(Mg,Fe)SiO3-perovskite stability under lower mantle conditions

In three different experiments up to 100 gigapascals and 3000 kelvin, (Mg,Fe)SiO3-perovskite, the major component of the lower mantle, remained stable and did not decompose to its component oxides (Mg, Fe)O and SiO2. Perovskite was formed from these oxides when heated in a diamond anvil cell at pressures up to 100 gigapascals. Both MgSiO3 crystals and glasses heated to 3000 kelvin at 75 gigapascals also formed perovskite as a single phase, as evident from Raman spectra. Moreover, fluorescence measurements on chromium-doped samples synthesized at these conditions gave no indication of the presence of MgO.     

Synthesis and Equation of State of (Mg,Fe) SiO3 Perovskite to Over 100 Gigapascals

Silicate perovskite of composition (Mg(0.88)Fe(0.12)) SiO(3) has been synthesized in a laser-heated diamond-anvil cell to a pressure of 127 gigapascals at temperatures exceeding 2000 K. The perovskite phase was identified and its unit-cell dimensions measured by in situ x-ray diffraction at elevated pressure and room temperature. An analysis of these data yields the first high-precision equation of state for this mineral, with values of the zero-pressure isothermal bulk modulus and its pressure derivative being K(0T) = 266 +/- 6 gigapascals and K'(0T) = 3.9 +/- 0.4. In addition, the orthorhombic distortion of the silicate-perovskite structure away from ideal cubic symmetry remains constant with pressure: the lattice parameter ratios are b/a = 1.032 +/- 0.002 and c/a = 1.444 +/- 0.006. These results, which prove that silicate perovskite is stable to ultrahigh pressures, demonstrate that perovskite can exist throughout the pressure range of the lower mantle and that it is therefore likely to be the most abundant mineral in Earth.     

High-Temperature Phase Transition and Dissociation of (Mg, Fe)SiO3 Perovskite at Lower Mantle Pressures

To study the crystallography of Earth's lower mantle, techniques for measuring synchrotron x-ray diffraction from a laser-heated diamond anvil cell have been developed. Experiments on samples of (Mg, Fe)SiO(3) show that silicate perovskite maintains its orthorhombic symmetry at 38 gigapascals and 1850 kelvin. Measurements at 65 and 70 gigapascals provide evidence for a temperature-induced orthorhombic-to-cubic phase transition and dissociation to an assemblage of perovskite and mixed oxides. If these phase transitions occur in Earth, they will require a significant change in mineralogical models of the lower mantle.     

Reduced radiative conductivity of low-spin (Mg,Fe)O in the lower mantle

Optical absorption spectra have been measured at pressures up to 80 gigapascals (GPa) for the lower-mantle oxide magnesiowüstite (Mg,Fe)O. Upon reaching the high-spin to low-spin transition of Fe2+ at about 60 GPa, we observed enhanced absorption in the mid- and near-infrared spectral range, whereas absorption in the visible-ultraviolet was reduced. The observed changes in absorption are in contrast to prediction and are attributed to d-d orbital charge transfer in the Fe2+ ion. The results indicate that low-spin (Mg,Fe)O will exhibit lower radiative thermal conductivity than high-spin (Mg,Fe)O, which needs to be considered in future geodynamic models of convection and plume stabilization in the lower mantle.     

Deformation of (Mg,Fe)SiO3 post-perovskite and D' anisotropy

Polycrystalline (Mg(0.9),Fe(0.1))SiO3 post-perovskite was plastically deformed in the diamond anvil cell between 145 and 157 gigapascals. The lattice-preferred orientations obtained in the sample suggest that slip on planes near (100) and (110) dominate plastic deformation under these conditions. Assuming similar behavior at lower mantle conditions, we simulated plastic strains and the contribution of post-perovskite to anisotropy in the D' region at the Earth core-mantle boundary using numerical convection and viscoplastic polycrystal plasticity models. We find a significant depth dependence of the anisotropy that only develops near and beyond the turning point of a downwelling slab. Our calculated anisotropies are strongly dependent on the choice of elastic moduli and remain hard to reconcile with seismic observations.     

Precision Lattice-Parameter Determination of (Mg,Fe)SiO3 Tetragonal Garnets

The tetragonal garnet (Mg,Fe)SiO(3) is a high-pressure phase of pyroxene that is thought to be a major constituent of the earth's upper mantle. Its crystal structure is similar to that of cubic garnet, but it is slightly distorted to tetragonal symmetry so that its x-ray powder diffraction pattern shows a very small line splitting. A suite of tetragonal garnets with different compositions in the MgSiO(3)-rich portion of the MgSiO(3)-FeSiO(3) system was synthesized at about 20 gigapascals and 2000 degrees C. The lattice parameters a and c of quenched samples were determined by whole-powder-pattern decomposition analysis of Fe Kalpha x-ray powder diffraction data, which has the capacity to resolve to a high degree heavily overlapping reflections. It was found that the lattice parameters can be obtained from the following equations; a (in angstroms) = 11.516 + 0.088x and c (in angstroms) = 11.428 + 0.157x, where x, teh mole fraction of FeSiO(3), is 0.0 相似文献   

Multivariable dependence of Fe-Mg partitioning in the lower mantle

High-pressure diamond-cell experiments indicate that the iron-magnesium partitioning between (Fe,Mg)SiO3-perovskite and magnesiowustite in Earth's lower mantle depends on the pressure, temperature, bulk iron/magnesium ratio, and ferric iron content. The perovskite stability field expands with increasing pressure and temperature. The ferric iron component preferentially dissolves in perovskite and raises the apparent total iron content but had little effect on the partitioning of the ferrous iron. The ferrous iron depletes in perovskite at the top of the lower mantle and gradually increases at greater depth. These changes in iron-magnesium composition should affect geochemical and geophysical properties of the deep interior.     

Comparative Compressibilities of Silicate Spinels: Anomalous Behavior of (Mg,Fe)2SiO4

Compressibilities of five silicate spinels, including gamma-Mg(2)SiO(4), gamma-Fe(2)SiO(4), Ni(2)SiO(4) and two ferromagnesian compositions, were determined on crystals positioned in the same high-pressure mount. Subjection of all crystals simultaneously to the same pressure revealed differences in compressibility that resulted from compositional differences. Ferromagnesian silicate spinels showed an anomalous 13 percent increase in bulk modulus with increasing iron content, from Mg(2)SiO(4) (184 gigapascals) to Fe(2)SiO(4) (207 gigapascals). This result suggests that ferrous iron and magnesium, which behave similarly under crustal conditions, are chemically more distinct at high pressures characteristic of the transition zone and lower mantle.     

Melting of (Mg,Fe)2SiO4 at the Core-Mantle Boundary of the Earth

The lower mantle of the Earth is believed to be largely composed of (Mg,Fe)O (magnesiowustite) and (Mg,Fe)SiO3 (perovskite). Radiative temperatures of single-crystal olivine [(Mg0.9,Fe0.1)2SiO4] decreased abruptly from 7040 +/- 315 to 4300 +/- 270 kelvin upon shock compression above 80 gigapascals. The data indicate that an upper bound to the solidus of the magnesiowustite and perovskite assemblage at 4300 +/- 270 kelvin is 130 +/- 3 gigapascals. These conditions correspond to those for partial melting at the base of the mantle, as has been suggested occurs within the ultralow-velocity zone beneath the central Pacific.     

Melting of (Mg, Fe)SiO3-Perovskite to 625 Kilobars: Indication of a High Melting Temperature in the Lower Mantle

The melting curves of two compositions of (Mg,Fe) SiO3-perovskite, the likely dominant mineral phase in the lower mantle, have been measured in a C02 laser-heated diamond cell with direct temperature measurements and in situ detection of melting. At 625 kilobars, the melting temperature is 5000 +/- 200 kelvin, independent of composition. Extrapolation to the core-mantle boundary pressure of 1.35 megabar with three different melting relations yields melting temperatures between 7000 and 8500 kelvin. Thus, the temperature at the base of the lower mantle, accepted to lie between 2550 and 2750 kelvin, is only at about one-third of the melting temperature. The large difference between mantle temperature and corresponding melting temperature has several important implications; particularly the temperature sensitivity of the viscosity is reduced thus allowing large lateral temperature variations inferred from seismic tomographic velocity anomalies and systematics found in measured velocity-density functions. Extensive melting of the lower mantle can be ruled out throughout the history of the Earth.     

Phase Transitions Between beta and ggr (Mg, Fe)2SiO4 in the Earth's Mantle: Mechanisms and Rheological Implications

The mechanisms of the phase transformations between the spinel (gamma) and modified spinel (beta) polymorphs of Mg(2)SiO(4) have been studied experimentally between 15 and 20 gigapascals and 800 degrees to 950 degrees C. The gamma to beta transformation occurs by a shear mechanism, whereas the beta to gamma transformation involves grain-boundary nucleation and interface-controlled growth. These contrasting mechanisms are a consequence of the number of independent slip systems that are available in the respective crystal structures. This result leads to the prediction that in subduction zones and perhaps also rising plumes in the Earth's mantle, the gamma to beta transformation should be accompanied by a transient reduction in strength.     

Structure and freezing of MgSiO3 liquid in Earth's lower mantle

First-principles molecular-dynamics simulations show that over the pressure regime of Earth's mantle the mean silicon-oxygen coordination number of magnesium metasilicate liquid changes nearly linearly from 4 to 6. The density contrast between liquid and crystal decreases by a factor of nearly 5 over the mantle pressure regime and is 4% at the core-mantle boundary. The ab initio melting curve, obtained by integration of the Clausius-Clapeyron equation, yields a melting temperature at the core-mantle boundary of 5400 +/- 600 kelvins.     

A High-Temperature Electrical Conduction Mechanism in the Lower Mantle Phase (Mg,Fe)1-xO

Measurements of electrical conductivity at high pressure and temperature were taken on the lower mantle phase magnesiowustite with varying Fe3+ content. Although previous measurements at atmospheric pressure suggest Fe2+-Fe3+ hopping (small polaron) as the dominant conductivity mechanism, the present experiments show a change in charge transport mechanism with temperature. The lower temperature measurements are consistent with small polaron conduction, but at higher temperatures, which are more applicable to the lower mantle, a large polaron mechanism is suggested. Because these mechanisms have different temperature and compositional dependencies, this transition has important implications for extrapolation to mantle conditions.     

Stability of Perovskite (MgSiO3) in the Earth's Mantle

Available thermodynamic data and seismic models favor perovskite (MgSiO3) as the stable phase in the mantle. MgSiO3 was heated at temperatures from 1900 to 3200 kelvin with a Nd-YAG laser in diamond-anvil cells to study the phase relations at pressures from 45 to 100 gigapascals. The quenched products were studied with synchrotron x-ray radiation. The results show that MgSiO3 broke down to a mixture of MgO (periclase) and SiO2 (stishovite or an unquenchable polymorph) at pressures from 58 to 85 gigapascals. These results imply that perovskite may not be stable in the lower mantle and that it might be necessary to reconsider the compositional and density models of the mantle.     

近红外光谱法快速测定烤烟中钙、镁、铁、锰和锌的含量

运用傅立叶变换近红外漫反射光谱仪(NIRS)对初烤烟叶部分无机元素含量进行检测研究。通过应用化学计量学方法,建立了近红外光谱与初烤烟叶中钙、镁、铁、锰、锌5种无机元素含量间关系的数学模型。结果表明:钙、镁模型稳定性好,NIRS法测定的结果与化学方法测试结果相吻合,已具有实用性;铁、锰、锌模型也可用来快速预测初烤烟叶样品中的铁、锰、锌含量趋势,但模型精度还有待提高。     

Phonon Density of States and Specific Heat of Forsterite, Mg2SiO4

The phonon density of states of the geophysically important mineral forsterite has been calculated with a rigid-ion model, which gives good agreement with an experimental measurement by inelastic neutron scattering. The density of states has been used to calculate the specific heat as a function of temperature, the results of which are in excellent agreement with calorimetrically measured values. The rigid-ion model takes account of the interatomic interactions and normal modes of vibration on a detailed microscopic basis, and is therefore more realistic than the Debye and other empirical models used previously.     

Pyroxferroite: Stability and X-ray Crystallography of Synthetic Ca0.15Fe0.85SiO3 Pyroxenoid

Synthetic Ca(0.15)Fe(0.85)SiO(3) pyroxenoid has the same (pyroxmangite) structure and very nearly the same composition as pyroxferroite, a new mineral found in Apollo 11 lunar samples. The synthetic material is not stable below pressures of approximately 10 kilobars. It appears likely that the lunar pyroxferroite has persisted in a metastable state for some billions of years.     

麻醉手术对老年人血清锌、铜、铁、镁的影响

目的：了解老年人围手术期血清锌、铜、铁、镁浓度的变化规律。方法：选择ＡＳＡ１～２级择期手术老年患者４０例。以原子吸收分光光度计检测麻醉手术期间及手术后１周内血清锌、铜、铁、镁的变化。结果;血清锌在术毕时较麻醉前明显下降（Ｐ〈０．０１）,术后第１、２天进一步下降（Ｐ〈０．００１）,第３天所有回升,但仍明显低于麻醉前（Ｐ〈０．０１）,第７天恢复至术前水平;血清铁于术中开始下降,术后第１天降至最低值９Ｐ