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Compositional heterogeneity in the mantle transition zone

Abstract

Earth’s mantle transition zone (MTZ) is characterized by several sharp increases in seismic wave speed between ~300 km and ~850 km depth. These seismic discontinuities are generally attributed to solid-state phase transitions that lead to density and viscosity increases, which could cause a barrier to convection by segregating thermally and chemically heterogeneous material. This Review discusses insights into the role of MTZ compositional heterogeneity in mantle convection, derived from the joint constraints of MTZ discontinuity-reflected, discontinuity-refracted and discontinuity-transmitted seismic waves and thermodynamic and convection models. Growing seismic data sets and advances in analysis techniques show that the topography of these discontinuities mainly reflects variations in mantle temperature and, hence, present-day mantle flow. However, the discordant behaviour of the 410 km and 660 km discontinuities shows that the thermal structure is not vertically coherent across the MTZ in many areas, indicating that the MTZ delays the convective transport of cold material from above and hot material from below. Variable reflectivity of the MTZ discontinuity provides evidence of lateral and vertical heterogeneity in major element chemistry and volatile content. Seismic results are consistent with whole-mantle mechanical mixing of tectonic plates, with segregated material accumulating in the MTZ over multiple mantle convection cycles.

Key points

  • The two main global mantle transition zone (MTZ) discontinuities, near 410 km and 660 km depth, and their topography are largely compatible with, respectively, exothermic and endothermic phase transformations in an olivine-rich composition at a potential temperature of ~1,600 K. In a few locations, the potential temperature seems to be high enough (>1,900 K) for exothermic garnet transitions to define the 660 km discontinuity.

  • Variable amplitudes of the 410 and 660 km discontinuities, frequent detection of a discontinuity near 300 km, infrequent detection of seismic reflectors between 700 km and 850 km depth and seismic complexity of the 520 km and 660 km discontinuities are all consistent with the mantle being largely a mechanical mixture of basaltic and refractory peridotitic material but having a larger diversity of peridotites than just harzburgite.

  • Geodynamic models indicate that whole-mantle convection of such mechanically mixed material will lead to compositional gradients in the MTZ, with accumulations of basalt just above 660 km and accumulations of refractory material below this depth. The current quality and quantity of seismic constraints are insufficient to confirm or rule out widespread vertical compositional gradients.

  • Some evidence indicates variable hydration of the MTZ and partial melting near the top of the MTZ, due to differences in water solubility of material above and below the 410 km discontinuity.

  • The absence of a clear correlation between observations of compositional and thermal heterogeneity in the MTZ is consistent with models suggesting that MTZ compositional heterogeneity accumulates over multiple convective cycles and is, therefore, largely disconnected from present-day flow.

  • Seismic tomography, numerical models and geochemical constraints support large-scale mantle flow, but evidence of the flattening of subducting lithospheric slabs and the accumulation of hot material derived from lower-mantle upwellings near the base of the MTZ indicates that the MTZ hampers flow more than would be expected for unobstructed whole-mantle convection, potentially owing to compositional gradients.

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Fig. 1: Mantle discontinuities and possible convection modes.
Fig. 2: Seismic wave speed and density in the mantle transition zone.
Fig. 3: Ray paths of seismic phases and synthetic waveforms.
Fig. 4: Synthetic reflectivity of SS and PP seismic wave precursors at the mantle transition zone 660 km discontinuity.
Fig. 5: Output of a tectonic plate recycling model.
Fig. 6: Characteristics of synthetic mantle transition zone seismic wave speed structures in a tectonic plate recycling model.
Fig. 7: Images of mantle transition zone seismic wave speed and thickness.

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Acknowledgements

The authors thank the reviewers for their thoughtful comments. C.Y. is supported by National Natural Science Foundation of China (NSFC) grant no. 42174058. This Review is based on previously published material and data.

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S.G., C.Y., M.D.B. and J.Y. researched data for the article. S.G., C.Y., M.D.B. and R.D.v.d.H. contributed substantially to discussions of the content. The manuscript was written by S.G. with input from all co-authors. All authors reviewed and/or edited the manuscript before submission. The original figures were made by S.G., C.Y. and J.Y.

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Correspondence to Saskia Goes.

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Glossary

Seismic discontinuities

Changes in seismic wave speed over a depth interval that is sufficiently narrow (usually 1–30 km) to produce refractions, reflections and conversions of seismic waves.

Solid-state phase transitions

Changes in mineral phase to a more energetically favoured atomic structure occurring as a function of pressure and temperature (such as to a more densely packed structure in response to an increase in pressure).

Partial melting

Rocks made up of multiple components with different melting temperatures can melt in part; for example, basalt is formed when a fertile peridotite or pyrolite melts by about 18%.

Major element

An element with a concentration exceeding 1 wt%; the most common building blocks of mantle rocks are magnesium, silicon, iron, oxygen, aluminium and calcium.

Mantle convection models

Numerical models that solve fluid dynamic equations of the conservation of mass, momentum and, sometimes, also energy, plus equations of state that capture how viscosity, density, thermal expansivity and conductivity vary with pressure and temperature (and sometimes other parameters such as strain rate or composition), used to study the physical behaviour of convection in the mantle.

Basalt

The main rock making up the crust of oceanic plates, formed by partial melting of mantle rocks (peridotites): includes mid-ocean-ridge basalt (MORB), island-arc basalt and ocean-island basalt.

Fertile

Rocks that have not been affected by partial melting.

Peridotite

Ultramafic rock that is representative of the composition of Earth’s mantle, including (from most to least refractory) dunites, harzburgites and lherzolites, all comprising ≥40% olivine (or its high-pressure polymorphs) along with pyroxenes, spinels and garnets.

Refractory

Residual rock remaining after partial melting: when a fertile peridotite partially melts at average mantle temperatures, the melt is basaltic and the refractory residue is harzburgitic.

Pyrolite

The theoretical composition of the mantle, based on melting models, compositions of meteorites and the least-differentiated mantle peridotite samples found at the surface.

Equilibrium assemblage

A mixture of rock types with different compositions that chemically react to form a single homogeneous bulk composition that is in thermodynamic equilibrium at ambient pressure and temperature.

Thermodynamic modelling

Prediction of mineral phase stability fields and physical properties by solving Gibbs free energy minimization and high-pressure–temperature equations for density and elastic parameters under the constraints of mineral-relevant solid-solution models; usually based on data from high-pressure–high-temperature experiments or ab initio property modelling.

Potential temperature

The temperature that a material would have if brought to the Earth’s surface in solid state while decompressing adiabatically (that is, without exchanging heat with the surroundings); this parameter facilitates the comparison of temperatures at different depths in the mantle.

Adiabat

A temperature–depth profile in convecting material that flows rapidly enough that no heat is exchanged with the surroundings; that is, temperatures increase with depth only because of isentropic compression and solid-state phase transitions.

Post-spinel

Refers to the (magnesium, iron) bridgmanite–magnesiowüstite high-pressure assemblage of minerals that ringwoodite (formerly known as γ-spinel) transforms to at lower-mantle pressures.

Post-garnet

Refers to the (magnesium, iron, calcium, aluminium) perovskite–magnesiowüstite high-pressure assemblage of minerals that garnet transforms to at lower-mantle pressures.

Precursors

A term used in this Review to refer to seismic phases that arrive before a surface-reflected phase because they are reflected off a subsurface discontinuity instead of the surface.

Receiver functions

Functions derived using a method of analysing seismic waveforms that enables the signal from a discontinuity structure below the recording station to be isolated.

Primordial

Originating from the time of Earth’s formation.

Seismic tomography

An inversion method used to image Earth’s volumetric interior structure using natural or human-induced seismic waves that traverse the Earth, preferably recorded along many different and crossing paths.

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Goes, S., Yu, C., Ballmer, M.D. et al. Compositional heterogeneity in the mantle transition zone. Nat Rev Earth Environ 3, 533–550 (2022). https://doi.org/10.1038/s43017-022-00312-w

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