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1.
The crystal structure of hydrous wadsleyite, Mg1.75SiH0.5O4 synthesized in an MA 8-type apparatus at conditions of 1300°C and 15.5 GPa, has been analyzed and refined in space group Imma, using the X-ray intensities measured on a 60X60X10 μm single crystal. The composition (Z=8) and unit cell are Mg1.74Si0.97H0.65O4 by E.P.M.A. analysis and a=5.663(1) Å, b= 11.546(2) Å, c=8.247(4) Å, V=539.2(5) Å3. The partial M-site occupancies were determined; vacancies associated with the incorporation of water are strongly concentrated on the Mg 3 site. The OH in the structure was confirmed by Raman and FTIR spectroscopies. The result of valence sum calculation based on the refined bond lengths indicates that O1 is a hydroxyl. The formula of hydrous wadsleyite can be expressed as Mg2-xSiH2xO4, where 0≤x≤0.25. When x=0.25, all of the O1 site is hydroxyl and the maximum solubility of 3.3 wt% H2O is realized. Structural relations to other dense hydrous phases are discussed.  相似文献   

2.
The crystal structures of the two hydrous wadsleyite crystals with formulae, Mg1.75SiH0.50O4 (0.5H–β) and Mg1.86SiH0.28O4 (0.3H–β) have been analyzed in this study. The single-crystal X-ray diffraction data showed that the unit cells of the 0.3H–β and the 0.5H–β are metrically monoclinic with a slight distortion from the orthorhombic cell but their intensity distributions conform to the orthorhombic symmetry within the limit of experimental errors. The Fourier and the difference Fourier syntheses were calculated. Small but significant Fourier peaks were found at the site, Si2, in a normally vacant tetrahedral void adjacent to Mg3 site as reported for the monoclinic hydrous wadsleyite by Smyth et al.. From the comparison of the hydrous and anhydrous wadsleyite structures, the Mg-vacant structural modules were found to be the building units for the structure of hydrous wadsleyite. The dilution of symmetry from orthorhombic to monoclinic in the hydrous wadsleyite structure is interpreted qualitatively due to lack of mirror perpendicular to the a axis in the module. The mode of arrangement of the Mg-vacant structural modules interprets the symmetry and hydrogen content of the hydrous wadsleyite and gives the structural relationship between hydrous wadsleyite and hydrous ringwoodite. Received: 8 May 1998 / Revised, accepted: 3 October 1998  相似文献   

3.
Raman spectra of monoclinic Fo90 hydrous wadsleyite with 2.4 wt% H2O have been measured in a diamond-anvil cell with helium as a pressure-transmitting medium to 58.4 GPa at room temperature. The most intense, characteristic wadsleyite modes, the Si–O–Si symmetric stretch at 721 cm−1 and the symmetric stretch of the SiO3 unit at 918 cm−1, shift continuously to 58.4 GPa showing no evidence of a first order change in the crystal structure despite compression well beyond the stability field of wadsleyite in terms of pressure. The pressure dependence of these two modes is nearly identical for Fo90 hydrous and Fo100 anhydrous wadsleyite. A striking feature in the high-pressure Raman spectra of Fo90 hydrous wadsleyite is the appearance of new Raman modes above 9 GPa in the mid-frequency range (300–650 cm−1 at 1-bar and shifted to 500–850 cm−1 at 58.4 GPa) accompanied by a significant growth in their intensities under further compression. In the OH stretching frequency range Fo90 hydrous wadsleyite exhibits a larger number of modes than the Mg end-member phase. The higher number of modes may be due to either additional protonation sites or simply that we observe a different subset of all possible OH modes for each sample. The high-pressure behaviour of the OH stretching modes of Fo90 and Fo100 hydrous wadsleyite is consistent: OH stretching modes with frequencies <3,530 cm−1 decrease with increasing pressure whereas the higher-frequency OH modes show a close to constant pressure dependence to at least 13.2 GPa. The approximately constant pressure dependence of the OH modes above 3,530 cm−1 is consistent with protons being located at the O1···O edges around M3.  相似文献   

4.
The mechanism of the high pressure transformation of olivine in the presence of aqueous fluid was investigated by high pressure experiments conducted nominally at the wadsleyite + ringwoodite stability field at 14.5 GPa and 700 and 800°C. The microstructures of recovered samples were observed using an analytical transmission electron microscope (ATEM) for which foils were prepared using a focused ion beam technique. Glass films approximately 1 μm in width always occupied the interface between olivine and hydrous ringwoodite. ATEM measurements showed that the chemical compositions of the glass films had approximately the same Mg/Fe ratio as that of olivine, but a higher Si content. Micro-structural and -chemical observations suggest that these glass films formed as quenched glass from the aqueous fluid dissolving olivine and that hydrous ringwoodite was crystallized from the fluid. This indicates that the transformation of olivine to hydrous ringwoodite was prompted by the dissolution–reprecipitation process. The dissolution–reprecipitation process is considered an important mineral replacement mechanism in the Earth’s crust by which one mineral is replaced by a more stable phase or phases. However, this process has not previously been reported for deep mantle conditions.  相似文献   

5.
 We carried out a series of melting experiments with hydrous primitive mantle compositions to determine the stability of dense hydrous phases under high pressures. Phase relations in the CaO–MgO–Al2O3–SiO2 pyrolite with ˜2 wt% of water have been determined in the pressure range of 10–25 GPa and in the temperature range between 800 and 1400 °C. We have found that phase E coexisting with olivine is stable at 10–12 GPa and below 1050 °C. Phase E coexisting with wadsleyite is stable at 14–16 GPa and below 900 °C. A superhydrous phase B is stable in pyrolite below 1100 °C at 18.5 GPa and below 1300 °C at 25 GPa. No hydrous phases other than wadsleyite are stable in pyrolite at 14–17 GPa and 900–1100 °C, suggesting a gap in the stability of dense hydrous magnesium silicates (DHMS). We detected an expansion in the stability field of wadsleyite to lower pressures (12 GPa and 1000 °C). The H2O content of wadsleyite was found to decrease not only with increasing temperature but also with increasing pressure. The DHMS phases could exist in a pyrolitic composition only under the conditions present in the subducting slabs descending into the lower mantle. Under the normal mantle and hot plume conditions, wadsleyite and ringwoodite are the major H2O-bearing phases. The top of the transition zone could be enriched in H2O in accordance with the observed increase in water solubility in wadsleyite with decreasing pressure. As a consequence of the thermal equilibration between the subducting slabs and the ambient mantle, the uppermost lower mantle could be an important zone of dehydration, providing fluid for the rising plumes. Received: 9 September 2002 / Accepted: 11 January 2003 Acknowledgements The authors are thankful to Y. Ito for the assistance with the EPMA measurement, A. Suzuki, T. Kubo and T. Kondo for technical help with the high-pressure experiments and Raman and X-ray diffraction measurements and C.R. Menako for technical support. K. Litasov thanks H. Taniguchi for his continuous encouragement and the Center for Northeast Asian Studies of Tohoku University and the Japanese Society for the Promotion of Science for the research fellowships. This work was partially supported by the Grant-in-Aid of Scientific Research of the Priority Area (B) of the Ministry of Education, Science, Sport, and Culture of the Japanese government (no. 12126201) to E. Ohtani.  相似文献   

6.
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7.
The structure of a single crystal hydrous ringwoodite, Mg1.89Si0.98H0.30O4 synthesized at conditions of 1300?°C and 20?GPa has been analyzed. Crystallographic data for hydrous ringwoodite obtained are; Cubic with Space group: Fd3m (no. 227). a= 8.0693(5)?Å, V=526.41(9)?Å3, Z=8, Dcalc= 3.48?g?cm?3. The results of site occupancy refinement using higher angle reflections showed the existence of a small degree of Mg2+-Si4+ disorder in the structure such as (Mg1.84Si0.05□0.11)(Si0.93Mg0.05□0.02)H0.30O4. The IR and Raman spectra were measured and OH vibration spectra were observed. A broad absorption band was observed in the IR spectrum and the maxima were observed at 3160?cm?1 in the IR and at 3165?cm?1 and 3685?cm?1 in relatively sharp Raman spectra, which suggest that locations between O-O pairs around the octahedral 16c and 16d sites are possible sites for hydrogen.  相似文献   

8.
The unique cation-disordered crystal structures of two samples of phase E, a non-stoichiometric, hydrous silicate synthesized in a uniaxial, split-sphere, multi-anvil apparatus at conditions above 13 GPa and 1000° C, have been solved and refined in space group $\bar 3$ . The compositions and unit cells for the two materials, assuming six oxygens per cell, are Mg2.08Si1.16H3.20O6, a=2.9701(1) Å, c=13.882(1) Å V = 106.05(4) Å3 for sample 1, and Mg2.17Si1.01H3.62O6, a=2.9853(6) Å, c=13.9482(7) Å, V= 107.65(4) Å3 for sample 2. The structure contains layers with many features of brucite-type units, with the layers stacked in a rhombohedral arrangement. The layers are cross linked by silicon in tetrahedral coordination and magnesium in octahedral coordination, as well as hydrogen bonds. Interlay er octahedra share edges with intralayer octahedra. Interlayer tetrahedra would share faces with intralayer octahedra. To avoid this situation, there are vacancies within the layers. There is, however, no long-range order in the occupation of these sites, as indicated by the lack of a superstructure. Selected-area electron diffraction patterns show walls of diffuse intensity similar in geometry and magnitude to those observed in short-range-ordered alloys and Hågg phases. Phase E thus appears to represent a new class of disordered silicates, which may be thermodynamically metastable.  相似文献   

9.
The thermodynamical stability of a newly observed wadsleyite II phase in the Mg2SiO4 system is studied by the density functional theory. The wadsleyite II equation of state has been derived. The phase boundaries of Mg2SiO4 polymorphs: wadsleyite, wadsleyite II and ringwoodite are studied using the quasi-harmonic approximation at high external pressures. Clapeyron slopes determined for wadsleyite II–ringwoodite and wadsleyite–wadsleyite II boundaries are 0.0047 and 0.0058 GPa/K, respectively. It is shown that the wadsleyite II phase is not thermodynamically preferred in the pure Mg2SiO4 system and will probably not occur between wadsleyite and ringwoodite phases.  相似文献   

10.
 Recently, the Hy-2a hydrous olivine (MgH2 SiO4)·3(Mg2SiO4) occurring as nanometre-sized inclusions in mantle olivines has been found by TEM, and has been suggested to be a new DHMS phase (Khisina et al. 2001). A model of the crystal structure of Hy-2a has been proposed as a 2a-superstructure of olivine with one Me2+ -vacant octahedral layer in the (1 0 0) plane per Hy-2a unit cell (Khisina and Wirth 2002). In the present study the crystal structure of Hy-2a hydrous olivine is optimized by ab initio calculations. The aims of this study are: (1) verification of the suggested models of Hy-2a hydrous olivine structure; (2) calculation of the most stable configurations for Hy-2a structure with minimum static lattice energy, by assuming a possible formation of Me2+ vacancies in either M1 or M2 octahedral sites; (3) determination of the position of protons and hydrogen bonds in the Hy-2a structure. Several different possible configurations of the Hy-2a structure are optimized. The results support the idea of a stable olivine structure with ordered planar-segregated OH-bearing defects oriented parallel to (1 0 0). The data obtained indicate a preferred stability of the Hy-2a structure with the protons associated with M1 vacancies and bonded with O1 and O2 oxygen sites. The relative energy values of the optimized Hy-2a structure configurations correlate as a rule with the average shifts of atoms from their positions in pure forsterite structure. Received: 7 February 2002 / Accepted: 23 October 2002  相似文献   

11.
Hydrogen site positions and occupancy in the crystal structure of dense hydrous magnesium silicate (DHMS) phase E were determined for the first time by pulsed neutron powder diffraction. A fully deuterated pure phase E powder sample, which had space group \(R\overline{3} m\) and lattice parameters of a = 2.97065(8) Å and c = 13.9033(4) Å, was synthesized at 15 GPa and 1100 °C. Through quantitative evaluation of refined structure parameters obtained with sufficient spatial resolution and very high signal-to-background ratio, we conclude that the O–D dipoles in the refined phase E structure are tilted by 24° from the direction normal to the layers of edge-shared MgO6 octahedra (octahedral layers). The tilted dipole structure of phase E is in remarkable contrast to that of brucite, Mg(OH)2, which has dipoles exactly normal to the octahedral layer. This contrast exists because the O–Si–O bonding unique in the phase E structure connects two adjacent octahedral layers and thereby reduces the interlayer O···O distance. This shrinkage of the interlayer distance induces the tilting of the O–D dipole and also generates unique O–D···O hydrogen bonding connecting all the layers in the phase E structure.  相似文献   

12.
Stress-induced proton disorder in hydrous ringwoodite   总被引：1，自引：1，他引：0  
We have measured in situ high-pressure IR absorption of synthetic hydrous (MgxFe1−x)2SiO4 ringwoodites (x = 0.00 to 0.61) up to a maximum pressure of 30 GPa. In our study, we combined the megabar-type diamond-anvil cell (DAC) with conventional and synchrotron FTIR spectroscopy. The high-pressure measurements were performed in three different pressure-transmitting environments: (1) CsI powder, (2) cryogenically loaded liquid argon, and (3) cryogenically loaded liquid argon annealed at 8.6 GPa at temperature of 120°C before further pressure increase. Between 10 and 12 GPa, all the samples loaded with methods (1) and (2), independent on composition, showed a sudden disappearance of the prominent OH-stretching feature and simultaneous discontinuities and/or changes in the pressure dependence of lattice vibrations compared with spectra of samples loaded with method (3). In experiments performed with method (3) the OH-stretching vibrations as well as lattice vibrations could be observed up to 30 GPa and their pressure behavior (dν/dP) can well be described by linear fits. Molecular vibrations, such as the OH stretching, are sensitive to non-hydrostatic conditions, especially in minerals with highly symmetric structures. We interpret the disappearance of the OH bands using methods (1) and (2) as a stress-induced proton disordering in hydrous ringwoodite. Our results confirm that argon pressure medium produces strongly non-hydrostatic conditions comparable to CsI or KBr, if it is not thermally annealed at pressures above 8 GPa. Our results suggest that the transition observed in hydrous Mg-ringwoodite end member is not present in compositions containing Fe. By comparing the behavior of samples compressed in different environments, we suggest that sudden disappearance of the OH-stretching band in hydrous ringwoodite could be driven by deterioration of the quasi-hydrostatic stress condition instead of a pressure-induced effect.  相似文献   

13.
Crystal chemistry of wadsleyite II and water in the Earth’s interior   总被引：1，自引：1，他引：0  
Wadsleyite II is a variably hydrous magnesium-iron silicate phase similar to spinelloid IV and a potential host for H in the Transition Zone of the Earths mantle. Two separate samples of wadsleyite II synthesized at 17.5 GPa and 1400°C and at 18 GPa and 1350°C have been characterized by electron microprobe, single-crystal X-ray diffraction, visible, IR, Raman, and Mössbauer spectroscopies, and transmission electron microscopy including electron energy-loss spectroscopy. The two samples have the following chemical formulae: Mg1.71Fe0.18Al0.01H0.33Si0.96O4 and Mg1.60Fe0.22Al0.01 H0.44Si0.97O4. Mössbauer spectroscopy and electron energy loss spectroscopy (EELS) indicate that about half of the iron present is ferric. Refinement of the structures shows them to be essentially the same as spinelloid IV. Calculated X-ray powder diffraction patterns show only subtle differences between wadsleyite and wadsleyite II. The hydration mechanism appears to be protonation of the non-silicate oxygen (O2) and possibly the oxygens surrounding the partially vacant tetrahedral site Si2, charge-balanced by cation vacancies in Si2, M5 and M6. The unit cell volume of this phase and its synthesis conditions indicate that it may be an intermediate phase occurring between the fields of wadsleyite and ringwoodite, if sufficient trivalent cations are available. The unit cell parameters have been refined at pressures up to 10.6 GPa by single-crystal X-ray diffraction in the diamond anvil cell. The refined bulk modulus for the sample containing 2.8 wt% H2O is 145.6 ± 2.8 GPa with a K of 6.1 ± 0.7. Similar to wadsleyite and ringwoodite, hydration has a large effect on the bulk modulus. The presence of this phase in the mantle could serve to obscure the seismic expression of the phase boundary between wadsleyite and ringwoodite near 525 km. The large apparent effect of hydration on bulk modulus is consistent with hydration having a larger effect on seismic velocities than temperature in the Transition Zone.  相似文献   

14.
The new synthetic phase Mg2Al3O[BO4]2(OH) provisionally named “pseudosinhalite” is optically, chemically, and structurally similar to the mineral sinhalite, MgAl[BO4], isostructural with forsterite. It grows hydrothermally from appropriate bulk compositions in the range 4–40?kbar at temperatures that increase with pressure (～650?→?900?°C), and it breaks down at higher temperatures to sinhalite?+?corundum?+?H2O. At P?≥?20?kbar single-phase products of euhedral twinned crystals could often be obtained. Pseudosinhalite is monoclinic with a?=?7.455 (1) Å, b?=?4.330 (1) Å, c?=?9.825 (2) Å, β?=?110.68 (1)°, and space group P21/c. Crystal structure analysis reveals that pseudosinhalite is also based on hexagonal close packing (hcp) of oxygen atoms with Mg and Al in octahedral and B in tetrahedral coordination. In pseudosinhalite the winged octahedral chains in the plane of hcp are not straight as in sinhalite but have a zigzag, 3-repeat period (Dreierkette), and only 1/10 instead of 1/8 of all tetrahedral sites are filled by boron. Hydrogen is located at a split position between two oxygen atoms O5—O5, which are only 2.550 Å apart and thus generate strong hydrogen bonding. This may be responsible for the absence of an hydroxyl absorption band between 2800?cm?1 and 3500?cm?1 in the powder IR spectrum. The equilibrium breakdown curve of pseudosinhalite to form sinhalite, corundum, and water was determined by bracketing experiments to pass through 10?kbar, 745?°C and 35?kbar, 950?°C, giving a slope of about 8?°C/kbar, similar to dehydration curves of some silicates at high pressure. In nature pseudosinhalite could have been misidentified as sinhalite. A possible appearance, like sinhalite in boron-rich skarns, would require more aluminous bulk compositions than for sinhalite at relatively low temperatures. However, pseudosinhalite might also form as a hydrous alteration product of sinhalite at low temperatures, perhaps in association with szaibelyite, MgBO2(OH).  相似文献   

15.
Ni, Co, and Zn are widely distributed in the Earth’s mantle as significant minor elements that may offer insights into the chemistry of melting in the mantle. To better understand the distribution of Ni2+, Co2+, and Zn2+ in the most abundant silicate phases in the transition zone and the upper mantle, we have analyzed the crystal chemistry of wadsleyite (Mg2SiO4), ringwoodite (Mg2SiO4), forsterite (Mg2SiO4), and clinoenstatite (Mg2Si2O6) synthesized at 12–20 GPa and 1200–1400 °C with 1.5–3 wt% of either NiO, CoO, or ZnO in starting materials. Single-crystal X-ray diffraction analyses demonstrate that significant amounts of Ni, Co, and Zn are incorporated in octahedral sites in wadsleyite (up to 7.1 at%), ringwoodite (up to 11.3 at%), olivine (up to 2.0 at%), and clinoenstatite (up to 3.2 at%). Crystal structure refinements indicate that crystal field stabilization energy (CFSE) controls both cation ordering and transition metal partitioning in coexisting minerals. According to electron microprobe analyses, Ni and Co partition preferentially into forsterite and wadsleyite relative to coexisting clinoenstatite. Ni strongly prefers ringwoodite over coexisting wadsleyite with \({D}_{\text{Ni}}^{\text{Rw}/\text{Wd}}\)?=?4.13. Due to decreasing metal–oxygen distances with rising pressure, crystal field effect on distribution of divalent metal ions in magnesium silicates is more critical in the transition zone relative to the upper mantle. Analyses of Ni partitioning between the major upper-mantle phases implies that Ni-rich olivine in ultramafic rocks can be indicative of near-primary magmas.  相似文献   

16.
A possible structure for hydrous modified olivine (HyM-α) has been obtained by the subtraction of Mg3SiO5 from forsterite by crystallographic shear along a direction parallel to the [010] direction of olivine. The subtraction of Mg3SiO5 results in the subtraction of MgO from bulk chemistry (?Mg3SiO5=?Mg2SiO4?MgO). A possible structure for HyM-α thus obtained has the chemical formula Mg9Si5H2O20 (= 5 × Mg1.8SiH0.4O4) with monoclinic unit cell a=4.754 Å, b=10.19 Å, c=29.90 Å, ρ=3.126 g cm?3, and space group=Ac2m (no. 39). Since the X-ray powder diffraction pattern of HyM-α proposed in this study is very close to that of clinohumite, there is the possibility of this phase having been undiscovered. The humite group minerals and HyM-α proposed in this study make a homologous series as recombination structures: Mg(2 m + n )Si m H2 n O2(2 m + n ) for the humite group and Mg(2 m + n )Si( m + n )H2 n O4( m + n ) for HyM-α A characteristic feature is that Mg/Si > 2 for the humite group and Mg/Si < 2 for HyM-α. Forsterite specimens containing around 100 ppm H2O reported in mantle xenoliths might be the disordered case with n=1 and m=1200 of the humite group or HyM-α.  相似文献   

17.
 Raman spectra of a single-crystal fragment of hydrous γ-Mg2SiO4, synthesized in a multianvil press, have been measured in a diamond-anvil cell with helium as pressure-transmitting medium to 56.5 GPa at room temperature. All five characteristic spinel Raman modes shift continuously up to the highest pressure, showing no evidence for a major change in the crystal structure despite compression well beyond the stability field of ringwoodite in terms of pressure. At pressures above ∼30 GPa a new mode on the low-frequency site of the two silicate-stretching modes is clearly identifiable, indicating a modification in the spinel structure which is reversible on pressure release. The frequency of the new mode (802 cm−1 extrapolated to 1 bar) suggests the presence of Si–O–Si linkages and/or a partial increase in the coordination of Si. Direct determination of the subtle structural change causing the new Raman mode would require high-pressure, single-crystal synchrotron X-ray diffraction experiments. The Raman modes of hydrous and anhydrous Mg-end-member ringwoodite are nearly identical up to 20 GPa, suggesting that protonation has only minor effect on the lattice dynamics over the entire pressure stability range for ringwoodite in the mantle. Received: 7 December 2001 / Accepted: 16 April 2002  相似文献   

18.
 Raman spectra of hydrous β-Mg2SiO4 (1.65 wt% H2O) have been measured in a diamond-anvil cell with helium as a pressure-transmitting medium at room temperature to 50 GPa. We observe three OH-stretching modes, a doublet with components at 3329 and 3373 cm−1, which decrease linearly with pressure, and a single mode at 3586 cm−1, which remains nearly constant up to 24 GPa before decreasing at higher pressures. Assessment of the mode frequencies and their pressure dependence, together with previous results from X-ray and IR data, are consistent with protonation of the O1 site in agreement with previous studies. Strict assignment of Raman activity awaits detailed structural models. The nature of the protonation in wadsleyite may require more specific experimental probes for full solution of the hydrogen-site problem. Received: 18 July 2000 / Accepted: 22 November 2000  相似文献   

19.
The structure and energies of the cores of [100] and [001] screw dislocations in wadsleyite (β-Mg2SiO4) are calculated using a cluster-based combined elastic-atomistic method and a new parameterized interatomic potential model. For a core radius of 10 Å, core energies are found to be 2.5 and 4.4 eV/Å for the [100] and [001] dislocations, respectively. Both dislocations are associated with significant non-elastic displacement fields extending beyond the core with a radial component toward the dislocation line. The core of the [100] dislocation contains tetrahedrally coordinated magnesium, has a simple 2D structure and is spread parallel to (011) in a manner that suggests high mobility. In contrast, the core of the [001] dislocation has an extended and complex 3D structure involving the formation a large Si6O19 unit twisted around the dislocation line. This implies that movement of the [001] dislocation will be inhibited by the need to cleave Si–O bonds. These observations, combined with the anomalously low core energy of the [100] dislocation, explain the regular occurrence of [100] dislocations and very rare observation of [001] dislocations in experimentally deformed wadsleyite samples.  相似文献   

20.
The growth rate of ringwoodite reaction rims between MgSiO3 perovskite and periclase was investigated at 22.5 GPa and 1,800 °C for 1–24 h using the Kawai-type high-pressure apparatus. The reaction was likely to proceed by a diffusion-controlled mechanism in which the dominant diffusion mechanism was grain-boundary diffusion. The reaction constant (the width of the ringwoodite reaction rim squared divided by time) determined from these experiments was between 1.3 × 10?15 and 5.6 × 10?15 m2/s. A Pt inert marker experiment indicated that the MgO component migrated faster than the SiO2 component in ringwoodite. Thus, either Mg or O having the slower diffusion rate controlled the reaction. Because previous diffusion studies have shown that diffusion rates of O are slower than those of Mg, O would be a rate-controlling element for ringwoodite formation from MgSiO3 perovskite and periclase. The growth rate appeared to be too fast to explain the observed topographic rise (~10 km) inside mantle plumes at the 660-km discontinuity.  相似文献   

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