Abstract δ-AlOOH is regarded as a potential water carrier that is stable in the Earth’s lower mantle down to the core-mantle boundary along the cold slab geotherm; thus, knowledge of its structural evolution under high pressure is very important for understanding water transport in the Earth’s interior. In this work, we conducted Raman scattering and luminescence spectroscopic experiments on δ-AlOOH at pressures up to 34.6 and 22.1 GPa, respectively. From the collected Raman spectra, significant changes in the pressure dependence of the frequencies of Raman-active modes were observed at ~8 GPa, with several modes displaying softening behavior. In particular, the soft A1 mode, which corresponds to a lattice vibration of the AlO6 octahedron correlated to OH stretching vibrations, decreases rapidly with increasing pressure and shows a trend of approaching 0 cm−1 at ~9 GPa according to a quadratic polynomial extrapolation. These results provide clear Raman-scattering spectroscopic evidence for the P21nm-to-Pnnm structural transition. Similarly, the phase transition was also observed in the luminescence spectra of Cr3+ in both powder and single-crystal δ-AlOOH samples, characterized by abrupt changes in the pressure dependences of the wavelength of the R-lines and sidebands across the P21nm-to-Pnnm transition. The continuous decrease in R2-R1 splitting with pressure indicated that the distortion of the AlO6 octahedron was suppressed under compression. No abnormal features were clearly observed in our Raman or luminescence spectra at ~18 GPa, where the ordered symmetrization or fully centered state with hydrogen located at the midpoint of the hydrogen bond was observed by a previous neutron diffraction study. However, some subtle changes in Raman and luminescence spectra indicated that the ordered symmetrization state might form at around 16 GPa.
Nearly all displacive transitions have been considered to be continuous or second order, and the rigid unit mode (RUM) provides a natural candidate for the soft mode. However, in-situ X-ray diffraction and Raman measurements show clearly the first-order evidences for the scheelite-to-fergusonite displacive transition in BaWO4: a 1.6% volume collapse, coexistence of phases, and hysteresis on release of pressure. Such first-order signatures are found to be the same as the soft modes in BaWO4, which indicates the scheelite-to-fergusonite displacive phase transition hides a deeper physical mechanism. By the refinement of atomic displacement parameters, we further show that the first-order character of this phase transition stems from a coupling of large compression of soft BaO8 polyhedrons to the small displacive distortion of rigid WO4 tetrahedrons. Such a coupling will lead to a deeper physical insight in the phase transition of the common scheelite-structured compounds.
Zircon is a widely used accessary mineral, and zircon saturation in silicate melts has many applications in geology. Based on the original model proposed by Watson and Harrison (Earth Planet. Sci. Lett. 64:295–304, 1983), the quantification of the zircon saturation in melts with felsic and intermediate compositions has recently become a major research topic, resulting in some disagreements and different models. Theoretically, the addition of new data, especially regarding the zircon solubility in a basaltic (peralkaline) melt that can provide an upper limit below which zircon is likely to crystallize in igneous rocks, is thus critical to the development of a new, refined model that can be applied to a wide range of compositions and provide a resolution to the ongoing debate. Here, the zircon saturation in a terrestrial basaltic melt was systematically investigated for the first time using a piston-cylinder apparatus across the temperature range of 1050–1350 °C at pressures of 0.5, 1.0 and 1.5 GPa. We combined our new data on mafic melts with data from previous studies on mafic to felsic melts to investigate the factors affecting zircon saturation in basaltic melts. Our results confirm an extremely high zircon saturation in mafic (peralkaline) melts in addition to its strong dependence on the melt composition and temperature and its weak dependence on the pressure and water content. We used all available data that can be used to calculate compositional parameter G [=(3·Al2O3 + SiO2)/(Na2O + K2O + CaO + MgO + FeO), molar ratio] to evaluate fit to a previous model designed to work with more alkaline compositions and proposed a new refined model, given by (with 1σ errors): lnCZr(melt) = (3.313 ± 0.349)–(1.35 ± 0.10)·lnG + (0.0065 ± 0.0003)·T, where CZr(melt) is the Zr concentration in the melt at zircon saturation and T is temperature in °C. Additionally, we reintroduced the previously proposed dominance of the temperature and melt composition on the degree of polymerization and thus the ability of zircon to enter the melt, thereby leading to large differences in the zircon saturation between intermediate-felsic magmas and mafic magmas. Although mantle-derived basaltic magmas have a low Zr concentration, they are capable of dissolving the zircon in surrounding rocks during their ascent. Zircons are generally rare in extrusive or hypabyssal basaltic rocks but can form during the late crystallization stage of plutonic basaltic magmas; thus, zircon crystallization has little impact on the distribution of trace elements in mafic magmas.
Abstract To explore the rheology of dolomite and investigate recent findings regarding the so‐called inversion of activation energy between dislocation and diffusion creep, we compressed medium‐grained Fangshan dolomite (113 ± 42 µm) at effective confining pressures of 50–300 MPa, temperatures of 27°C–900°C, and strain rates of 10 −6 to 2 × 10 −4 s −1 using a Paterson gas‐medium apparatus. Two end‐member deformation regimes with corresponding diagnostic flow laws and microstructures were identified. At temperatures ≤500°C, low‐temperature plasticity (LTP), which is characterized by microstructures of predominant abrupt undulatory extinctions and f‐twins, was determined to dominate the deformation of Fangshan dolomite. The corresponding flow behavior can be described by an with and (Regime 1). At temperatures ≥800°C, dislocation creep, which shows characteristic microstructures of smooth undulating extinction and new recrystallized grains, dominated the deformation of Fangshan dolomite. The corresponding flow behavior can be expressed by a power law equation, with , , and (Regime 2). At temperatures between ∼500 and 800°C, a transition regime between LTP and dislocation creep was identified (Regime 3) with the dependence of flow stress on strain rate increasing gradually with increasing temperature. When extrapolated to natural conditions, our flow law of dislocation creep for dolomite in combination with that of diffusion creep reported by Davis et al. (2008) suggests that the dislocation creep regime of dolomite is limited to a relatively narrow region of high temperature and relatively high stress, whereas the diffusion creep regime dominates the deformation of dolomite in tectonic settings with low stress levels.
This is the dataset for the paper "Single-crystal elasticity of phase E at high pressure and temperature: Implications for the low-velocity layer atop the 410-km depth".
A new cubic perovskite polymorph of PbGeO3 (Phase II) was synthesized by laser heating in the diamond-anvil cell (DAC) at the pressure of 36 GPa. Fitting the Birch-Murnaghan equation of state against its observed P-V data yields a bulk modulus K0 of 196(6) GPa and the volume V0 of 56.70(13) Å3 when K0′ is assumed being 4. After the pressure is released, the PbGeO3 Phase II changes gradually into an amorphous phase, which contains mainly fourfold-coordinated germanium. It indicates that the PbGeO3 Phase II with a GeO6 octahedron framework transforms to a GeO4 tetrahedron network during the amorphization. The existence of PbGeO3 cubic perovskite Phase II at high pressures indicates that the polarized character of the Pb2+ ion induced by its 6s2 lone pair electrons would be totally reduced in the environment of major silicate perovskites inside the lower mantle, and thus the Pb atom would substitute the Ca atom to enter the CaSiO3 perovskite.
Abstract The crystallographic preferred orientation (CPO) of olivine, specifically the type‐B characterized by c‐axes aligned parallel to lineation and b ‐axes concentrated perpendicular to foliation, is essential for explaining the trench‐parallel seismic anisotropy in the forearc regions of subduction zones. However, its origin remains a subject of ambiguity and controversy. In this study, we present experimental findings on the formation of a type‐B olivine CPO through the dehydration of foliated serpentinite under a compressive stress at a pressure of 300 MPa and temperature of 700–750°C. Our results reveal a progressive evolution of olivine CPO, transitioning from a type‐C fabric to a type‐B fabric, with increasing grain size and dehydration level. The type‐B CPO observed in coarse‐grained olivine within fully dehydrated samples primarily arises from mechanisms involving anisotropic growth, grain rotation, and oriented coalescence of newly formed, small olivine grains following the decomposition of antigorite under a compressive stress. This study provides the first experimental evidence for a novel, low‐temperature dynamic dehydration mechanism, in contrast to the mechanism of high‐temperature plastic flow, for explaining the development of type‐B olivine CPO in forearc regions. Hence, it contributes significantly to our understanding of the formation of olivine CPO with implications for seismic anisotropy in subduction zone forearcs.
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