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.[1][2] It was first proposed by Ted Ringwood (1962)[3] as being 1 part basalt and 4 parts dunite, but later was revised to being 1 part tholeiitic basalt and 3 parts dunite.[1][4] The term is derived from the mineral names PYR-oxene and OL-ivine.[5] However, whether pyrolite is representative of the Earth's mantle remains debated.[6]

Chemical composition and phase transition
Fig.1 The mineral volume fraction in a pyrolitic mantle up to 1000 km depth.[7][8] Ol: olivine; Opx: orthopyroxene; Cpx: clinopyroxene; Gt: garnet; Wad: wadsleyite; Ring: ringwoodite; Pv: perovskite; Fp: ferropericlase; Ca-Pv: calcium perovskite.


The major elements composition of pyrolite is about 44.71 weight percent (wt%) SiO2, 3.98 wt% Al2O3, 8.18 wt% FeO, 3.17 wt% CaO, 38.73 wt% MgO, 0.13 wt% Na2O.[9]

1) A pyrolitic Upper Mantle is mainly composed of olivine (~60 volume percent (vol%)), clinopyroxene, orthopyroxene, and garnet.[7] Pyroxene would gradually dissolved into garnet and form majoritic garnet.[10]

2) A pyrolitic Mantle Transition Zone is mainly composed of 60 vol% olivine-polymorphs (wadsleyite, ringwoodite) and ~40 vol% majoritic garnet. The top and bottom boundary of the Mantle Transition zone are mainly marked by olivine-wadsleyite transition and ringwoodite-perovskite transition, respectively.

3) A pyrolitc Lower Mantle is mainly composed of magnesium perovskite (~80 vol%), ferroperclase (~13 vol%), and calcium perovskite (~7%). In addition, post-perovskite may present at the bottom of the Lower Mantle.

Seismic velocity and density properties
Fig. 2 Vp and Vs profiles of pyrolite along the 1600 K adiabatic geotherm[2]
Fig. 3 Density profile of pyrolite along the 1600 K adiabatic geotherm[2]

The P-wave and S-wave velocities (Vp and Vs) of pyrolite along the 1600 K adiabatic geotherm are shown in Fig. 2,[2] and its density profile is shown in Fig. 3.[2]

At the boundary between the Upper Mantle and the Mantle Transition Zone (~410 km), Vp, Vs, and density jump by ~6%, ~6%, and ~4% in a pyrolite model,[2] respectively, which are mainly attributed to the olivine-wadsleyite phase transition. [11]

At the boundary between the Mantle Transition Zone and the Lower Mantle, Vp, Vs, and density jump by ~3%, ~6%, and ~6% in a pyrolite model, respectively.[2] With more elasticity parameters available, the Vp, Vs, and density profiles of pyrolite would be updated.

Shortcomings

Whether pyrolite could represent the ambient mantle remains debated.

In the geochemical aspect, it does not satisfy trace elements or isotopic data of Mid-Ocean Ridge Basalts because the pyrolite hypothesis is based on major elements and some arbitrary assumptions (e.g. amounts of basalt and melting in the source).[1] It may also violate mantle heterogeneity.[12]

In the geophysical aspect, some studies suggest that seismic velocities of pyrolite do not match well with the observed global seismic models (such as PREM) in the Earth's interior, [6] whereas some studies support the pyrolite model.[13]

Other Mantle Rock models
Fig. 4. Mineral proportion of a MORB-transformed eclogite at 250-500 km depth[14]

There are other rock models for the Earth's mantle:

(1) Piclogite: by contrast to the olivine-enriched pyrolite, piclogite is an olivine-poor model (~20% olivine) proposed to provide a better match to the seismic velocity observations in the transition zone.[15][16] The piclogite phase composition is similar as 20% olivine + 80% eclogite.[17]

(2) Eclogite, it is transformed from the Mid-Ocean Ridge Basalt at a depth of ~60 km,[18] exists in the Earth's mantle mainly within the subducted slabs. It is mainly composed of garnet and clinopyroxene (mainly omphacite) up to ~500 km depth (Fig. 4).

(3) Harzburgite, it mainly exists under the Mid-Ocean Ridge basalt layer of the oceanic lithosphere, and can enter into the deep mantle along with the subducted oceanic lithosphere. Its phase composition is similar as pyrolite, but shows higher olivine proportion (~70 vol%) than pyrolite.[19]

Overall, pyrolite and piclogite are both rock models for the ambient mantle, eclogite and harzburgite are rock models for subducted oceanic lithosphere. Formed from partial melting of pyrolite, the oceanic lithosphere is mainly composed of the basalt layer, harzburgite layer, and depleted pyrolite from top to bottom.[20] The subducted oceanic lithospheres contribute to the heterogeneity in the Earth's mantle because they have different composition (eclogite and harzburgite) from the ambient mantle (pyrolite).[2][14]

See also

References
  1. Anderson, Don L. (1989-01-01). Theory of the Earth. Boston, MA: Blackwell Scientific Publications. ISBN 978-0-86542-335-0.
  2. Xu, Wenbo; Lithgow-Bertelloni, Carolina; Stixrude, Lars; Ritsema, Jeroen (October 2008). "The effect of bulk composition and temperature on mantle seismic structure". Earth and Planetary Science Letters. 275 (1–2): 70–79. doi:10.1016/j.epsl.2008.08.012. ISSN 0012-821X.
  3. Ringwood, A. E. (Feb 1962). "A model for the upper mantle". Journal of Geophysical Research. 67 (2): 857–867. doi:10.1029/jz067i002p00857. ISSN 0148-0227.
  4. Ringwood, A.E.; Major, Alan (Sep 1966). "High-pressure transformations in pyroxenes". Earth and Planetary Science Letters. 1 (5): 351–357. doi:10.1016/0012-821x(66)90023-9. ISSN 0012-821X.
  5. D.H. Green. Pyrolite. In: Petrology. Encyclopedia of Earth Science. Springer, 1989
  6. Katsura, Tomoo; Shatskiy, Anton; Manthilake, M. A. Geeth M.; Zhai, Shuangmeng; Yamazaki, Daisuke; Matsuzaki, Takuya; Yoshino, Takashi; Yoneda, Akira; Ito, Eiji; Sugita, Mitsuhiro; Tomioka, Natotaka (2009-06-12). "P-V-Trelations of wadsleyite determined by in situ X-ray diffraction in a large-volume high-pressure apparatus". Geophysical Research Letters. 36 (11). doi:10.1029/2009gl038107. ISSN 0094-8276.
  7. Frost, Daniel J. (2008-06-01). "The Upper Mantle and Transition Zone". Elements. 4 (3): 171–176. doi:10.2113/GSELEMENTS.4.3.171. ISSN 1811-5209.
  8. Stixrude, Lars; Lithgow‐Bertelloni, Carolina (2005). "Mineralogy and elasticity of the oceanic upper mantle: Origin of the low-velocity zone". Journal of Geophysical Research: Solid Earth. 110 (B3). doi:10.1029/2004JB002965. ISSN 2156-2202.
  9. Workman, Rhea K.; Hart, Stanley R. (Feb 2005). "Major and trace element composition of the depleted MORB mantle (DMM)". Earth and Planetary Science Letters. 231 (1–2): 53–72. doi:10.1016/j.epsl.2004.12.005. ISSN 0012-821X.
  10. Irifune, Tetsuo (May 1987). "An experimental investigation of the pyroxene-garnet transformation in a pyrolite composition and its bearing on the constitution of the mantle". Physics of the Earth and Planetary Interiors. 45 (4): 324–336. doi:10.1016/0031-9201(87)90040-9. ISSN 0031-9201.
  11. SAWAMOTO, H.; WEIDNER, D. J.; SASAKI, S.; KUMAZAWA, M. (1984-05-18). "Single-Crystal Elastic Properties of the Modified Spinel (Beta) Phase of Magnesium Orthosilicate". Science. 224 (4650): 749–751. doi:10.1126/science.224.4650.749. ISSN 0036-8075.
  12. Don L. Anderson, New Theory of the Earth, Cambridge University Press, 2nd ed. 2007, p. 193 ISBN 978-0-521-84959-3
  13. Irifune, T.; Higo, Y.; Inoue, T.; Kono, Y.; Ohfuji, H.; Funakoshi, K. (2008). "Sound velocities of majorite garnet and the composition of the mantle transition region". Nature. 451 (7180): 814–817. doi:10.1038/nature06551. ISSN 0028-0836.
  14. Hao, Ming; Zhang, Jin S.; Pierotti, Caroline E.; Zhou, Wen-Yi; Zhang, Dongzhou; Dera, Przemyslaw (Aug 2020). "The seismically fastest chemical heterogeneity in the Earth's deep upper mantle—implications from the single-crystal thermoelastic properties of jadeite". Earth and Planetary Science Letters. 543: 116345. doi:10.1016/j.epsl.2020.116345. ISSN 0012-821X.
  15. Bass, Jay D.; Anderson, Don L. (Mar 1984). "Composition of the upper mantle: Geophysical tests of two petrological models". Geophysical Research Letters. 11 (3): 229–232. doi:10.1029/gl011i003p00229. ISSN 0094-8276.
  16. Bass, Jay D.; Anderson, Don L. (1988), "Composition of the upper mantle: Geophysical tests of two petrological models", Elastic Properties and Equations of State, Washington, D. C.: American Geophysical Union, pp. 513–516, ISBN 0-87590-240-5, retrieved 2020-10-03
  17. Irifunea, T.; Ringwood, A. E. (1987), "Phase transformations in primitive MORB and pyrolite compositions to 25 GPa and some geophysical implications", High‐Pressure Research in Mineral Physics: A Volume in Honor of Syun‐iti Akimoto, Washington, D. C.: American Geophysical Union, pp. 231–242, ISBN 0-87590-066-6, retrieved 2020-10-03
  18. Makar, A. B.; McMartin, K. E.; Palese, M.; Tephly, T. R. (Jun 1975). "Formate assay in body fluids: application in methanol poisoning". Biochemical Medicine. 13 (2): 117–126. doi:10.1016/0006-2944(75)90147-7. ISSN 0006-2944. PMID 1.
  19. Ishii, Takayuki; Kojitani, Hiroshi; Akaogi, Masaki (Apr 2019). "Phase Relations of Harzburgite and MORB up to the Uppermost Lower Mantle Conditions: Precise Comparison With Pyrolite by Multisample Cell High‐Pressure Experiments With Implication to Dynamics of Subducted Slabs". Journal of Geophysical Research: Solid Earth. 124 (4): 3491–3507. doi:10.1029/2018jb016749. ISSN 2169-9313.
  20. Ringwood, A. E.; Irifune, T. (Jan 1988). "Nature of the 650–km seismic discontinuity: implications for mantle dynamics and differentiation". Nature. 331 (6152): 131–136. doi:10.1038/331131a0. ISSN 1476-4687.
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