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

mid-ocean ridge A mid-ocean ridge (MOR) is a undersea mountain range, seafloor mountain system formed by plate tectonics. It typically has a depth of about and rises about above the deepest portion of an ocean basin. This feature is where seafloor spreading ...

basalts Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90% of a ...

(MORB) by partial melting. It was first proposed by Ted Ringwood (1962) as being 1 part basalt and 4 parts

harzburgite Harzburgite, an ultramafic, igneous rock, is a variety of peridotite consisting mostly of the two minerals olivine and low-calcium (Ca) pyroxene ( enstatite); it is named for occurrences in the Harz Mountains of Germany. It commonly contains a ...

, but later was revised to being 1 part

tholeiitic basalt The tholeiitic magma series () is one of two main magma series in subalkaline igneous rocks, the other being the calc-alkaline series. A magma series is a chemically distinct range of magma compositions that describes the evolution of a mafic ...

and 3 parts

dunite Dunite (), also known as olivinite (not to be confused with the mineral olivenite), is an intrusive igneous rock of ultramafic composition and with phaneritic (coarse-grained) texture. The mineral assemblage is greater than 90% olivine, wit ...

. The term is derived from the mineral names PYR-oxene and OL-ivine. However, whether pyrolite is entirely representative of the Earth's mantle remains debated.

Chemical composition and
phase transition In physics, chemistry, and other related fields like biology, a phase transition (or phase change) is the physical process of transition between one state of a medium and another. Commonly the term is used to refer to changes among the basic Sta ...

The major elements composition of pyrolite is about 44.71

molar percent The atomic ratio is a measure of the ratio of atoms of one kind (i) to another kind (j). A closely related concept is the atomic percent (or at.%), which gives the percentage of one kind of atom relative to the total number of atoms. The molecula ...

(mol%) SiO₂, 3.98 % Al₂O₃, 8.18 % FeO, 3.17 % CaO, 38.73 % MgO, 0.13 % Na₂O. 1) A pyrolitic

Upper Mantle The upper mantle of Earth is a very thick layer of rock inside the planet, which begins just beneath the crust (geology), crust (at about under the oceans and about under the continents) and ends at the top of the lower mantle (Earth), lower man ...

is mainly composed of olivine (~60

volume percent In chemistry and fluid mechanics, the volume fraction \varphi_i is defined as the volume of a constituent ''V'i'' divided by the volume of all constituents of the mixture ''V'' prior to mixing: :\varphi_i = \frac . Being dimensionless, its un ...

(vol%)),

clinopyroxene The pyroxenes (commonly abbreviated Px) are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula , where X represents ions of calcium (Ca), sodium (Na), iron (Fe ...

orthopyroxene The pyroxenes (commonly abbreviated Px) are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula , where X represents ions of calcium (Ca), sodium (Na), iron (Fe( ...

, and

garnet Garnets () are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives. Garnet minerals, while sharing similar physical and crystallographic properties, exhibit a wide range of chemical compositions, de ...

. Pyroxene would gradually dissolved into

and form majoritic garnet. 2) A pyrolitic

Mantle Transition Zone The transition zone is the part of Earth's mantle that is located between the lower and the upper mantle, most strictly between the seismic-discontinuity depths of about , but more broadly defined as the zone encompassing those discontinuities ...

is mainly composed of 60 vol% olivine-polymorphs (

wadsleyite Wadsleyite is an orthorhombic mineral with the formula β-(Mg,Fe)2SiO4. It was first found in nature in the Peace River meteorite from Alberta, Canada. It is formed by a phase transformation from olivine (α-(Mg,Fe)2SiO4) under increasing press ...

ringwoodite Ringwoodite is a high-pressure phase of Mg2SiO4 (magnesium silicate) formed at high temperatures and pressures of the Earth's mantle between depth. It may also contain iron and hydrogen. It is polymorphous with the olivine phase forsterite (a ...

) 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 Post-perovskite (pPv) is a high-pressure phase of magnesium silicate (MgSiO3). It is composed of the prime oxide constituents of the Earth's rocky mantle (MgO and SiO2), and its pressure and temperature for stability imply that it is likely to occu ...

may present at the bottom of the Lower Mantle.

Seismic velocity and density properties

Seismic_velocities_of_pyrolite_along_the_1600_K_adiabatic_geotherm_

The

P-wave A P wave (primary wave or pressure wave) is one of the two main types of elastic body waves, called seismic waves in seismology. P waves travel faster than other seismic waves and hence are the first signal from an earthquake to arrive at any ...

and

S-wave __NOTOC__ In seismology and other areas involving elastic waves, S waves, secondary waves, or shear waves (sometimes called elastic S waves) are a type of elastic wave and are one of the two main types of elastic body waves, so named because t ...

velocities (Vp and Vs) of pyrolite along the 1600 K adiabatic geotherm are shown in Fig. 2, and its density profile is shown in Fig. 3. 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, respectively, which are mainly attributed to the olivine-wadsleyite

phase transition In physics, chemistry, and other related fields like biology, a phase transition (or phase change) is the physical process of transition between one state of a medium and another. Commonly the term is used to refer to changes among the basic Sta ...

. 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. 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). It may also violate mantle heterogeneity. 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, whereas some studies support the pyrolite model.

Other Mantle Rock models

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. The piclogite phase composition is similar as 20% olivine + 80% eclogite. (2)

Eclogite Eclogite () is a metamorphic rock containing garnet ( almandine- pyrope) hosted in a matrix of sodium-rich pyroxene ( omphacite). Accessory minerals include kyanite, rutile, quartz, lawsonite, coesite, amphibole, phengite, paragonite, zoisit ...

, it is transformed from the Mid-Ocean Ridge Basalt at a depth of ~60 km, exists in the Earth's mantle mainly within the subducted slabs. It is mainly composed of garnet and clinopyroxene (mainly

omphacite Omphacite is a member of the clinopyroxene group of silicate minerals with formula: ( Ca, Na)( Mg, Fe2+, Al) Si2 O6. It is a variably deep to pale green or nearly colorless variety of clinopyroxene. It normally appears in eclogite, which is t ...

) up to ~500 km depth (Fig. 4). (3) Harzburgite, it mainly exists under the Mid-Ocean Ridge basalt layer of the oceanic

lithosphere A lithosphere () is the rigid, outermost rocky shell of a terrestrial planet or natural satellite. On Earth, it is composed of the crust and the lithospheric mantle, the topmost portion of the upper mantle that behaves elastically on time ...

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

subducted Subduction is a geological process in which the oceanic lithosphere and some continental lithosphere is recycled into the Earth's mantle at the convergent boundaries between tectonic plates. Where one tectonic plate converges with a second plat ...

oceanic lithosphere A lithosphere () is the rigid, outermost rocky shell of a terrestrial planet or natural satellite. On Earth, it is composed of the crust and the lithospheric mantle, the topmost portion of the upper mantle that behaves elastically on time sc ...

. 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. 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).

References

{{Reflist Igneous rocks Petrology Geologic modelling

Chemical composition and phase transition In physics, chemistry, and other related fields like biology, a phase transition (or phase change) is the physical process of transition between one state of a medium and another. Commonly the term is used to refer to changes among the basic Sta ...

Seismic velocity and density properties

Shortcomings

Other Mantle Rock models

See also

References

Chemical composition and
phase transition In physics, chemistry, and other related fields like biology, a phase transition (or phase change) is the physical process of transition between one state of a medium and another. Commonly the term is used to refer to changes among the basic Sta ...