Pressure-induced transformations in deep mantle and core minerals

摘要

Recent experimental and theoretical studies provide new insight into the variety of high-pressuretransformations in minerals that comprise the Earth's deep mantle and core. Representative examples of reconstructive, displacive, electronic and magnetic transformations studied by new diamond-anvil cell techniques are examined. Despite reports for various transitions in (Mg, Fe)SiO_3-perovskite, the stability field of the orthorhombic phase expands relative to magnesiowustite + SiO_2 with increasing pressure and temperature. The partitioning of Fe and Mg between Mg-rich silicate perovskite and magnesiowustite depends strongly on pressure, temperature, bulk Fe/Mg ratio, and ferric iron content. The soft-mode transition in SiO_2 from the rutile- to CaCl_2-type structure, originally documented by X-ray powder diffraction, Raman scattering, and first-principles theory has been explored in detail by single crystal diffraction, and transitions to higher-pressure forms have been examined. The effect of H on the transformations of various nominally anhydrous phases and transitions in dense hydrous Mg-silicates are also examined. New studies of the phase diagram of FeO include the transition to rhombohedral and higher-pressure NiAs polymorphs, and provide prototypical examples of coupled structural, electronic, and magnetic transitions. High-spin/low-spin transitions in FeO have been examined by high-resolution X-ray emission spectroscopy to t50 GPa, and the results are compared with similar studies of Fe2O3 and FeS. Finally, laser-heating studies to above 150 GPa and 2500 K show that (hcp) c-Fe has a large P-T stability field. Radial XRD measurements carried out at room temperature to 220 GPa have constrained the elasticity, rheology and sound velocities of c-Fe at core pressures.