Pyrolite

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Pyrolite is a term used to characterize a model composition of the Earth's

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

molar percent (mol%) SiO2, 3.98 % Al2O3, 8.18 % FeO, 3.17 % CaO, 38.73 % MgO, 0.13 % Na2O.[9]

1) A pyrolitic

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 pyrolitic

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

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,[citation needed] 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.[18]

Overall, pyrolite and piclogite are both rock models for the ambient mantle, eclogite and harzburgite are rock models for

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.[19] 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. ^
    ISBN 978-0-86542-335-0
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  2. ^
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  3. ISSN 0148-0227
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  5. ^ D.H. Green. Pyrolite. In: Petrology. Encyclopedia of Earth Science. Springer, 1989
  6. ^
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  7. ^
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  8. ISSN 2156-2202
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  9. ISSN 0012-821X
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  10. ISSN 0031-9201
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  11. S2CID 6602306
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  12. ISBN 978-0-521-84959-3
  13. S2CID 205212051
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  14. ^
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  15. ISSN 0094-8276
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  16. ISBN 0-87590-240-5
    , retrieved 2020-10-03
  17. ISBN 0-87590-066-6
    , retrieved 2020-10-03
  18. S2CID 146787786
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  19. S2CID 4323081
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