Abstract
High-pressure polymorphism of olivine (α-phase of Mg2SiO4) is of particular interest for geophysicists aiming to understand the structure and dynamics of the Earth’s interior because of olivine’s prominent abundance in the upper mantle. Therefore, natural and synthetic olivine polymorphs have been actively studied in the past half century. Here, we report a new high-pressure polymorph, the ε*-phase, which was discovered in a heavily shocked meteorite. It occurs as nanoscale lamellae and has a topotaxial relationship with the host ringwoodite (γ-phase of Mg2SiO4). Olivine in the host rock entrapped in a shock-induced melt vein initially transformed into polycrystalline ringwoodite through a nucleation and growth mechanism. The ringwoodite grains then coherently converted into the ε*-phase by shear transformation during subsequent pressure release. This intermediate metastable phase can be formed by all Mg2SiO4 polymorphs via a shear transformation mechanism. Here, we propose high-pressure transformations of olivine that are enhanced by diffusionless processes, not only in shocked meteorites but also in thick and cold lithosphere subducting into the deep Earth.
Highlights
Phase equilibrium studies confirmed that olivine successively transforms into wadsleyite (β-phase with a spineloid structure) and ringwoodite (γ-phase with a spinel structure) with increasing pressure[1] (Supplementary Fig. S1)
Olivine is metastably preserved in thick and cold lithospheric slabs descending toward the mantle transition zone (MTZ) due to kinetically hindered high-pressure transformations at low temperature[14,15]
A lattice-coherent shear mechanism, promoted by coherent shear of oxygen sublattices associated with cation shuffling in interstices, was proposed to affect the olivine–ringwoodite phase transformation based on a topological study[16] and was assessed using first-principles energy calculations[17]
Summary
Phase equilibrium studies confirmed that olivine successively transforms into wadsleyite (β-phase with a spineloid structure) and ringwoodite (γ-phase with a spinel structure) with increasing pressure[1] (Supplementary Fig. S1). High-pressure transformations of mantle minerals in general mainly occur by a nucleation and growth mechanism, mostly at grain boundaries. Based on the occurrence of stacking faults in olivine polymorphs, a shear mechanism is proposed to promote the olivine–spinel transformation[21].
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