Mantle Phase Changes

Solid-state mineralogical changes that occur with increasing depth in a planet’s mantle. These are best understood for Earth’s mantle. The change at ~400 km corresponds to a transition from the α- to β-structures of multi-component olivine with ~6% increase in density. The small Fe concentration in natural mantle olivine results in a thin region where both phases coexist in equilibrium and where macroscopically observed density changes occur. Depending actual temperature-depth relationships, the thickness of the two-phase region varies. The ~670-km transformation is fundamentally different from the a-b transition, because it involves a change in chemical composition. The olivine and pyroxene-garnet components transform into magnesiowüstite, (Mg,Fe)O, and perovskite with ~10% density increase across the 1-2 km thick phase change region. An addition transformation has been proposed for lowermost 100s of km of the mantle in which perovskite transforms in to a “post-perovskite” phase. Mantle mineralogy and phase changes (after Ringwood for whom ringwoodite is named) are shown in the diagram below.

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Some or all content above used with permission from J. H. Wittke.

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