Search for: All records

The viscosity of iron alloy liquids is the key for the core dynamo and core‐mantle differentiation of terrestrial bodies. Here we measured the viscosity of Fe‐Ni‐C liquids up to 7 GPa using the floating sphere viscometry method and up to 330 GPa using first‐principles calculations. We found a viscosity increase at ∼3–5 GPa, coincident with a structural transition in the liquids. After the transition, the viscosity reaches ∼14–27 mPa·s, a factor of 2–4 higher than that of Fe and Fe‐S liquids. Our computational results from 5 to 330 GPa also indicate a high viscosity of the Fe‐Ni‐C liquids. For a carbon‐rich core in large terrestrial body, the level of turbulence in the outer core would be lessened approaching the inner core boundary. It is also anticipated that Fe‐Ni‐C liquids would percolate in Earth's deep silicate mantle at a much slower speed than Fe and Fe‐S liquids.

 

A comprehensive analysis of experimental data and theoretical simulations on the partial molar volume of water in silicate melt indicates that finite strain theory successfully describes the compression of the H₂O component dissolved in silicate melt at high pressures and temperatures. However, because of the high compressibility of the water component, a fourth order equation of state fit is required to accurately simulate experimental results on water's volume in silicate melts at a deep upper mantle, transition zone, and lower mantle pressures. Data from previous shock compression experiments on hydrous minerals in which melting occurs along the Hugoniot are used to provide an experimental constraint on the partial molar volume of water in silicate melt at deep mantle temperatures and pressures. The equation of state of the water component indicates that, depending on elastic averaging technique, the amount of water that could be present in neutrally or negatively buoyant mafic/ultramafic melts above the 410 km seismic discontinuity is upper‐bounded at 5.6 wt%: smaller than previously inferred, and consistent with melt being confined to a narrow depth range above the 410 km discontinuity. If melt is predominantly distributed along grain boundaries in low aspect ratio films, extents of melting as low as 2% could produce observed seismic velocity reductions. The ability of the lowermost mantle to contain negatively buoyant hydrous liquids hinges on the trade‐off between iron content and hydration: at these depths, substantially higher degrees of hydration could be present within partial melts.

 

The presence of light elements in the metallic cores of the Earth, the Moon, and other rocky planetary bodies has been widely proposed. Carbon is among the top candidates in light of its high cosmic abundance, siderophile nature, and ubiquity in iron meteorites. It is, however, still controversial whether carbon‐rich core compositional models can account for the seismic velocity observations within the Earth and lunar cores. Here, we report the density and elasticity of Fe₉₀Ni₁₀‐3 wt.% C and Fe₉₀Ni₁₀‐5 wt.% C liquid alloys using synchrotron‐based X‐ray absorption experiments and first‐principles molecular dynamics simulations. Our results show that alloying of 3 wt.% and 5 wt.% C lowers the density of Fe₉₀Ni₁₀liquid by ∼2.9–3.1% at 2 GPa, and ∼3.4–3.6% at 9 GPa. More intriguingly, our experiments and simulations both demonstrate that the bulk moduli of the Fe‐Ni‐C liquids are similar to or slightly higher than those of Fe‐Ni liquids. Thus, the calculated compressional velocities (v_p) of Fe‐Ni‐C liquids are higher than that of pure Fe‐Ni alloy, promoting carbon as a possible candidate to explain the elevatedv_pin the Earth's outer core. However, the values and slopes of both density andv_pof the studied two Fe‐Ni‐C liquids do not match the outer core seismic models, suggesting that carbon may not be the sole principal light element in Earth's outer core. The highv_pof Fe‐Ni‐C liquids does not match the presumptivev_pof the lunar outer core well, indicating that carbon is less likely to be its dominant light element.

 

The high-pressure structure and stability of the calcic amphibole tremolite (Ca2Mg5Si8O22(OH)2) was investigated to ~40 GPa at 300 K by single-crystal X-ray diffraction using synchrotron radiation. C2/m symmetry tremolite displays a broader metastability range than previously studied clinoamphiboles, exhibiting no first-order phase transition up to 40 GPa. Axial parameter ratios a/b and a/c, in conjunction with finite strain versus normalized pressure trends, indicate that changes in compressional behavior occur at pressures of ~5 and ~20 GPa. An analysis of the finite strain trends, using third-order Birch-Murnaghan equations of state, resulted in bulk moduli (𝐾) of 72(7), 77(2), and 61(1) GPa for the compressional regimes from 0-5 GPa (regime I), 5-20 GPa (II), and above 20 GPa (III), respectively, and accompanying pressure-derivatives of the bulk moduli (𝐾′) of 8.6(42), 6.0(3), and 10.0(2). The results are consistent with first-principle theoretical calculations of tremolite elasticity. The axial compressibility ratios of tremolite, determined as 𝛽a : 𝛽b : 𝛽c = 2.22:1.0:0.78 (regime I), 2.12:1.0:0.96 (II), and 1.03:1.0:0.75 (III), demonstrate a substantial reduction of the compressional anisotropy of tremolite at high pressures, which is a notable contrast with the increasingly anisotropic compressibility observed in the high-pressure polymorphs of the clinoamphibole grunerite. The shift in compression-regime at 5 GPa (I-II) transition is ascribed to stiffening along the crystallographic a-axis corresponding to closure of the vacant A-site in the structure, and a shift in the topology of the a-oriented surfaces of the structural I-beam from concave to convex. The II-III regime shift at 20 GPa corresponds to an increasing rate of compaction of the Ca-polyhedra and increased distortion of the Mg-octahedral sites, processes which dictate compaction in both high-pressure compression-regimes. Bond-valence analyses of the tremolite structure under pressure show dramatic overbonding of the Ca-cations (75% at 30 GPa), with significant Mg-cation overbonding as well (40%). These imply that tremolite’s notable metastability range hinges on the calcium cation’s bonding environment. The 8-fold coordinated Ca-polyhedron accommodates significant compaction under pressure, while the geometry of the Ca-O polyhedron becomes increasingly regular and inhibits the reorientation of the tetrahedral chains that generate phase transitions observed in other clinoamphiboles. Peak/background ratio of diffraction data collected above 40 GPa and our equation of state determination of bulk moduli and compressibilities of tremolite in regime III, in concert with the results of our previous Raman study, suggest that C2/m tremolite may be approaching the limit of its metastability above 40 GPa. Our results have relevance for both the metastable compaction of tremolite during impact events, and for possible metastable persistence of tremolite within cold subduction zones within the Earth. 

Garnet is an important mineral phase in the upper mantle as it is both a key component in bulk mantle rocks, and a primary phase at high-pressure within subducted basalt. Here, we focus on the strength of garnet and the texture that develops within garnet during accommodation of differential deformational strain. We use X-ray diffraction in a radial geometry to analyze texture development in situ in three garnet compositions under pressure at 300 K: a natural garnet (Prp60Alm37) to 30 GPa, and two synthetic majorite-bearing compositions (Prp59Maj41 and Prp42Maj58) to 44 GPa. All three garnets develop a modest (100) texture at elevated pressure under axial compression. Elasto-viscoplastic self-consistent (EVPSC) modeling suggests that two slip systems are active in the three garnet compositions at all pressures studied: {110}<1-21 11> and {001}<110>. We determine a flow strength of ~5 GPa at pressures between 10 to 15 GPa for all three garnets; these values are higher than previously measured yield strengths measured on natural and majoritic garnets. Strengths calculated using the experimental lattice strain differ from the strength generated from those calculated using EVPSC. Prp67Alm33, Prp59Maj41 and Prp42Maj58 are of comparable strength to each other at room temperature, which indicates that majorite substitution does not greatly affect the strength of garnets. Additionally, all three garnets are of similar strength as lower mantle phases such as bridgmanite and ferropericlase, suggesting that garnet may not be notably stronger than the surrounding lower mantle/deep upper mantle phases at the base of the upper mantle.