The high-pressure structural evolution of olivine along the forsterite–fayalite join

Structural refinements from single-crystal X-ray diffraction data are reported for olivine with a composition of Fo100 (forsterite Mg2SiO4, synthetic), Fo80 and Fo62 (~Mg1.6Fe0.4SiO4 and ~Mg1.24Fe0.76SiO4, both natural) at room temperature and high pressure to ~8 GPa. The new results, along with data from the literature on Fo0 (fayalite Fe2SiO4), were used to investigate the previously reported structural mechanisms which caused small variations of olivine bulk modulus with increasing Fe content. For all the investigated compositions, the M2 crystallographic site, with its bonding configuration and its larger polyhedral volume, was observed to control the compression mechanisms in olivine. From Fo100 to Fo0, the compression rates for M2–O and M1–O bond lengths were observed to control the relative polyhedral volumes, resulting in a less-compressible M1O6 polyhedral volume, likely causing the slight increase in bulk modulus with increasing Fe content

Evidence for a Fe ³⁺-rich pyrolitic lower mantle from (Al,Fe)-bearing bridgmanite elasticity data

The chemical composition of Earth's lower mantle can be constrained by combining seismological observations with mineral physics elasticity measurements(1-3). However, the lack of laboratory data for Earth's most abundant mineral, (Mg, Fe, Al)(Si, Al, Fe)O-3 bridgmanite (also known as silicate perovskite), has hampered any conclusive result. Here we report single-crystal elasticity data on (Al, Fe)-bearing bridgmanite (Mg0.9Fe0.1Si0.9Al0.1)O-3 measured using high-pressure Brillouin spectroscopy and X-ray diffraction. Our measurements show that the elastic behaviour of (Al, Fe)-bearing bridgmanite is markedly different from the behaviour of the MgSiO3 endmember2,4. We use our data to model seismic wave velocities in the top portion of the lower mantle, assuming a pyrolitic(5) mantle composition and accounting for depth-dependent changes in iron partitioning between bridgmanite and ferropericlase(6,7). We find excellent agreement between our mineral physics predictions and the seismic Preliminary Reference Earth Model(8) down to at least 1,200 kilometres depth, indicating chemical homogeneity of the upper and shallow lower mantle. A high Fe3+/Fe2+ ratio of about two in shallow-lower-mantle bridgmanite is required to match seismic data, implying the presence of metallic iron in an isochemical mantle. Our calculated velocities are in increasingly poor agreement with those of the lower mantle at depths greater than 1,200 kilometres, indicating either a change in bridgmanite cation ordering or a decrease in the ferric iron content of the lower mantle