The relative behaviour of bulk and shear modulus as an indicator of the iron spin transition in the lower mantle

Date

2021

Authors

Kennett, Brian

Journal Title

Journal ISSN

Volume Title

Publisher

Elsevier

Abstract

For a regular material, in a single state, the effect of increasing pressure is to produce a linear relation between shear modulus (G), bulk modulus (K) and pressure (p). Such a dependence on pressure fits the behaviour of individual mineral and mineral composites well. For the iron bearing minerals in the lower mantle, the transition from the high-spin state to the low-spin state with increasing pressure produces deviations from this simple behaviour, due to a much larger reduction of the bulk modulus than the shear modulus in the spin crossover. There are linear zones in the dependence of the modulus ratio G/K on pressure scaled by bulk modulus (p/K) above and below the transition, with a complex excursion to high ratios in between. With increasing temperature the spin transition is smoothed out in depth, and is expected to be smooth along a mantle geotherm. For the ak135 body-wave model of radial Earth structure there is a slight, but distinct, change of slope in the relation between the modulus ratio and scaled pressure corresponding to a depth range around 1550 km. This feature is strongly suggestive of a residual effect of a 3-D averaged spin transition in the ferropericlase component of the lower mantle.

Description

Keywords

Spin transition, Earth models, bulk modulus, shear modulus, deep Earth

Citation

Source

Earth and Planetary Science Letters

Type

Journal article

Book Title

Entity type

Access Statement

License Rights

DOI

10.1016/j.epsl.2021.116808

Restricted until

2099-12-31