Olivine Phase Transition Research Articles

Stabilizing inverse ringwoodite with defects, and a possible origin for the 560-km seismic discontinuity

Ringwoodite is an important mineral in the mantle transition zone, and its cationic disorder can profoundly affect its physicochemical properties, but there is currently much controversy about this disorder. In this study, we investigate the cation disorder states of pure Mg2SiO4-ringwoodite and defective ringwoodite under mantle transition zone conditions through DFT calculations and thermodynamic models. Two stable endmembers are seen, one with normal ringwoodite structure and the other with inverted structure (its Si atoms and half of its Mg atoms have swapped sites). Our results indicate that pure ringwoodite does not invert (swap Mg and Si cations) under normal mantle temperatures but the introduction of a Si-excess, Mg-deficient defect induces a swap at normal mantle temperatures and this swap is likely induced by a wide range of defects including water. Thus, in the presence of such a defect or similar defects the olivine phase transition sequence may then go from olivine to wadsleyite to inverse ringwoodite, and then normal ringwoodite. We calculate the seismic properties of normal and inverse ringwoodite and find significantly slower wave speeds in inverted ringwoodite. Due to this difference the presence of inverse ringwoodite may provide a potential explanation for the discontinuous interface of seismic waves at the depth of ∼560 km.

High precision measurement of trace F and Cl in olivine by electron probe microanalysis

Fluorine (F) and chlorine (Cl) are important volatiles in olivine and its high-pressure polymorphs, which would significantly affect olivine phase transition, melting temperature, and physical property of the mantle. F and Cl concentrations in olivine can be detected by electron probe microanalysis (EPMA). However, the analytical accuracy and precision can be impeded by severe peak overlaps, low peak intensities of traditional analytical crystals, and secondary fluorescence effects. In this study, we constructed an optimized analytical method with high accuracy and precision to analyze trace F and Cl in olivine. Key parameters of analytical crystals, beam conditions, peak overlaps, and secondary fluorescence effects were discussed. Variations in the levels of the analyzed trace elements fall within ± 10%. The detection limits (3σ) for F and Cl are lowered to 30 ppm and 5 ppm, respectively. This method can provide precise F and Cl analysis for natural olivine samples and help to provide significant information on its formation process.

Roles of partial hydration of mantle transition zone and wadsleyite–olivine phase transition in wet plume development: Origin of Quaternary intraplate volcanoes in Korea

The Quaternary intraplate volcanoes in Korea have a sparser distribution (~330–563 km) than that of Japanese arc volcanoes (~80 km), geochemistry indicating mixing of components from the stagnant slab and mantle transition zone, and a longer elapsed time (~10–20 Myr) of the Quaternary eruption after the stagnation of the Pacific Plate. Although volcanism can be explained by wet plumes originating from the hydrous layer of the stagnant slab, previous numerical studies could not consistently explain the spatiotemporal and geochemical observations because of the neglected partial hydration of the mantle transition zone and wadsleyite–olivine phase transition at the 410-km discontinuity, both of which affect the viscosity and buoyancy structures. Thus, we constructed a series of two-dimensional numerical models by considering the partial hydration of the mantle transition zone and phase transition at the 410-km discontinuity. The results showed that the wet plumes are retarded at the 410-km discontinuity by the phase transition and merge into batches forming hybrid plumes in the asthenosphere, explaining the mixing of components from the hydrous slab and mantle transition zone, evidenced by the geochemical observations. The spacing (~485 km) and elapsed time (~24 Myr) of the hybrid plumes beneath the lithosphere are consistent with the spatiotemporal observations. Furthermore, the degree of mixing of the components from the hydrous slab and mantle transition zone found in the hybrid plumes explains the spatial distribution of water in the mantle transition zone beneath Korea.

X-discontinuity and transition zone structure beneath Hawaii suggests a heterogeneous plume

The Hawaiian Island chain in the middle of the Pacific Ocean is a well-studied example of hotspot volcanism caused by an underlying upwelling mantle plume. The thermal and compositional nature of the plume alters the mantle phase transitions, which can be seen in the depth and amplitude of seismic discontinuities. This study utilises >5000 high quality receiver functions from Hawaiian island stations to detect P-to-s converted phases to image seismic discontinuities between 200 to 800 km depth. Common-conversion point stacks of the data are used to map out lateral variations in converted phase observations, while slowness stacks allow differentiation between true conversions from discontinuities and multiples. We find that the 410 discontinuity is depressed by 20 km throughout our study region, while the main 660 is around average depth throughout most of the area. To the southwest of the Big Island we observe splitting of the 660, with a major peak at 630 km, and a minor peak appearing at 675 km depth. This is inferred to represent the position of the hot plume at depth, with the upper discontinuity caused by an olivine phase transition and the lower by a garnet phase transition. In the upper mantle, a discontinuity is found across the region at depths varying between 290 to 350 km. Identifying multiples from this depth confirms the presence of a so-called X-discontinuity. To the east of the Big Island the X-discontinuity lies around 336 km and the associated multiple is particularly coherent and strong in amplitude. Strikingly, the discontinuity around 410 km disappears in this area. Synthetic modelling reveals that such observations can be explained by a silica phase transition from coesite to stishovite, consistent with widespread ponding of silica-saturated material at these depths around the plume. This material could represent eclogite enriched material, which is relatively silica-rich compared to pyrolite, spreading out from the plume to the east as a deep eclogite pool, a hypothesis which is consistent with dynamical models of thermochemical plumes. Therefore these results support the presence of a significant garnet and eclogite component within the Hawaiian mantle plume.

Polymerization of methane molecules and phase transition of san carlos olivine under the Earth's mantle conditions

High-pressure and high-temperature experiments of the olivine-methane-water system were performed using a laser-heated diamond anvil cell at pressure range from 5.8 GPa to 29.4 GPa and temperatures up to 2000K. The samples were examined by X-ray diffractometry and Raman spectroscopy under high pressures and room temperature. The heated areas of the samples changed to black color. Raman spectroscopy revealed the existence of ethane, heavier hydrocarbons, graphite and glassy carbon besides methane molecules. X-ray diffractometry showed that olivine was remained in the sample heated at 5. 8 GPa, 2000K. Wadsleyite and ringwoodite were observed in the samples heated at 14.5 GPa and 19.5 GPa, respectively. At 29.4 GPa, the diffraction line of Mg-perovskite and magnesiowustite were observed. The observed phase changes were similar to those observed in anhydrous and hydrous conditions. The present results suggest that polymerization of methane molecules occurred and that phase transition of olivine occurred even under the existence of methane-water fluid in the deeper part of the mantle.

Detailed nature of the 660 km region of the mantle from global receiver function data

We use a global data set of receiver functions to study the range of observations for the region at the base of the mantle transition zone, around the 660 km discontinuity. A variety of signals has already been observed in regional studies; here we characterize the breadth of such observations on a global scale. While the 410 km discontinuity is usually found to be simple in shape, the 660 km discontinuity often shows broad or asymmetric peaks and sometimes multiple signals. Unlike previous studies on the subject, we show that complexity of structure is not limited to subduction zones. Reflectivity synthetics are used to show that the observed signals can be generated by structures with combinations of sharp discontinuities and velocity gradients around 660 km depth. Mineral physics suggests that at 410 km depth only the olivine phase transition is important. In contrast, at 660 km depth both the olivine and nonolivine transitions generate multiple discontinuities and gradients, which could be visible to Pds signals. Seismic observations of a simple 410 km signal and a complex 660 km signal might therefore be expected from the predictions of mineral physics. However, a mantle of uniform composition fails to explain the variety of signals seen in our data set, requiring laterally varying chemical composition at the base of the mantle transition zone.

Mantle plumes pinched in the transition zone

We have employed an axisymmetric, spherical shell model with the two major phase transitions and variable viscosity to study the dynamical processes associated with both regular mantle plumes and superplumes, which can develop spontaneously in strongly time-dependent situations. This model is heated both from below and from within and the viscosity varies according to the temperature, pressure and the phase boundaries. We have studied the physical processes due to the interactions between the different types of plumes and superplumes with the transition zone, which can shed light on some new understanding of intraplate volcanism. The interaction between the plume and the two major phase transitions gives rise to the production of a reservoir of plume material, localized in the transition zone. The spinel to olivine phase transition induces the ejection of very fast plume material within a very thin vertical channel towards the surface. The spreading of this plume is controlled by the upper-mantle circulation. Stable plumes, which extend throughout the mantle for several hundred million years, can produce volcanism with an age progression of the seamounts. Other plumes, which are bent and move relative to the global circulation, may generate chaotic surface volcanism with a very limited age progression of the seamounts. Plume–plume collision produces a superplume, growing from the core–mantle boundary (CMB). The interaction between superplumes and the perovskite to spinel phase transition generates a thick thermal boundary layer, in which the upper-mantle can serve as the site for the launching of new smaller plumes. Stress fields associated with the superplume have the largest magnitude at the CMB and just below the 670 km discontinuity. They may be associated with seismic anisotropy found there.

Fine structure of the 410‐km discontinuity

The April 14, 1995, earthquake in western Texas (Mw 5.7) produced a strong topside reflection off the 410‐km discontinuity which was recorded on a multitude of seismic arrays throughout the southwestern United States. Data from 394 vertical short‐period and 24 broadband instruments provide dense coverage of this event from distances of 11° to 19° and provide a detailed look at the subcontinental 410‐km structure. The salient features of this data set are (1) the strong dependence on wavelength of the 410‐km triplication range, (2) the uniform amplitude ratio of the direct P and reflected P410 phases on both short‐period and broadband recordings throughout the triplication, and (3) the abrupt termination of the short‐period P410 phase at 13.3°. These features are best modeled by a composite discontinuity in which a sharp velocity jump of 3% is overlain by a linear velocity jump of 3.5% spread over 14 km. The interference of energy turning in the diffuse and sharp portions of this discontinuity structure reproduces both the long‐ and short‐period triplication range and the step‐like behavior of the P410 short‐period amplitude, which cannot be reproduced with either a simple linearly diffuse or a purely sharp discontinuity. This composite structure produces a triplication range which depends on source frequency and has an apparent depth which depends on observation frequency. Additionally, this is the structure expected from mineralogical arguments for the α to β olivine phase transition.

高温高圧鉱物物性 マントル物質の相平衡および鉱物物性に対するＨ２Ｏの影響

This paper reviews the effect of H2O in the mantle mineralsbased on the recent author's studies. The topics include the melting relations and the melt compositions of the mantle materials under hydrous conditions, the hydrous minerals in the earth interiors, especially hydrous wadsleyite and hydrous ringwoodite, the effect of H2O in the phase transition of olivine, the elastic properties of hydrous minerals, the rheology of olivine polymorphs between anhydrous and hydrous conditions, the dehydration of the slab constituentminerals, and the generation of komatiite magma in the hydrous mantle.

Electrical conductivity of dunite during shock compression from 12.5 to 45 GPa

The electrical conductivity of 7 dunites (Addie, Åheim, Balsam Gap, Corundum Hill, Dun Mountain, Mooihoek and Twin Sisters) in the pressure range 12.5 ‐ 45 GPa has been studied using the shock wave technique. In this pressure range the measured conductivity varies from 2.8·10−5 to 2.6·10−2Ω−1cm−1. Differences in the total olivine content, Fe‐content of the olivines, Fe2O3− and chromite content and grain sizes of the dunites lead to differences in their conductivity. All dunites show an increase of about 105Ω−1cm−1 of the electrical conductivity in the pressure range 0 ‐ 40 GPa, thus approaching semiconductor behavior. This increase is due to both temperature and pressure effects and possibly also to a partial phase transition of olivine.