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  1. Abstract Olivine is the most abundant mineral in the Earth's upper mantle and subducting slabs. Studying the structural evolution and equation of state of olivine at high-pressure is of fundamental importance in constraining the composition and structure of these regions. Hydrogen can be incorporated into olivine and significantly influence its physical and chemical properties. Previous infrared and Raman spectroscopic studies indicated that local structural changes occur in Mg-rich hydrous olivine (Fo ≥ 95; 4883–9000 ppmw water) at high-pressure. Since water contents of natural olivine are commonly <1000 ppmw, it is inevitable to investigate the effects of such water contents on the equation of state (EoS) and structure of olivine at high-pressure. Here we synthesized a low water content hydrous olivine (Fo95; 1538 ppmw water) at low SiO2 activity and identified that the incorporated hydrogens are predominantly associated with the Si sites. We performed high-pressure single-crystal X-ray diffraction experiments on this olivine to 29.9 GPa. A third-order Birch-Murnaghan equation of state (BM3 EoS) was fit to the pressure-volume data, yielding the following EoS parameters: VT0 = 290.182(1) Å3, KT0 = 130.8(9) GPa, and K′T0 = 4.16(8). The KT0 is consistent with those of anhydrous Mg-rich olivine, which indicates that such lowmore »water content has negligible effects on the bulk modulus of olivine. Furthermore, we carried out the structural refinement of this hydrous olivine as a function of pressure to 29.9 GPa. The results indicate that, similar to the anhydrous olivine, the compression of the M1-O and M2-O bonds are comparable, which are larger than that of the Si-O bonds. The compression of M1-O and M2-O bonds of this hydrous olivine are comparable with those of anhydrous olivine, while the Si-O1 and Si-O2 bonds in the hydrous olivine are more compressible than those in the anhydrous olivine. Therefore, this study suggests that low water content has negligible effects on the EoS of olivine, though the incorporation of water softens the Si-O1 and Si-O2 bond.« less
  2. Fe‐Al‐bearing bridgmanite may be the dominant host for ferric iron in Earth's lower mantle. Here we report the synthesis of (Mg0.5Fe3+0.5)(Al0.5Si0.5)O3 bridgmanite (FA50) with the highest Fe3+‐Al3+ coupled substitution known to date. X‐ray diffraction measurements showed that at ambient conditions the FA50 adopted the LiNbO3 structure. Upon compression at room temperature to 18 GPa, it transformed back into the bridgmanite structure, which remained stable up to 102 GPa and 2600 K. Fitting Birch‐Murnaghan equation of state of FA50 bridgmanite yields V 0 = 172.1(4) Å3, K 0 = 229(4) GPa with K 0′ = 4(fixed). The calculated bulk sound velocity of the FA50 bridgmanite is ~7.7% lower than MgSiO3 bridgmanite, mainly because the presence of ferric iron increases the unit‐cell mass by 15.5%. This difference likely represents the upper limit of sound velocity anomaly introduced by Fe3+‐Al3+ substitution. X‐ray emission and synchrotron Mössbauer spectroscopy measurements showed that after laser annealing ~6% of Fe3+ cations exchanged with Al3+ and underwent the high‐spin to low‐spin transition at 59 GPa. The low‐spin proportion of Fe3+ increased gradually with pressure and reached 17‐31% at 80 GPa. Since the cation exchange and spin transition in this Fe3+‐Al3+‐enriched bridgmanite do not cause resolvable unit‐cell volume reduction,more »and the increase of low‐spin Fe3+ fraction with pressure occurs gradually, the spin transition would not produce a distinct seismic signature in the lower mantle. However, it may influence iron partitioning and isotopic fractionation, thus introducing chemical heterogeneity in the lower mantle.« less
  3. A suite of hydrous orthoenstatite crystals were synthesized at 5–7 GPa and 1100–1300 °C, corresponding to the mid upper mantle conditions in continental regions. The synthetic crystals presented a clear, inclusion-free, and euhedral form with a size range from 100 to a few hundred microns. The Al- and water content of crystals were less than 2 wt. % and ranging from ~500 ppm to 1000 ppm, respectively, characterized by Raman and IR spectroscopy, electron microscopy, and SIMS. The analysis shows that the capability of water incorporation for this suite of hydrous orthoenstatite is correlated to the Al-content in the crystal structure. To understand how the detailed crystal structure reflects the influence of water and Al, single crystal X ray diffraction analysis was performed for this suite of hydrous orthoenstatite. By comparing the results obtained at ambient conditions, we find that for low-level of Al-content, <2 wt. %, the Al3+ cation tends to occupy one of tetrahedral sites (TB) only. Analysis of the X ray diffraction results under high pressure indicates that the elasticity of orthoenstatites is insensitive to the presence of low-level water and Al. We use this finding to evaluate the velocity profile at the mid upper mantle ofmore »continental regions to compare with seismic observation. The comparison indicates that the cause of the low velocity zone in continental regions originates from the geotherm profile rather than the effect of water on the elasticity of mantle phases.« less
  4. Clinopyroxene (Cpx) is commonly believed to be the best structural water (hydrogen) carrier among all major upper mantle nominally anhydrous minerals (NAMs). In this study, we have measured the single-crystal elastic properties of a Cpx, a natural omphacite with ~710 ppm water at ambient pressure (P) and temperature (T) conditions. Utilizing the single-crystal X-ray diffraction (XRD) and electron microprobe data, the unit cell parameters and density were determined as a = 9.603(9) Å, b = 8.774(3) Å, c = 5.250(2) Å, β = 106.76(5)o, V = 255.1(4) Å3, and ρ = 3.340(6) g/cm3. We performed Brillouin spectroscopy experiments on four single crystals along a total of 52 different crystallographic directions. The best-fit single-crystal elastic moduli (Cijs), bulk and shear moduli were determined as: C11 = 245(1) GPa, C22 = 210(2) GPa, C33 = 249.6(9) GPa, C44 = 75.7(9) GPa, C55 = 71.2(5) GPa, C66 = 76(1) GPa, C12 = 85(2) GPa, C13 = 70(1) GPa, C23 = 66(2) GPa, C15 = 8.0(6) GPa, C25 = 6(1) GPa, C35 = 34.7(6) GPa, and C46 = 8.7(7) GPa, KS0 = 125(3) GPa, and G0 = 75(2) GPa, respectively. Compared with the anticipated elastic properties of an anhydrous omphacite with the same chemicalmore »composition, our results indicate that the incorporation of ~710 ppm structural water has no resolvable effect on the aggregate elastic properties of omphacite, although small differences (up to ~9 GPa) were observed in C13, C25, C44, and C66.« less
  5. High-pressure diamond-anvil cell synchrotron X-ray diffraction experiments were conducted on single-crystal samples of natural orthoamphibole; gedrite; with composition; (K0.002Na0.394)(Mg2)(Mg1.637Fe2.245Mn0.004Ca0.022Cr0.003Na0.037Al1.052)(Si6.517Al1.483)O22(OH)2. The samples were compressed at 298 K up to a maximum pressure of 27(1) GPa. In this pressure regime, we observed a displacive phase transition between 15.1(7) and 21(1) GPa from the orthorhombic Pnma phase to a new structure with space group P21/m; which is different from the familiar P21/m structure of cummingtonite and retains the (+, +, −, −) I-beam stacking sequence of the orthorhombic structure. The unit cell parameters for the new phase at 21(1) GPa are a = 17.514(3), b = 17.077(1), c = 4.9907(2) Å and β = 92.882(6)°. The high-pressure P21/m phase is the first amphibole structure to show the existence of four crystallographically distinct silicate double chains. The orthorhombic to monoclinic phase transition is characterized by an increase in the degree of kinking of the double silicate chains and is analogous to displacive phase changes recently reported in orthopyroxenes, highlighting the parallel structural relations and phase transformation behavior of orthorhombic single- and double-chain silicates.