simulations.

History

Mesosphere (not to be confused with mesosphere, a layer of the atmosphere) is derived from "mesospheric shell", coined by Reginald Aldworth Daly, a Harvard University geology professor. In the pre-plate tectonics era, Daly (1940) inferred that the outer Earth consisted of three spherical layers: lithosphere (including the crust), asthenosphere, and mesospheric shell.BOOK, Daly, Reginald Aldworth, 1940, Strength and Structure of the Earth,weblink registration, Prentice Hall, New York, Daly's hypothetical depths to the lithosphere-asthenosphere boundary ranged from {{convert|abbr=on|80|to|100|km|mi|0}}, and the top of the mesospheric shell (base of the asthenosphere) were from {{convert|abbr=on|200|to|480|km|mi|0|round=5}}. Thus, Daly's asthenosphere was inferred to be {{convert|abbr=on|120|to|400|km|mi|0|round=5}} 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, was introduced as a heuristic, based on a combination of "mesosphere" and "plate", for postulated reference frames in which mantle hotspots exist.BOOK, Kumazawa, M, Fukao, Y, Manghnani, Murli, Akimoto, Syun-Iti, High-Pressure Research: Applications in Geophysics, 1977, Academic Press, 978-0-12-468750-9, 127, 10.1016/B978-0-12-468750-9.50014-0, Dual Plate Tectonics Model,

See also

• Large low-shear-velocity provinces

References

{{Reflist}}{{Earthsinterior}}