ALBERT

Description: Mantle wedge regions in subduction zone settings show anomalously high electrical conductivity (~1 S/m) that has often been attributed to the presence of aqueous fluids released by slab dehydration. Laboratory-based measurements of the electrical conductivity of hydrous phases and aqueous fluids are significantly lower and cannot readily explain the geophysically observed anomalously high electrical conductivity. The released aqueous fluid also rehydrates the mantle wedge and stabilizes a suite of hydrous phases, including serpentine and chlorite. In this present study, we have measured the electrical conductivity of a natural chlorite at pressures and temperatures relevant for the subduction zone setting. In our experiment, we observe two distinct conductivity enhancements when chlorite is heated to temperatures beyond its thermodynamic stability field. The initial increase in electrical conductivity to ~3 x 10 –3 S/m can be attributed to chlorite dehydration and the release of aqueous fluids. This is followed by a unique, subsequent enhancement of electrical conductivity of up to 7 x 10 –1 S/m. This is related to the growth of an interconnected network of a highly conductive and chemically impure magnetite mineral phase. Thus, the dehydration of chlorite and associated processes are likely to be crucial in explaining the anomalously high electrical conductivity observed in mantle wedges. Chlorite dehydration in the mantle wedge provides an additional source of aqueous fluid above the slab and could also be responsible for the fixed depth (120 ± 40 km) of melting at the top of the subducting slab beneath the subduction-related volcanic arc front.

Description: 〈span〉〈div〉Abstract〈/div〉Phase egg, [AlSiO〈sub〉3〈/sub〉(OH)], is an aluminosilicate hydrous mineral that is thermodynamically stable in lithological compositions represented by Al〈sub〉2〈/sub〉O〈sub〉3〈/sub〉-SiO〈sub〉2〈/sub〉-H〈sub〉2〈/sub〉O (ASH) ternary, i.e., a simplified ternary for the mineralogy of subducted sediments and continental crustal rocks. High-pressure and high-temperature experiments on lithological compositions resembling hydrated sedimentary layers in subducting slabs show that phase egg is stable up to pressures of 20–30 GPa, which translates to the transition zone to lower mantle depths. Thus, phase egg is a potential candidate for transporting water into the Earth's mantle transition zone. In this study, we use 〈span〉first-principles〈/span〉 simulations based on density functional theory to explore the pressure dependence of crystal structure and how it influences energetics and elasticity. Our results indicate that phase egg exhibits anomalous behavior of the pressure dependence of the elasticity at mantle transition zone depths (~15 GPa). Such anomalous behavior in the elasticity is related to changes in the hydrogen bonding O-H· · ·O configurations, which we delineate as a transition from a low-pressure to a high-pressure structure of phase egg. Full elastic constant tensors indicate that phase egg is very anisotropic resulting in a maximum anisotropy of compressional wave velocity, 〈span〉AvP〈/span〉 ≈ 30% and of shear wave velocity, 〈span〉AvS〈/span〉 ≈ 17% at zero pressures. Our results also indicate that the phase egg has one of the fastest bulk sound velocities (〈span〉vP〈/span〉 and 〈span〉vS〈/span〉) compared to other hydrous aluminous phases in the ASH ternary, which include topaz-OH, phase Pi, and δ-AlOOH. However, the bulk sound velocity of phase egg is slower than that of stishovite. At depths corresponding to the base of mantle transition zone, phase egg decomposes to a mixture of δ-AlOOH and stishovite. The changes in compressional Δ〈span〉vP〈/span〉 and shear Δ〈span〉vS〈/span〉 velocity associated with the decomposition is ~0.42% and –1.23%, respectively. Although phase egg may be limited to subducted sediments, it could hold several weight percentages of water along a normal mantle geotherm.〈/span〉

Description: The 3.65 Å phase [MgSi(OH) 6 ] is likely to be formed by decomposition of the hydrous 10 Å phase [Mg 3 Si 4 O 10 (OH) 2 ·H 2 O] at pressures of 9–10 GPa. In this study, we use a combination of X-ray diffraction and first-principles simulations to constrain the equation of state and elasticity of the 3.65 Å phase. We find that the equation of state results for the 3.65 Å phase, from X-ray diffraction data are well represented by a third-order Birch-Murnaghan formulation, with K 0 = 83.0 (±1.0) GPa, K 0 ' = 4.9 (±0.1), and V 0 = 194.52 (±0.02) Å 3 . Based on the first-principles simulations, the full single-crystal elastic constant tensor with monoclinic symmetry shows significant anisotropy with the compressional c 11 = 156.2 GPa, c 22 = 169.4 GPa, c 33 = 189.3 GPa, the shear components c 44 = 55.9 GPa, c 55 = 58.5 GPa, c 66 = 74.8 GPa, and c 46 = 1.6 GPa; the off-diagonal components c 12 = 38.0 GPa, c 13 = 26.5 GPa, c 23 = 22.9 GPa, c 15 = 1.5 GPa, c 25 = 1.5 GPa, and c 35 = –1.9 GPa at zero pressure. At depths corresponding to 270–330 km, the seismological X-discontinuity has been observed in certain regions. We find that the formation of 3.65 Å from layered hydrous magnesium silicates (LHMS) such as 10 Å angstrom phases occurs at around 9 GPa, i.e., coinciding with the seismic X-discontinuity. The LHMS phases have significant seismic anisotropy. Based on the full elastic constant tensor, although among the dense hydrous magnesium silicate (DHMS) phases, the 3.65 Å phase reveals considerably larger elastic anisotropy, it is significantly smaller than the LHMS phases. This change in seismic anisotropy in hydrous phases might be one of the plausible explanations for the seismic X-discontinuity.

Description: Garnet is the second most abundant mineral phase in the upper mantle and transition zone settings. The crystal structure of garnet is quite flexible and hence it is able to accommodate various cations, including large incompatible cation such as sodium. We used first-principles simulation based on density functional theory and two widely used approximations—local density approximation (LDA) and generalized gradient approximation (GGA)—to explore the crystal structure, equation of state, and elasticity of sodium bearing majorite garnet with Na 2 MgSi 5 O 12 stoichiometry at pressures relevant to the upper mantle and transition zone. We find that the pressure-volume results based on LDA can be explained by a Birch Murnaghan finite strain equation of state with V 0 = 1447.6 (± 0.1) Å 3 , K 0 = 177.4 (± 0.4) GPa, and K ' 0 = 3.93 (± 0.02). The results based on GGA can be explained by a Birch Murnaghan finite strain equation of state with V 0 = 1525.8 (± 0.2) Å 3 , K 0 = 160.2 (± 0.4) GPa, and K ' 0 = 3.96 (± 0.02). The full elastic moduli tensor for Na-majorite with tetragonal symmetry exhibits slight deviation from the cubic symmetry with C 11 〈 C 33 , C 12 ~ C 13 , and C 44 ~ C 66 . The magnitude of the tetragonal strain also captures the slight deviation from the cubic symmetry. At pressures corresponding to the upper mantle and mantle transition zone, the compressional wave velocity, v P , and shear wave velocity, v S , for the Na-majorite garnet are fast compared to a wide variety of garnets such as pyrope, grossular, almandine, and majorite garnet. Although single-crystal anisotropy of Na-majorite is greater than pyrope, it is still low compared to the major mantle phases.

Description: Geochemical, cosmochemical, geophysical, and mineral physics data suggest that iron (or iron-nickel alloy) is the main component of the Earth's core. The inconsistency between the density of pure iron at pressure and temperature conditions of the Earth's core and seismological observations can be explained by the presence of light elements. However, the low shear wave velocity and high Poisson's ratio of the Earth's core remain enigmatic. Here we experimentally investigate the effect of carbon on the elastic properties of iron at high pressures and temperatures and report a high-pressure orthorhombic phase of iron carbide, Fe7C3. We determined the crystal structure of the material at ambient conditions and investigated its stability and behaviour at pressures up to 205 GPa and temperatures above 3,700 K using single-crystal and powder X-ray diffraction, Mössbauer spectroscopy, and nuclear inelastic scattering. Estimated shear wave and compressional wave velocities show that Fe7C3 exhibits a lower shear wave velocity than pure iron and a Poisson's ratio similar to that of the Earth's inner core. We suggest that carbon alloying significantly modifies the properties of iron at extreme conditions to approach the elastic behaviour of rubber. Thus, the presence of carbon may explain the anomalous elastic properties of the Earth's core. © 2015 Macmillan Publishers Limited.

Description: A natural phengite-2M1 of composition (K0.95Na0.05)(Al0.76Fe0.14Mg0.10)2 (Si3.25Al0.75)O10(OH1.96F0.04) [a= 5.2173(1) Å, b= 9.0493(2) Å, c= 19.989 (1) Å and β = 95.734(4)°] was studied using in situ high-temperature FTIR. Correlations to structural changes were made using previously-reported neutron diffraction data from the same sample. Correlations have been made between the microscopic atomic displacements (arising from thermal effects) and analogous macroscopic properties, such as bond strain and ditrigonal distortions. Spectra were collected in the far-infrared region to study the behaviour of the interlayer (K+) cation and also in the mid-infrared region to distinguish the Si–O stretching modes. We found anisotropic thermal expansion of the interlayer site. The K O bond length is divided into K Oouter and K Oinner, and the K–Oinner bond length is correlated with the far-infrared spectra. The thermal dependence of the correlation between K–O bond length and corresponding far-infrared stretching frequency is different from the effect of the chemical composition. We also found that the K–O bond strain could be successfully resolved into the sum of inner strain and lattice strain. The Si–O stretching mode, obtained from the mid-infrared measurements, showed only weak changes. However, the neutron refinement data showed different thermal behaviour for distinct crystallographic T-sites.