Lower mantle explained

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.[1] 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).[2] 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,[3] 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,[4] which may affect both mantle plume dynamics[5] [6] and lower mantle chemistry.

The upper boundary is defined by the sharp increase in seismic wave velocities and density at a depth of 660km (410miles).[7] At a depth of 660 km, ringwoodite decomposes into Mg-Si perovskite and magnesiowüstite.[7] 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 NaN2700. The base of the lower mantle is about 2700 km.[7]

Physical properties

The lower mantle was initially labelled as the D-layer in Bullen's spherically symmetric model of the Earth.[8] 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:

The temperature of the lower mantle ranges from at the topmost layer to at a depth of .[9] 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 0.47K/km at the uppermost lower mantle to 0.24K/km 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,

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.[10] 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.[11] 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%.[12] 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.[13] 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.[14] 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.[15] 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.[16] Daly's hypothetical depths to the lithosphere-asthenosphere boundary ranged from NaN80, and the top of the mesospheric shell (base of the asthenosphere) were from NaN200. Thus, Daly's asthenosphere was inferred to be NaN120 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.[17]

See also

Notes and References

  1. Book: The Earth's lower mantle: composition and structure. Kaminsky, Felix V.. 2017. Springer. 9783319556840. Cham. 988167555.
  2. 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.
  3. Book: Ringwood, Alfred E. . Composition and petrology of the earth's mantle . 1976 . McGraw-Hill . 0070529329 . 16375050 . registration.
  4. 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.
  5. 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. free . 0956-540X.
  6. 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. free.
  7. Book: Condie, Kent C. . 2001 . 'Mantle Plumes and Their Record in Earth History . . 3 - 10 . 0-521-01472-7 .
  8. Bullen. K.E.. 1942. The density variation of the earth's central core. Bulletin of the Seismological Society of America. 32. 1 . 19–29. 10.1785/BSSA0320010019 . 1942BuSSA..32...19B .
  9. 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.
  10. 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.
  11. 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.
  12. 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.
  13. 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.
  14. 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.
  15. 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.
  16. Book: Daly, Reginald Aldworth . 1940 . Strength and Structure of the Earth . registration . . New York.
  17. 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.