Reports

Phase Transitions Between β and γ (Mg, Fe)2SiO4 in the Earth's Mantle: Mechanisms and Rheological Implications

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Science  03 Jun 1994:
Vol. 264, Issue 5164, pp. 1445-1448
DOI: 10.1126/science.264.5164.1445

Abstract

The mechanisms of the phase transformations between the spinel (γ) and modified spinel (β) polymorphs of Mg2SiO4 have been studied experimentally between 15 and 20 gigapascals and 800° to 950°C. The γ to β transformation occurs by a shear mechanism, whereas the β to γ transformation involves grain-boundary nucleation and interface-controlled growth. These contrasting mechanisms are a consequence of the number of independent slip systems that are available in the respective crystal structures. This result leads to the prediction that in subduction zones and perhaps also rising plumes in the Earth's mantle, the γ to β transformation should be accompanied by a transient reduction in strength.