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Abstract Mineral phase transitions can either hinder or accelerate mantle flow. In the present day, the formation of the bridgmanite + ferropericlase assemblage from ringwoodite at 660 km depth has been found to cause weak and intermittent layering of mantle convection. However, for the higher temperatures in Earth's past, different phase transitions could have controlled mantle dynamics. We investigate the potential changes in convection style during Earth's secular cooling using a new numerical technique that reformulates the energy conservation equation in terms of specific entropy instead of temperature. This approach enables us to accurately include the latent heat effect of phase transitions for mantle temperatures different from the average geotherm, and therefore fully incorporate the thermodynamic effects of realistic phase transitions in global‐scale mantle convection modeling. We set up 2‐D models with the geodynamics softwareAspect, using thermodynamic properties computed by HeFESTo, while applying a viscosity profile constrained by the geoid and mineral physics data and a visco‐plastic rheology to reproduce plate‐like behavior and Earth‐like subduction morphologies. Our model results reveal the layering of plumes induced by the wadsleyite to garnet (majorite) + ferropericlase endothermic transition (between 450 and 590 km depth and over the 2000–2500 K temperature range). They show that this phase transition causes a large‐scale and long‐lasting temperature elevation in a depth range of 500–650 km depth if the potential temperature of the mantle is higher than 1800 K, indicating that mantle convection may have been partially layered in Earth's early history.
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Thermal expansivity, heat capacity and bulk modulus of the mantle
SUMMARY We derive exact expressions for the thermal expansivity, heat capacity and bulk modulus for assemblages with arbitrarily large numbers of components and phases, including the influence of phase transformations and chemical exchange. We illustrate results in simple two-component, two-phase systems, including Mg–Fe olivine-wadsleyite and Ca–Mg clinopyroxene-orthopyroxene and for a multicompontent model of mantle composition in the form of pyrolite. For the latter we show results for the thermal expansivity and heat capacity over the entire mantle pressure–temperature regime to 40 GPa, or a depth of 1000 km. From the thermal expansivity, we derive a new expression for the phase buoyancy parameter that is valid for arbitrarily large numbers of phases and components and which is defined at every point in pressure–temperature space. Results reveal regions of the mantle where the magnitude of the phase buoyancy parameter is larger in magnitude than for those phase transitions that are most commonly included in mantle convection simulations. These regions include the wadsleyite to garnet and ferropericlase transition, which is encountered along hot isentropes (e.g. 2000 K potential temperature) in the transition zone, and the ferropericlase and stishovite to bridgmanite transition, which is encountered along cold isentropes (e.g. 1000 K potential temperature) in the shallow lower mantle. We also show the bulk modulus along a typical mantle isentrope and relate it to the Bullen inhomogeneity parameter. All results are computed with our code HeFESTo, updates and improvements to which we discuss, including the implementation of the exact expressions for the thermal expansivity, heat capacity and bulk modulus, generalization to allow for pressure dependence of non-ideal solution parameters and an improved numerical scheme for minimizing the Gibbs free energy. Finally, we present the results of a new global inversion of parameters updated to incorporate more recent results from experiment and first principles theory, as well as a new phase (nal phase), and new species: Na-majorite and the NaAlO2 end-member of ferropericlase.
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- Award ID(s):
- 1853388
- PAR ID:
- 10321503
- Date Published:
- Journal Name:
- Geophysical Journal International
- Volume:
- 228
- Issue:
- 2
- ISSN:
- 0956-540X
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1093/gji/ggab394
