Spin crossover equation of state and sound velocities of (Mg_(0.65)Fe_(0.35))O ferropericlase to 140 GPa

Abstract

We have determined the elastic and vibrational properties of periclase-structured (Mg_(0.65)Fe_(0.35))O ("FP35"), a composition representative of deep mantle "pyrolite" or chondrite-pyroxenite models, from nuclear resonant inelastic x-ray scattering (NRIXS) and x-ray diffraction (XRD) measurements in diamond-anvil cells at 300 K. Combining with in situ XRD measurements, the Debye sound velocity of FP35 was determined from the low-energy region of the partial phonon density of states (DOS) obtained from NRIXS measurements in the pressure range of 70 to 140 GPa. In order to obtain an accurate description of the equation of state (EOS) for FP35, separate XRD measurements were performed up to 126 GPa at 300 K. A new spin crossover EOS was introduced and applied to the full P-V data set, resulting in a zero-pressure volume V_0 = 77.24 ± 0.17 Å^3, bulk modulus K_0 = 159 ± 8 GPa and its pressure-derivative K′_0 = 4.12 ± 0.42 for high-spin FP35 and K_(0,LS) = 72.9 ± 1.3 Å^3, K_(0,LS) = 182 ± 17 GPa with K′_(0,LS) fixed to 4 for low-spin FP35. The high-spin to low-spin transition occurs at 64 ± 3 GPa. Using the spin crossover EOS and Debye sound velocity, we derived the shear (V_S) and compressional (V_P) velocities for FP35. Comparing our data with previous results on (Mg,Fe)O at similar pressures, we find that the addition of iron decreases both V_P and V_S, while elevating their ratio (V_P/V_S). Small amounts (<10%) of low-spin FP35 mixed with silicates could explain moderate reductions in wave speeds near the core mantle boundary (CMB), while a larger amount of FP35 near the CMB would not allow a large structure to maintain neutral buoyancy.

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