Single-Crystal Equations of State and Hyperfine Fields of Magnesiowüstite at High Pressures

Abstract:

In recent years, seismic observations have provided increasing evidence for significant heterogeneity in Earth’s lower mantle at both large (i.e. large low shear velocity provinces, or LLSVPs) and comparatively small (ultra-low velocity zones, or ULVZs) scales. One possible source of heterogeneity is variation in the Fe-content of the (Mg,Fe)O component of the lower mantle due to melting events and/or reactions with Earth’s outer core. Most previous studies have focused on compositions containing ~10-20 mol% Fe, but small amounts of compositions with an enhanced Fe concentration may strongly impact the elastic properties of the bulk phase assemblage. Here, we present results from two high-precision single-crystal x-ray diffraction studies on (Fe_0.78Mg_0.22)O magnesiowüstite to pressures of about 55 GPa at 300 K, one using neon and the other using helium as pressure-transmitting media. We observe a noticeably different compression behavior in the two pressure media at pressures greater than about 20 GPa, and compare to previous work on similar compositions. We also conducted a complementary single-crystal time domain synchrotron Mössbauer spectroscopy (SMS) study on the same composition in a helium medium to about 70 GPa to gain insight into the atom-scale properties of the Fe sublattice. We discuss the resulting hyperfine fields as a function of pressure, including the isomer shift, quadrupole splitting, magnetic, and texturing parameters. The advantages of using single crystals for such investigations will also be discussed. Finally, implications for the elastic properties of magnesiowüstite in the deep mantle will be considered.