Jump to content

Lower mantle: Difference between revisions

From Wikipedia, the free encyclopedia
Browse history interactively
← Previous editNext edit →
Content deleted Content added
VisualWikitext
Darahok (talk | contribs)
40 edits
→Spin Transition Zone: Added text to STZ section. Note to self: maybe switch the last two sections to (density variation then as a result mantle plume instability)
Darahok (talk | contribs)
40 edits
Made several edits including clarification. Will be adding section on Mantle Convection later in the week and clarifying how spin transition remain seismically invisible
Line 1: Line 1:
= Lower Mantle =
= Lower Mantle =
The lower mantle comprise of approximately 56% of the Earth's total volume located 660-2900 km beneath the crust <ref name=":2">{{Cite book|url=https://www.worldcat.org/oclc/988167555|title=The Earth's lower mantle : composition and structure|last=V.|first=Kaminsky, Felix|date=2017|publisher=Springer|isbn=9783319556840|location=Cham|oclc=988167555}}</ref>. 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)<ref name=":0">{{Cite journal|last=Dziewonski|first=Adam M.|last2=Anderson|first2=Don L.|date=1981-06|title=Preliminary reference Earth model|url=http://dx.doi.org/10.1016/0031-9201(81)90046-7|journal=Physics of the Earth and Planetary Interiors|volume=25|issue=4|pages=297–356|doi=10.1016/0031-9201(81)90046-7|issn=0031-9201}}</ref>. Pressures and temperature at the lower mantle ranges from 24-127 GPa<ref name=":0" /> and 1900-2630K<ref>{{Cite journal|last=Katsura|first=Tomoo|last2=Yoneda|first2=Akira|last3=Yamazaki|first3=Daisuke|last4=Yoshino|first4=Takashi|last5=Ito|first5=Eiji|date=2010-11|title=Adiabatic temperature profile in the mantle|url=http://dx.doi.org/10.1016/j.pepi.2010.07.001|journal=Physics of the Earth and Planetary Interiors|volume=183|issue=1-2|pages=212–218|doi=10.1016/j.pepi.2010.07.001|issn=0031-9201}}</ref>. It is widely accepted that the composition of the lower mantle is pyrolitic<ref name=":4">{{Cite book|url=http://worldcat.org/oclc/16375050|title=Composition and petrology of the earth's mantle|last=Edward)|first=Ringwood, A. E. (Alfred|date=[1976]|publisher=McGraw-Hill|isbn=0070529329|oclc=16375050}}</ref> containing three major phases of Bridgmanite, Ferropericlase and Calcium-silicate perovskite. The pressures at the lower mantle was shown to induce a spin transition of iron-bearing ferropericlase<ref name=":1">{{Cite journal|last=Badro|first=J.|date=2003-04-03|title=Iron Partitioning in Earth's Mantle: Toward a Deep Lower Mantle Discontinuity|url=http://dx.doi.org/10.1126/science.1081311|journal=Science|volume=300|issue=5620|pages=789–791|doi=10.1126/science.1081311|issn=0036-8075}}</ref> affecting both mantle plume dynamics<ref>{{Cite journal|last=Shahnas|first=M.H.|last2=Pysklywec|first2=R.N.|last3=Justo|first3=J.F.|last4=Yuen|first4=D.A.|date=2017-05-09|title=Spin transition-induced anomalies in the lower mantle: implications for mid-mantle partial layering|url=http://dx.doi.org/10.1093/gji/ggx198|journal=Geophysical Journal International|volume=210|issue=2|pages=765–773|doi=10.1093/gji/ggx198|issn=0956-540X}}</ref><ref>{{Cite journal|last=Bower|first=Dan J.|last2=Gurnis|first2=Michael|last3=Jackson|first3=Jennifer M.|last4=Sturhahn|first4=Wolfgang|date=2009-05-28|title=Enhanced convection and fast plumes in the lower mantle induced by the spin transition in ferropericlase|url=http://doi.wiley.com/10.1029/2009GL037706|journal=Geophysical Research Letters|language=en|volume=36|issue=10|doi=10.1029/2009GL037706|issn=0094-8276}}</ref> and the mineralogy<ref name=":1" />.
The lower mantle comprise of approximately 56% of the Earth's total volume located 660-2900 km below the crust in between the transition zone and the outer core<ref name=":2">{{Cite book|url=https://www.worldcat.org/oclc/988167555|title=The Earth's lower mantle : composition and structure|last=V.|first=Kaminsky, Felix|date=2017|publisher=Springer|isbn=9783319556840|location=Cham|oclc=988167555}}</ref>. 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)<ref name=":0">{{Cite journal|last=Dziewonski|first=Adam M.|last2=Anderson|first2=Don L.|date=1981-06|title=Preliminary reference Earth model|url=http://dx.doi.org/10.1016/0031-9201(81)90046-7|journal=Physics of the Earth and Planetary Interiors|volume=25|issue=4|pages=297–356|doi=10.1016/0031-9201(81)90046-7|issn=0031-9201}}</ref>. Pressures and temperature at the lower mantle ranges from 24-127 GPa<ref name=":0" /> and 1900-2630K<ref>{{Cite journal|last=Katsura|first=Tomoo|last2=Yoneda|first2=Akira|last3=Yamazaki|first3=Daisuke|last4=Yoshino|first4=Takashi|last5=Ito|first5=Eiji|date=2010-11|title=Adiabatic temperature profile in the mantle|url=http://dx.doi.org/10.1016/j.pepi.2010.07.001|journal=Physics of the Earth and Planetary Interiors|volume=183|issue=1-2|pages=212–218|doi=10.1016/j.pepi.2010.07.001|issn=0031-9201}}</ref>. It is widely accepted that the composition of the lower mantle is [[Pyrolite|pyrolitic]]<ref name=":4">{{Cite book|url=http://worldcat.org/oclc/16375050|title=Composition and petrology of the earth's mantle|last=Edward)|first=Ringwood, A. E. (Alfred|date=[1976]|publisher=McGraw-Hill|isbn=0070529329|oclc=16375050}}</ref> containing three major phases of [[Silicate perovskite|Bridgmanite]], [[Ferropericlase]] and Calcium-silicate perovskite. The pressures at the lower mantle was shown to induce a spin transition of iron-bearing [[ferropericlase]]<ref name=":1">{{Cite journal|last=Badro|first=J.|date=2003-04-03|title=Iron Partitioning in Earth's Mantle: Toward a Deep Lower Mantle Discontinuity|url=http://dx.doi.org/10.1126/science.1081311|journal=Science|volume=300|issue=5620|pages=789–791|doi=10.1126/science.1081311|issn=0036-8075}}</ref> affecting both [[Mantle plume|mantle plume]] dynamics<ref>{{Cite journal|last=Shahnas|first=M.H.|last2=Pysklywec|first2=R.N.|last3=Justo|first3=J.F.|last4=Yuen|first4=D.A.|date=2017-05-09|title=Spin transition-induced anomalies in the lower mantle: implications for mid-mantle partial layering|url=http://dx.doi.org/10.1093/gji/ggx198|journal=Geophysical Journal International|volume=210|issue=2|pages=765–773|doi=10.1093/gji/ggx198|issn=0956-540X}}</ref><ref>{{Cite journal|last=Bower|first=Dan J.|last2=Gurnis|first2=Michael|last3=Jackson|first3=Jennifer M.|last4=Sturhahn|first4=Wolfgang|date=2009-05-28|title=Enhanced convection and fast plumes in the lower mantle induced by the spin transition in ferropericlase|url=http://doi.wiley.com/10.1029/2009GL037706|journal=Geophysical Research Letters|language=en|volume=36|issue=10|doi=10.1029/2009GL037706|issn=0094-8276}}</ref> and chemistry<ref name=":1" />.


== Physical properties ==
== Physical properties ==
The lower mantle was initially labelled as the D-layer in Bullen's spherically symmetric model of the Earth<ref>{{Cite journal|last=Bullen|first=K.E.|date=1942|title=The density variation of the earth's central core|url=https://pubs.geoscienceworld.org/ssa/bssa/article/32/1/19/115369/the-density-variation-of-the-earth-s-central-core|journal=Bulletin of the Seismological Society of America|volume=32|pages=19-29|via=}}</ref>. 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<ref name=":0" />,
The lower mantle was initially labelled as the D-layer in Bullen's spherically symmetric model of the Earth<ref>{{Cite journal|last=Bullen|first=K.E.|date=1942|title=The density variation of the earth's central core|url=https://pubs.geoscienceworld.org/ssa/bssa/article/32/1/19/115369/the-density-variation-of-the-earth-s-central-core|journal=Bulletin of the Seismological Society of America|volume=32|pages=19-29|via=}}</ref>. 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<ref name=":0" />,


* 660-770 km: A discontinuity in 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.
* 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 [[Silicate perovskite|Bridgmanite]] and [[Ferropericlase]] and the transition between the [[Transition zone (Earth)|Transition Zone]] layer to the lower mantle.
* 770-2700 km: A gradual increase in velocity indicative of the adiabatic compression of the mineral phases at the lower mantle.
* 770-2700 km: A gradual increase in velocity indicative of the adiabatic compression of the mineral phases at the lower mantle.
* 2700-2900 km: The [[Core–mantle boundary|D" layer]] is considered the transition between the lower mantle to the outer core.
* 2700-2900 km: The [[Core–mantle boundary|D" layer]] is considered the transition between the lower mantle to the [[Outer core|outer core]].


Temperatures of the lower mantle ranges from 1960 K to 2630 K<ref name=":3">{{Cite journal|last=Katsura|first=Tomoo|last2=Yoneda|first2=Akira|last3=Yamazaki|first3=Daisuke|last4=Yoshino|first4=Takashi|last5=Ito|first5=Eiji|date=2010-11|title=Adiabatic temperature profile in the mantle|url=http://dx.doi.org/10.1016/j.pepi.2010.07.001|journal=Physics of the Earth and Planetary Interiors|volume=183|issue=1-2|pages=212–218|doi=10.1016/j.pepi.2010.07.001|issn=0031-9201}}</ref>. Models of the temperature of the lower mantle approximates convection as the primary heat transport contribution while conduction and radiative heat is considered negligible. As a result, the Mantle's temperature gradient as a function of depth is approximately adiabatic<ref name=":2" />. Calculation of the geothermal gradient observed a decrease from 0.47 K/km at the uppermost lower mantle to 0.24 K/km at 2600km<ref name=":3" />.
Temperatures of the lower mantle ranges from 1960 K at the topmost layer to 2630 K at a depth of 2700 km<ref name=":3">{{Cite journal|last=Katsura|first=Tomoo|last2=Yoneda|first2=Akira|last3=Yamazaki|first3=Daisuke|last4=Yoshino|first4=Takashi|last5=Ito|first5=Eiji|date=2010-11|title=Adiabatic temperature profile in the mantle|url=http://dx.doi.org/10.1016/j.pepi.2010.07.001|journal=Physics of the Earth and Planetary Interiors|volume=183|issue=1-2|pages=212–218|doi=10.1016/j.pepi.2010.07.001|issn=0031-9201}}</ref>. Models of the temperature of the lower mantle approximates [[convection]] as the primary heat transport contribution while conduction and radiative heat is considered negligible. As a result, the Mantle's temperature gradient as a function of depth is approximately adiabatic<ref name=":2" />. Calculation of the geothermal gradient observed a decrease from 0.47 K/km at the uppermost lower mantle to 0.24 K/km at 2600km<ref name=":3" />.


== Composition ==
== Composition ==
The lower mantle is mainly composed of three components, Bridgmanite, Ferropericlase and Calcium-Silicate Perovskite (CaSiO<sub>3</sub>-perovskite). The proportion of each component has been a subject of discussion historically where the bulk composition is suggested to be,
The lower mantle is mainly composed of three components, [[Silicate perovskite|Bridgmanite]], [[Ferropericlase]] and Calcium-Silicate Perovskite (CaSiO<sub>3</sub>-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% CaSiO<sub>3</sub>-perovskite by volume<ref name=":4" />.
* 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% [[Silicate perovskite|Bridgmanite]], 17% [[Ferropericlase]] and 8% CaSiO<sub>3</sub>-perovskite by volume<ref name=":4" />.
* Chondritic: suggests that the Earth's lower mantle was accreted from composition of chondritic meteorite suggesting a Mg/Si ratio of approximately 1. This infers that Bridgmanite and CaSiO<sub>3</sub>-perovskites are major components.
* Chondritic: suggests that the Earth's lower mantle was accreted from composition of [[Chondrite|chondritic meteorite]] suggesting a Mg/Si ratio of approximately 1. This infers that [[Silicate perovskite|Bridgmanite]] and CaSiO<sub>3</sub>-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<ref>{{Cite journal|last=Irifune|first=T.|last2=Shinmei|first2=T.|last3=McCammon|first3=C. A.|last4=Miyajima|first4=N.|last5=Rubie|first5=D. C.|last6=Frost|first6=D. J.|date=2010-01-08|title=Iron Partitioning and Density Changes of Pyrolite in Earth's Lower Mantle|url=http://www.sciencemag.org/cgi/doi/10.1126/science.1181443|journal=Science|language=en|volume=327|issue=5962|pages=193–195|doi=10.1126/science.1181443|issn=0036-8075}}</ref>. 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<ref>{{Cite journal|last=Wang|first=Xianlong|last2=Tsuchiya|first2=Taku|last3=Hase|first3=Atsushi|date=2015-7|title=Computational support for a pyrolitic lower mantle containing ferric iron|url=http://www.nature.com/articles/ngeo2458|journal=Nature Geoscience|language=en|volume=8|issue=7|pages=556–559|doi=10.1038/ngeo2458|issn=1752-0894}}</ref>. Furthermore, shear wave velocity calculations of a pyrolitic lower mantle compositions considering minor elements resulted in a match with the PREM shear velocity profile within 1%<ref>{{Cite journal|last=Hyung|first=Eugenia|last2=Huang|first2=Shichun|last3=Petaev|first3=Michail I.|last4=Jacobsen|first4=Stein B.|date=2016-4|title=Is the mantle chemically stratified? Insights from sound velocity modeling and isotope evolution of an early magma ocean|url=https://linkinghub.elsevier.com/retrieve/pii/S0012821X16300127|journal=Earth and Planetary Science Letters|language=en|volume=440|pages=158–168|doi=10.1016/j.epsl.2016.02.001}}</ref>. Thus, it is widely accepted that the bulk composition of the lower mantle is pyrolitic<ref name=":2" />.
Laboratory multi-anvil compression experiments of [[pyrolite]] simulated conditions of the adiabatic [[Geothermal gradient|geotherm]] and measured the density using ''in situ'' [[X-ray crystallography|X-ray diffraction]]. It was shown that the density profile along the [[Geothermal gradient|geotherm]] is in agreement with the [[Preliminary reference Earth model|PREM]] model<ref>{{Cite journal|last=Irifune|first=T.|last2=Shinmei|first2=T.|last3=McCammon|first3=C. A.|last4=Miyajima|first4=N.|last5=Rubie|first5=D. C.|last6=Frost|first6=D. J.|date=2010-01-08|title=Iron Partitioning and Density Changes of Pyrolite in Earth's Lower Mantle|url=http://www.sciencemag.org/cgi/doi/10.1126/science.1181443|journal=Science|language=en|volume=327|issue=5962|pages=193–195|doi=10.1126/science.1181443|issn=0036-8075}}</ref>. First principle calculation of the density and velocity profile across the lower mantle [[Geothermal gradient|geotherm]] of varying [[Silicate perovskite|Bridgmanite]] and [[Ferropericlase]] proportion observed a match to the [[Preliminary reference Earth model|PREM]] model at an 8:2 proportion. This proportion is consistent with the pyrolitic bulk composition at the lower mantle<ref>{{Cite journal|last=Wang|first=Xianlong|last2=Tsuchiya|first2=Taku|last3=Hase|first3=Atsushi|date=2015-7|title=Computational support for a pyrolitic lower mantle containing ferric iron|url=http://www.nature.com/articles/ngeo2458|journal=Nature Geoscience|language=en|volume=8|issue=7|pages=556–559|doi=10.1038/ngeo2458|issn=1752-0894}}</ref>. Furthermore, shear wave velocity calculations of a pyrolitic lower mantle compositions considering minor elements resulted in a match with the PREM shear velocity profile within 1%<ref>{{Cite journal|last=Hyung|first=Eugenia|last2=Huang|first2=Shichun|last3=Petaev|first3=Michail I.|last4=Jacobsen|first4=Stein B.|date=2016-4|title=Is the mantle chemically stratified? Insights from sound velocity modeling and isotope evolution of an early magma ocean|url=https://linkinghub.elsevier.com/retrieve/pii/S0012821X16300127|journal=Earth and Planetary Science Letters|language=en|volume=440|pages=158–168|doi=10.1016/j.epsl.2016.02.001}}</ref>. Thus, it is widely accepted that the bulk composition of the lower mantle is pyrolitic<ref name=":2" />.


== Spin Transition Zone ==
== Spin Transition Zone ==
The electronic environment of iron (Fe) in Ferropericlase transitions from a high-spin (HS) to a low-spin (LS) state at 60-70 GPa<ref name=":1" /> while Fe in Bridgmanite remains in a HS-state up to 1 TPa<ref>{{Cite journal|last=Cohen|first=R. E.|date=1997-01-31|title=Magnetic Collapse in Transition Metal Oxides at High Pressure: Implications for the Earth|url=http://dx.doi.org/10.1126/science.275.5300.654|journal=Science|volume=275|issue=5300|pages=654–657|doi=10.1126/science.275.5300.654|issn=0036-8075}}</ref>. The transition discrepancy between Ferropericlase and Bridgmanite results in the increase in partition coefficient from 0 (below HS-LS transition) to 10-14 (above HS-LS transition) implying that Bridgmanite will be depleted of Fe<ref name=":1" />. The spin transition alongside the pressure and temperature condition will result in the exothermic Perovskite to post-Perovskite transition (Pv-pPv)<ref name=":5">{{Cite journal|last=Bower|first=Dan J.|last2=Gurnis|first2=Michael|last3=Jackson|first3=Jennifer M.|last4=Sturhahn|first4=Wolfgang|date=2009-05-28|title=Enhanced convection and fast plumes in the lower mantle induced by the spin transition in ferropericlase|url=http://doi.wiley.com/10.1029/2009GL037706|journal=Geophysical Research Letters|language=en|volume=36|issue=10|doi=10.1029/2009GL037706|issn=0094-8276}}</ref>. Numerical simulations observed the heat released from the Pv-pPv transition destabilizes mantle plumes<ref name=":5" /><ref>{{Cite journal|last=Shahnas|first=M. H.|last2=Peltier|first2=W. R.|date=2010-11-17|title=Layered convection and the impacts of the perovskite-postperovskite phase transition on mantle dynamics under isochemical conditions|url=http://doi.wiley.com/10.1029/2009JB007199|journal=Journal of Geophysical Research|language=en|volume=115|issue=B11|doi=10.1029/2009JB007199|issn=0148-0227}}</ref>. The spin transition varied density and enhanced buoyancy of the mantle plume, and as a result destabilizing mantle plume convection. For example, as the cold downwelling anomaly traverses through the spin transition zone at approximately 900 km, the anomaly begins to gain negative buoyancy and accelerates (as compared to fully HS state models) until 1600 km. From 1600-1900 km, the cold anomaly decelerates, and finally below 1900 km the anomaly begins to accelerate again<ref>{{Cite journal|last=Shahnas|first=M. H.|last2=Peltier|first2=W. R.|last3=Wu|first3=Z.|last4=Wentzcovitch|first4=R.|date=2011-08-09|title=The high-pressure electronic spin transition in iron: Potential impacts upon mantle mixing|url=http://doi.wiley.com/10.1029/2010JB007965|journal=Journal of Geophysical Research|language=en|volume=116|issue=B8|doi=10.1029/2010JB007965|issn=0148-0227}}</ref>.<br />
The electronic environment of iron (Fe) in [[Ferropericlase]] transitions from a high-spin (HS) to a low-spin (LS) state at 60-70 GPa<ref name=":1" /> while Fe in [[Silicate perovskite|Bridgmanite]] remains in a HS-state up to 1 TPa<ref>{{Cite journal|last=Cohen|first=R. E.|date=1997-01-31|title=Magnetic Collapse in Transition Metal Oxides at High Pressure: Implications for the Earth|url=http://dx.doi.org/10.1126/science.275.5300.654|journal=Science|volume=275|issue=5300|pages=654–657|doi=10.1126/science.275.5300.654|issn=0036-8075}}</ref>. The transition discrepancy between [[Ferropericlase]] and [[Silicate perovskite|Bridgmanite]] results in the increase in [[Partition coefficient|partition coefficient]] to 10-14 (above HS-LS transition) implying that [[Silicate perovskite|Bridgmanite]] will be depleted of Fe<ref name=":1" />. The spin transition alongside the pressure and temperature condition will result in the exothermic Perovskite to post-Perovskite transition (Pv-pPv)<ref name=":5">{{Cite journal|last=Bower|first=Dan J.|last2=Gurnis|first2=Michael|last3=Jackson|first3=Jennifer M.|last4=Sturhahn|first4=Wolfgang|date=2009-05-28|title=Enhanced convection and fast plumes in the lower mantle induced by the spin transition in ferropericlase|url=http://doi.wiley.com/10.1029/2009GL037706|journal=Geophysical Research Letters|language=en|volume=36|issue=10|doi=10.1029/2009GL037706|issn=0094-8276}}</ref>. Numerical simulations observed the heat released from the Pv-pPv transition destabilizes mantle plumes<ref name=":5" /><ref>{{Cite journal|last=Shahnas|first=M. H.|last2=Peltier|first2=W. R.|date=2010-11-17|title=Layered convection and the impacts of the perovskite-postperovskite phase transition on mantle dynamics under isochemical conditions|url=http://doi.wiley.com/10.1029/2009JB007199|journal=Journal of Geophysical Research|language=en|volume=115|issue=B11|doi=10.1029/2009JB007199|issn=0148-0227}}</ref>. The spin transition varied density and enhanced buoyancy of the [[Mantle plume|mantle plume]], and as a result destabilizing [[Mantle plume|mantle plume]] convection. For example, as the cold downwelling anomaly traverses through the spin transition zone at approximately 900 km, the anomaly begins to gain negative buoyancy and accelerates (as compared to fully HS state models) until 1600 km. From 1600-1900 km, the cold anomaly decelerates, and finally below 1900 km the anomaly begins to accelerate again<ref>{{Cite journal|last=Shahnas|first=M. H.|last2=Peltier|first2=W. R.|last3=Wu|first3=Z.|last4=Wentzcovitch|first4=R.|date=2011-08-09|title=The high-pressure electronic spin transition in iron: Potential impacts upon mantle mixing|url=http://doi.wiley.com/10.1029/2010JB007965|journal=Journal of Geophysical Research|language=en|volume=116|issue=B8|doi=10.1029/2010JB007965|issn=0148-0227}}</ref>.<br />

Revision as of 06:06, 26 February 2019

Lower Mantle

The lower mantle comprise of approximately 56% of the Earth's total volume located 660-2900 km below the crust in 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]. Pressures and temperature at the lower mantle ranges from 24-127 GPa[2] and 1900-2630K[3]. It is widely accepted that the composition of the lower mantle is pyrolitic[4] containing three major phases of Bridgmanite, Ferropericlase and Calcium-silicate perovskite. The pressures at the lower mantle was shown to induce a spin transition of iron-bearing ferropericlase[5] affecting both mantle plume dynamics[6][7] and chemistry[5].

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[2],

  • 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 at the lower mantle.
  • 2700-2900 km: The D" layer is considered the transition between the lower mantle to the outer core.

Temperatures of the lower mantle ranges from 1960 K at the topmost layer to 2630 K at a depth of 2700 km[9]. Models of the temperature of the lower mantle approximates convection as the primary heat transport contribution while conduction and radiative heat is considered negligible. As a result, the Mantle's temperature gradient as a function of depth is approximately adiabatic[1]. Calculation of the geothermal gradient observed a decrease from 0.47 K/km at the uppermost lower mantle to 0.24 K/km at 2600km[9].

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[4].
  • Chondritic: suggests that the Earth's lower mantle was accreted from 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[10]. 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 a pyrolitic lower mantle compositions considering minor elements resulted in a match with the PREM shear velocity profile within 1%[12]. Thus, it is widely accepted that the bulk composition of the lower mantle is pyrolitic[1].

Spin Transition Zone

The electronic environment of iron (Fe) in Ferropericlase transitions from a high-spin (HS) to a low-spin (LS) state at 60-70 GPa[5] while Fe in Bridgmanite remains in a HS-state up to 1 TPa[13]. The transition discrepancy between Ferropericlase and Bridgmanite results in the increase in partition coefficient to 10-14 (above HS-LS transition) implying that Bridgmanite will be depleted of Fe[5]. The spin transition alongside the pressure and temperature condition will result in the exothermic Perovskite to post-Perovskite transition (Pv-pPv)[14]. Numerical simulations observed the heat released from the Pv-pPv transition destabilizes mantle plumes[14][15]. The spin transition varied density and enhanced buoyancy of the mantle plume, and as a result destabilizing mantle plume convection. For example, as the cold downwelling anomaly traverses through the spin transition zone at approximately 900 km, the anomaly begins to gain negative buoyancy and accelerates (as compared to fully HS state models) until 1600 km. From 1600-1900 km, the cold anomaly decelerates, and finally below 1900 km the anomaly begins to accelerate again[16].

  1. ^ a b c V., Kaminsky, Felix (2017). The Earth's lower mantle : composition and structure. Cham: Springer. ISBN 9783319556840. OCLC 988167555.{{cite book}}: CS1 maint: multiple names: authors list (link)
  2. ^ a b c Dziewonski, Adam M.; Anderson, Don L. (1981-06). "Preliminary reference Earth model". Physics of the Earth and Planetary Interiors. 25 (4): 297–356. doi:10.1016/0031-9201(81)90046-7. ISSN 0031-9201. {{cite journal}}: Check date values in: |date= (help)
  3. ^ Katsura, Tomoo; Yoneda, Akira; Yamazaki, Daisuke; Yoshino, Takashi; Ito, Eiji (2010-11). "Adiabatic temperature profile in the mantle". Physics of the Earth and Planetary Interiors. 183 (1–2): 212–218. doi:10.1016/j.pepi.2010.07.001. ISSN 0031-9201. {{cite journal}}: Check date values in: |date= (help)
  4. ^ a b Edward), Ringwood, A. E. (Alfred ([1976]). Composition and petrology of the earth's mantle. McGraw-Hill. ISBN 0070529329. OCLC 16375050. {{cite book}}: Check date values in: |date= (help)CS1 maint: multiple names: authors list (link)
  5. ^ a b c d Badro, J. (2003-04-03). "Iron Partitioning in Earth's Mantle: Toward a Deep Lower Mantle Discontinuity". Science. 300 (5620): 789–791. doi:10.1126/science.1081311. ISSN 0036-8075.
  6. ^ 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. doi:10.1093/gji/ggx198. ISSN 0956-540X.{{cite journal}}: CS1 maint: unflagged free DOI (link)
  7. ^ 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). doi:10.1029/2009GL037706. ISSN 0094-8276.
  8. ^ Bullen, K.E. (1942). "The density variation of the earth's central core". Bulletin of the Seismological Society of America. 32: 19–29.
  9. ^ a b Katsura, Tomoo; Yoneda, Akira; Yamazaki, Daisuke; Yoshino, Takashi; Ito, Eiji (2010-11). "Adiabatic temperature profile in the mantle". Physics of the Earth and Planetary Interiors. 183 (1–2): 212–218. doi:10.1016/j.pepi.2010.07.001. ISSN 0031-9201. {{cite journal}}: Check date values in: |date= (help)
  10. ^ Irifune, T.; Shinmei, T.; McCammon, C. A.; Miyajima, N.; Rubie, D. C.; Frost, D. J. (2010-01-08). "Iron Partitioning and Density Changes of Pyrolite in Earth's Lower Mantle". Science. 327 (5962): 193–195. doi:10.1126/science.1181443. ISSN 0036-8075.
  11. ^ Wang, Xianlong; Tsuchiya, Taku; Hase, Atsushi (2015-7). "Computational support for a pyrolitic lower mantle containing ferric iron". Nature Geoscience. 8 (7): 556–559. doi:10.1038/ngeo2458. ISSN 1752-0894. {{cite journal}}: Check date values in: |date= (help)
  12. ^ Hyung, Eugenia; Huang, Shichun; Petaev, Michail I.; Jacobsen, Stein B. (2016-4). "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. doi:10.1016/j.epsl.2016.02.001. {{cite journal}}: Check date values in: |date= (help)
  13. ^ Cohen, R. E. (1997-01-31). "Magnetic Collapse in Transition Metal Oxides at High Pressure: Implications for the Earth". Science. 275 (5300): 654–657. doi:10.1126/science.275.5300.654. ISSN 0036-8075.
  14. ^ a b 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). doi:10.1029/2009GL037706. ISSN 0094-8276.
  15. ^ Shahnas, M. H.; Peltier, W. R. (2010-11-17). "Layered convection and the impacts of the perovskite-postperovskite phase transition on mantle dynamics under isochemical conditions". Journal of Geophysical Research. 115 (B11). doi:10.1029/2009JB007199. ISSN 0148-0227.
  16. ^ Shahnas, M. H.; Peltier, W. R.; Wu, Z.; Wentzcovitch, R. (2011-08-09). "The high-pressure electronic spin transition in iron: Potential impacts upon mantle mixing". Journal of Geophysical Research. 116 (B8). doi:10.1029/2010JB007965. ISSN 0148-0227.
Retrieved from "https://en.wikipedia.org/enwiki/w/index.php?title=Lower_mantle&oldid=885141756"