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Jung-fu Lin - One of the best experts on this subject based on the ideXlab platform.
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Iron partitioning in natural Lower-Mantle minerals: toward a chemically heterogeneous Lower Mantle
American Mineralogist, 2017Co-Authors: Felix V. Kaminsky, Jung-fu LinAbstract:The concentrations of Fe, Al, and Ni and their distributions were determined for all known natural assemblages of ferropericlase (fPer) and bridgmanite (Bridg), coexisting as inclusions in deep-Mantle diamonds from Brazil, Canada, Guinea, and South Australia. Based upon these data, it is likely that some areas within the deep Lower Mantle are iron-rich and differ markedly from a pyrolitic composition. In the Lowermost Lower Mantle, Bridg is Al-rich and fPer is Ni-poor, witnessing the presence of a free metallic phase in the mineral-forming environment. The iron partitioning in the Bridg + fPer association [ K D Bridg-fPer = ( [ Fe / Mg ] Bridg ) / ( [ Fe / Mg ] fPer ) at in juvenile diamond inclusions is as low as 0.1–0.2. During ascent of the diamonds with their inclusions to the surface, the K D Bridg-fPer eventually increases to values of 0.4–0.5 and even as high as 0.7. The details of the element partitioning between natural Bridg and fPer in the Lower Mantle are as follows: iron in Bridg is ferrous Fe 2+ in the A site, substituting for Mg 2+ ; almost all iron in fPer is ferrous Fe 2+ ; the share of ferric Fe 3+ iron in fPer is Fe 3+ /ΣFe = 8–12 at%; iron concentrations in both Bridg and fPer increase with depth (pressure), reflecting the increasing Fe content in the Lower part of the Lower Mantle, different from that of a pyrolitic model. Al in Bridg is mainly in the cation site B and partly in the cation site A, in both cases substituting for Si, Mg, and Fe with vacancy formation; and in the case of Al positioning into both B and A sites, a charge-balanced reaction occurs. The natural samples show very diverse K D Bridg-fPer values and elemental distribution that cannot be simply explained by our current understanding on alumina dissolution in Bridg and the spin transition of Fe 2+ in fPer. These major differences between experimental results and observations in natural samples demonstrate the complex, inhomogeneous iron speciation and chemistry in the Lower Mantle.
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Iron Partitioning between Ferropericlase and Bridgmanite in the Earth's Lower Mantle
Journal of Geophysical Research: Solid Earth, 2017Co-Authors: Jung-fu Lin, Dane MorganAbstract:Earth's Lower Mantle is generally believed to be seismically and chemically homogeneous because most of the key seismic parameters can be explained using a simplified mineralogical model at expected press-temperature conditions. However, recent high-resolution tomographic images have revealed seismic and chemical stratification in the middle-to-Lower parts of the Lower Mantle. Thus far, the mechanism for the compositional stratification and seismic inhomogeneity, especially their relationship with the speciation of iron in the Lower Mantle, remains poorly understood. We have built a complete and integrated thermodynamic model of iron and aluminum chemistry for Lower Mantle conditions, and from this model has emerged a stratified picture of the valence, spin and composition profile in the Lower Mantle. Within this picture the Lower Mantle has an upper region with Fe3+ enriched bridgmanite with high-spin ferropericlase and metallic Fe, and a Lower region with low-spin, iron-enriched ferropericlase coexisting with iron-depleted bridgmanite and almost no metallic Fe. The transition between the regions occurs at a depth of around 1600km and is driven by the spin transition in ferropericlase, which significantly changes the iron partitioning and speciation to one that favors Fe2+ in ferropericlase and suppresses Fe3+ and metallic iron formation These changes lead to Lowered bulk sound velocity by 3-4% around the mid-Lower Mantle and enhanced density by ~1% toward the Lowermost Mantle. The predicted chemically and seismically stratified Lower Mantle differs dramatically from the traditional homogeneous model.
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Electronic spin transition of iron in the Earth’s Lower Mantle
Hyperfine Interactions, 2011Co-Authors: Jung-fu Lin, Andrea WheatAbstract:Silicate perovskite and ferropericlase are thought to be the primary constituents of the Lower Mantle, whereas silicate post-perovskite is more likely found in the Lowermost Mantle. Because these minerals contain certain amounts of iron, their properties and, consequently, those of the deep Mantle, are strongly influenced by iron’s spin and valence states. A high-spin to low-spin crossover in ferropericlase has been observed to occur in the middle part of the Lower-Mantle conditions. Recent Mossbauer results consistently show that Fe2 + predominantly exhibits extremely high quadrupole splittings in perovskite and post-perovskite, whereas a high-spin to low-spin transition of Fe3 + in the octahedral site occurs at high pressures. These results provide a new venue for discussion of the effects of the spin and valence states of iron on the physical and chemical properties of the Lower Mantle.
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Iron-rich perovskite in the Earth's Lower Mantle
Earth and Planetary Science Letters, 2011Co-Authors: Zhu Mao, Jung-fu Lin, Vitali B. Prakapenka, Heather C. Watson, Henry P. Scott, Yuming Xiao, Paul Chow, Catherine MccammonAbstract:The equations of state of perovskite with (Mg{sub 0.75},Fe{sub 0.25})SiO{sub 3} and MgSiO{sub 3} compositions have been investigated by synchrotron X-ray diffraction up to 130 GPa at 300 K in diamond anvil cells. Here we show that the addition of 25% Fe in MgSiO{sub 3} perovskite increases its density and bulk sound velocity (V{phi}) by 4-6% and 6-7%, respectively, at Lower-Mantle pressures. Based on concurrent synchrotron X-ray emission and Moessbauer spectroscopic studies of the samples, the increase in V{phi} and density can be explained by the occurrence of the low-spin Fe3+ and the extremely high-quadrupole component of Fe{sup 2+}. Combining these experimental results with thermodynamic modeling, our results indicate that iron-rich perovskite can produce an increase in density and a value of V{phi} that is compatible with seismic observations of reduced shear-wave velocity in regions interpreted as dense, stiff piles in the Lower Mantle. Therefore, the existence of the Fe-rich perovskite in the Lower Mantle may help elucidate the cause of the Lower-Mantle large low-shear-velocity provinces (LLSVPs) where enhanced density and V{phi} are seismically observed to anti-correlate with the reduced shear wave velocity.
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Spin transition of iron in the Earth's Lower Mantle
Physics of the Earth and Planetary Interiors, 2008Co-Authors: Jung-fu Lin, Taku TsuchiyaAbstract:Abstract Electronic spin-pairing transitions of iron and associated effects on the physical properties of host phases have been reported in Lower-Mantle minerals including ferropericlase, silicate perovskite, and possibly in post-perovskite at Lower-Mantle pressures. Here we evaluate current understanding of the spin and valence states of iron in the Lower-Mantle phases, emphasizing the effects of the spin transitions on the density, sound velocities, chemical behavior, and transport properties of the Lower-Mantle phases. The spin transition of iron in ferropericlase occurs at approximately 50 GPa and room temperature but turns into a wide spin crossover under Lower-Mantle temperatures. Current experimental results indicate a continuous nature of the spin crossover in silicate perovskite at high pressures, but which valence state of iron undergoes the spin crossover and what is its associated crystallographic site remain uncertain. The spin transition of iron results in enhanced density, incompressibility, and sound velocities, and reduced radiative thermal conductivity and electrical conductivity in the low-spin ferropericlase, which should be considered in future geophysical and geodynamic modeling of the Earth's Lower Mantle. In addition, a reduction in sound velocities within the spin transition is recently reported. Our evaluation of the experimental and theoretical pressure–volume results shows that the spin crossover of iron results in a density increase of 2–4% in ferropericlase containing 17–20% FeO. Here we have modeled the density and bulk modulus profiles of ferropericlase across the spin crossover under Lower-Mantle pressure–temperature conditions and shown how the ratio of the spin states of iron affects our understanding of the state of the Earth's Lower Mantle.
Leonid Dubrovinsky - One of the best experts on this subject based on the ideXlab platform.
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On origin of Lower-Mantle diamonds and their primary inclusions
Physics of the Earth and Planetary Interiors, 2014Co-Authors: Yuriy A. Litvin, A. V. Spivak, Natalia Solopova, Leonid DubrovinskyAbstract:Abstract Knowledge of mineralogy and petrology of unattainable Lower Mantle material is usually founded on high-pressure experiments with pyrolite (‘ in situ ’ material) and oceanic MORB basalt (subducted material). Primary inclusions in transition zone and Lower-Mantle ‘super-deep’ diamonds represent heterogeneous fragments of diamond-parental medium (not the unaltered Lower Mantle material). Inclusions of magnesiowustite and stishovite intergrowths (‘stishovite paradox’) give experimentally-supported evidence that stishovite, similarly to magnesiowustite, is not subducted but in situ Lower Mantle mineral. Primary Ca-, Mg-, Na-carbonate inclusions are symptomatic for multicomponent carbonatite (carbonate-oxide-silicate) parental melts for the Lower-Mantle diamonds and inclusions. We investigated melting phase relations of simple carbonates of Ca, Mg, Na and multicomponent Mg-Fe-Na-carbonate up to 60 GPa and 3500–4000 K (using multianvil press and diamond-anvil cell with laser heating) and determined a congruent melting of the carbonates and stability of PT -extended phase fields of the carbonate melts. ‘Super-deep’ diamonds are experimentally crystallized in melts of the Lower Mantle diamond-parental carbonate - magnesiowustite – Mg-perovskite – carbon system. Based on experimental and mineralogical evidence for the Lower Mantle diamonds inclusions, genetic links between diamonds and inclusions are determined and a generalized composition diagram of parental media for Lower Mantle diamonds and inclusions is constructed.
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Stable intermediate-spin ferrous iron in Lower-Mantle perovskite
Nature Geoscience, 2008Co-Authors: Catherine Mccammon, Vitali B. Prakapenka, Innokenty Kantor, O. Narygina, Jérôme Rouquette, U. Ponkratz, Ilya Sergueev, Mohamed Mezouar, Leonid DubrovinskyAbstract:Iron has the ability to adopt different electronic configurations, and transitions in its spin state in the Lower Mantle can significantly influence Mantle properties and dynamics. Experimental results for two Lower-Mantle perovskite compositions show that the intermediate spin state of iron is stable throughout the bulk of the Lower Mantle.
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Stable intermediate-spin ferrous iron in Lower-Mantle perovskite
Nature Geoscience, 2008Co-Authors: Catherine Mccammon, Innokenty Kantor, O. Narygina, Jérôme Rouquette, U. Ponkratz, Ilya Sergueev, Mohamed Mezouar, V. Prakapenka, Leonid DubrovinskyAbstract:The Lower Mantle is dominated by a magnesium- and iron-bearing mineral with the perovskite structure. Iron has the ability to adopt different electronic configurations, and transitions in its spin state in the Lower Mantle can significantly influence Mantle properties and dynamics. However, previous studies aimed at understanding these transitions have provided conflicting results1–4. Here we report the results of high-pressure (up to 110 GPa) and high-temperature (up to 1,000 K) experiments aimed at understanding spin transitions of iron in perovskite at Lower-Mantle conditions . Our M¨ossbauer and nuclear forward scattering data for two Lower-Mantle perovskite compositions demonstrate that the transition of ferrous iron from the high-spin to the intermediate-spin state occurs at approximately 30 GPa, and that high temperatures favour the stability of the intermediate-spin state. We therefore infer that ferrous iron adopts the intermediate-spin state throughout the bulk of the Lower Mantle. Our X-ray data show significant anisotropic compression of Lower-Mantle perovskite containing intermediate-spin ferrous iron, which correlates strongly with the spin transition. We predict spin-state heterogeneities in the uppermost part of the Lower Mantle associated with sinking slabs and regions of upwelling. These may affect local properties, including thermal and electrical conductivity, deformation (viscosity) and chemical behaviour, and thereby affect Mantle dynamics.
Catherine Mccammon - One of the best experts on this subject based on the ideXlab platform.
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Iron-rich perovskite in the Earth's Lower Mantle
Earth and Planetary Science Letters, 2011Co-Authors: Zhu Mao, Jung-fu Lin, Vitali B. Prakapenka, Heather C. Watson, Henry P. Scott, Yuming Xiao, Paul Chow, Catherine MccammonAbstract:The equations of state of perovskite with (Mg{sub 0.75},Fe{sub 0.25})SiO{sub 3} and MgSiO{sub 3} compositions have been investigated by synchrotron X-ray diffraction up to 130 GPa at 300 K in diamond anvil cells. Here we show that the addition of 25% Fe in MgSiO{sub 3} perovskite increases its density and bulk sound velocity (V{phi}) by 4-6% and 6-7%, respectively, at Lower-Mantle pressures. Based on concurrent synchrotron X-ray emission and Moessbauer spectroscopic studies of the samples, the increase in V{phi} and density can be explained by the occurrence of the low-spin Fe3+ and the extremely high-quadrupole component of Fe{sup 2+}. Combining these experimental results with thermodynamic modeling, our results indicate that iron-rich perovskite can produce an increase in density and a value of V{phi} that is compatible with seismic observations of reduced shear-wave velocity in regions interpreted as dense, stiff piles in the Lower Mantle. Therefore, the existence of the Fe-rich perovskite in the Lower Mantle may help elucidate the cause of the Lower-Mantle large low-shear-velocity provinces (LLSVPs) where enhanced density and V{phi} are seismically observed to anti-correlate with the reduced shear wave velocity.
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Stable intermediate-spin ferrous iron in Lower-Mantle perovskite
Nature Geoscience, 2008Co-Authors: Catherine Mccammon, Vitali B. Prakapenka, Innokenty Kantor, O. Narygina, Jérôme Rouquette, U. Ponkratz, Ilya Sergueev, Mohamed Mezouar, Leonid DubrovinskyAbstract:Iron has the ability to adopt different electronic configurations, and transitions in its spin state in the Lower Mantle can significantly influence Mantle properties and dynamics. Experimental results for two Lower-Mantle perovskite compositions show that the intermediate spin state of iron is stable throughout the bulk of the Lower Mantle.
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Stable intermediate-spin ferrous iron in Lower-Mantle perovskite
Nature Geoscience, 2008Co-Authors: Catherine Mccammon, Innokenty Kantor, O. Narygina, Jérôme Rouquette, U. Ponkratz, Ilya Sergueev, Mohamed Mezouar, V. Prakapenka, Leonid DubrovinskyAbstract:The Lower Mantle is dominated by a magnesium- and iron-bearing mineral with the perovskite structure. Iron has the ability to adopt different electronic configurations, and transitions in its spin state in the Lower Mantle can significantly influence Mantle properties and dynamics. However, previous studies aimed at understanding these transitions have provided conflicting results1–4. Here we report the results of high-pressure (up to 110 GPa) and high-temperature (up to 1,000 K) experiments aimed at understanding spin transitions of iron in perovskite at Lower-Mantle conditions . Our M¨ossbauer and nuclear forward scattering data for two Lower-Mantle perovskite compositions demonstrate that the transition of ferrous iron from the high-spin to the intermediate-spin state occurs at approximately 30 GPa, and that high temperatures favour the stability of the intermediate-spin state. We therefore infer that ferrous iron adopts the intermediate-spin state throughout the bulk of the Lower Mantle. Our X-ray data show significant anisotropic compression of Lower-Mantle perovskite containing intermediate-spin ferrous iron, which correlates strongly with the spin transition. We predict spin-state heterogeneities in the uppermost part of the Lower Mantle associated with sinking slabs and regions of upwelling. These may affect local properties, including thermal and electrical conductivity, deformation (viscosity) and chemical behaviour, and thereby affect Mantle dynamics.
Felix V. Kaminsky - One of the best experts on this subject based on the ideXlab platform.
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Basic problems concerning the composition of the Earth's Lower Mantle
Lithos, 2020Co-Authors: Felix V. KaminskyAbstract:Abstract The last decade has seen the publication of a number of new and highly pertinent studies on the composition of the Earth's Lower Mantle, leading to a better understanding of the Deep Earth. A series of new Lower-Mantle minerals were found, having formed under natural conditions and received the following names: bridgmanite, jeffbenite, breyite, and ellinaite. Some other, as yet, unnamed oxides, phosphates, and fluorides were also discovered for the first time. Among the new mineral phases, of particular interest are cubic nitrogen and ice-VII. Their presence demonstrates a significant role of both nitrogen and of water in the Deep Earth. This new data allows for creation of a principal model for the composition of the Earth's Lower Mantle. By various evidences, it differs greatly to that of the upper Mantle composition, and is heterogeneous.
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Water in the Earth’s Lower Mantle
Geochemistry International, 2018Co-Authors: Felix V. KaminskyAbstract:All major, rock-forming Lower-Mantle minerals (bridgmanite, CaSi-perovskite, ferropericlase and stishovite) are “nominally anhydrous minerals” (NAMs), in which hydrogen comprises less than 1 wt % and whose chemical formula would be normally written without hydrogen. In NAMs, hydrogen occupies various defects of the crystal lattice and is bonded to structural oxygen, forming hydroxyl groups. Currently, two main techniques can be used for water determination in the Mantle minerals: Fourier transform infrared spectrometry (FTIR) and secondary ion mass spectrometry (SIMS). They produce different results: determinations by SIMS are usually higher than quantifications of FTIR. As a result, the estimates of water concentrations in Lower-Mantle minerals vary widely. Most reliable concentrations of water are 1400–1800 ppm in bridgmanite, 10–80 ppm in ferropericlase, and 20–150 ppm in stishovite. The average concentration of water in the Lower Mantle is ~1500 ppm. Despite such minor concentrations in Lower-Mantle minerals, water forms a great reservoir within the Lower Mantle, probably amounting ~45.45 × 1023 grams H2O, i.e., ~3.3 times the mass of the Earth’s oceans. Some amount of water is transported into the Lower Mantle by subducting lithospheric slabs; this amount is balanced by the water flux from the Lower Mantle to the transition zone. Within areas of partial melting in the Lower and upper parts of the Lower Mantle, as well as in some local areas, stress and thermal increase initiate release of water from Lower-Mantle minerals into melt. The enrichment of partial melts with H2O depends on the P–T conditions, oxygen fugacity values, and percentage of melting. It causes major geodynamic processes that are initiated within the deep Earth. The major source of the water reservoir in the Lower Mantle is primordial water stored early in the Earth’s evolution.
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Lower-Mantle Mineral Associations
The Earth's Lower Mantle, 2017Co-Authors: Felix V. KaminskyAbstract:There are three major sources of information about the composition of the Lower Mantle: high P–T experiments, theoretical calculations, and geological observations. Experimental data, based on the use of diamond-anvil cell technique (DAC) , and theoretical calculations demonstrate that silicates, occurring in the upper Mantle and the transition zone , are replaced by predominantly perovskitic assemblage in the Lower Mantle. Depending on the starting substrate composition, two mineral associations should occur at pressures corresponding to the Lower-Mantle conditions: ultramafic (bridgmanite + CaSi-perovskite + ferropericlase ) and mafic (bridgmanite + CaSi-perovskite + ferropericlase + silica + Al-phase). Both iron-containing Lower-Mantle minerals, bridgmanite and ferropericlase, should be magnesium-rich. In recent decades, Lower-Mantle minerals were found as inclusions in diamonds from Brazil, Guinea, Canada, Australia and South Africa. They confirm the presence of ultramafic, mafic and carbonatitic mineral associations. Geological samples differ notably from the Lower Mantle compositions suggested on the basis of experimental and theoretical data for the pyrolitic composition. First, ferropericlase is the most common in the Lower-Mantle ultramafic association (averaging 55.6%), while bridgmanite comprises only 7.5%, about ten times Lower than has been suggested (c. 70–74%) in the Lower Mantle. Second, silica inclusions were identified in all sets of Lower-Mantle minerals observed in diamond from all regions and areas. Third, wide variations in ferropericlase compositions, reaching an iron index of up to fe = 0.64 were observed. Minerals from the ultramafic association overwhelmingly predominate in the Lower Mantle samples; only two samples of mafic mineral phases, phase Egg and δ−AlOOH are found to date.
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Diamond in the Lower Mantle
The Earth's Lower Mantle, 2017Co-Authors: Felix V. KaminskyAbstract:Diamond contains mineral inclusions of all three Lower-Mantle associations, juvenile ultramafic, mafic and carbonatitic; it is also an accessory mineral in all these associations. While the first two associations coexist with diamond, the carbonatitic association is a parental medium for the Lower-Mantle diamond. Physical and chemical characteristics of Lower-Mantle diamond differ from ones of lithospheric origin. Most of the Lower-Mantle diamonds are ‘nitrogen -free’ Type II variety. The others are usually low-nitrogen stones with the average nitrogen aggregation rate of 94%. The high proportion of nitrogen-aggregated diamonds suggests that they had a prolonged residence in the Lower Mantle under high-T conditions, which resulted in an almost complete transformation of single-atomic and paired nitrogen centers into polyatomic complexes. In contrast to lithospheric diamonds, almost all analyzed Lower-Mantle ones (70–89%) have noticeable levels of hydrogen centers (up to 4–6 cm−1). The isotopic compositions of Lower-Mantle diamonds are located within a narrow range: from −5.45 to −1.26‰ δ 13C VPDB, with an average value of −4.36‰ ± 2.28‰ (2σ). It may be considered as the juvenile Lower-Mantle carbon isotopic composition. The isotopic composition of nitrogen for Lower-Mantle diamonds is located within a close range, from −5.2 to −1.0‰ δ 15Natm, with an average value of δ 15Natm = −3.00‰ ± 2.37‰ δ 15Natm. Lower-Mantle diamond was formed in carbonate-oxide parental melts and fluids, which experienced fractional crystallization with the decrease of temperature and changes in the melt composition. The most important role in this process belongs to the carbonate component in the parental melt.
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Ultramafic Lower-Mantle Mineral Association
The Earth's Lower Mantle, 2017Co-Authors: Felix V. KaminskyAbstract:The juvenile ultramafic Lower Mantle is composed of the mineral association: bridgmanite + ferropericlase + CaSi-perovskite + free silica. Bridgmanite, with mg = 0.84–0.96 forms two compositional groups: low-Al and high-Al. High-Al bridgmanite is richer in Fe and infers the characteristic of deeper layers in the Lower Mantle. The crystal structure of bridgmanite is orthorhombic through the entire Lower Mantle down to the D″ layer. The chemical composition of ferropericlase is different from the predicted one with the magnesium index mg varying from 0.36 to 0.90. Low-Fe ferropericlase has a cubic rocksalt structure, which is stable throughout the entire Lower Mantle. Iron contents in both ferropericlase and bridgmanite and ferropericlase increase with pressure indicating the increase of Fe concentration in the Lower Mantle with depth. CaSi-perovskite is remarkably clean in its chemical composition with only minor admixtures of Ti, Al and Fe, but is enriched in trace elements. CaSi-perovskite within the Lower Mantle has a cubic structure which at low temperatures (in subsolidus conditions) may transfer into a tetragonal or orthorhombic structure. The presence of free silica in the Lower Mantle was identified in geological samples from all areas. In the upper part of the Lower Mantle it is represented by stishovite ; at a depth of 1600–1800 km stishovite transforms into the CaCl2-structured polymorph; and at the CMB, into a α-PbO2 phase seifertite. In addition to the major minerals, a variety of other mineral phases occurs in the Lower Mantle: Mg–Cr–Fe, Ca–Cr and other orthorhombic oxides , jeffbenite , ilmenite , native Ni and Fe, moissanite and some others.
David A. Yuen - One of the best experts on this subject based on the ideXlab platform.
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critical phenomena in thermal conductivity implications for Lower Mantle dynamics
Journal of Geodynamics, 2007Co-Authors: Anne M. Hofmeister, David A. YuenAbstract:Abstract Microscopic mechanisms for heat transport in dense minerals (phonon scattering and photon attenuation) exhibit aspects of threshold behavior, discussed qualitatively here. For all minerals examined so-far using laser-flash analysis, the lattice component of the thermal conductivity of the Mantle asymptotes to a constant above a critical temperature of ∼1500 K. Radiative transfer calculated from absorption spectra has thresholds in both grain-size and Fe content, and a rather complex dependence on temperature. These critical phenomena impact convection of the Lower Mantle, because the lattice contribution tends to destabilize the cold boundary layers, whereas radiative transfer mostly promotes stability in the Lower Mantle, unless the grains are large and Fe-rich, which makes convection chaotic and time-dependent. The specific behavior suggests that flow in the Lower Mantle is sluggish, whereas flow in the upper Mantle-transition zone is time-dependent. The decrease in krad as Fe/(Fe + Mg) increases beyond ∼0.1 may be connected with formation of Lower Mantle, thermo-chemical plumes through positive feedback.
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Lower-Mantle material properties and convection models of multiscale plumes
2007Co-Authors: Ctirad Matyska, David A. YuenAbstract:We present the results of numerical Mantle convection models demonstrating that dynamical effects induced by variable Mantle viscosity, depth-dependent thermal expansivity, radiative thermal conductivity at the base of the Mantle, the spinel to perovskite phase change and the perovskite to post-perovskite phase transition in the deep Mantle can result in multiscale Mantle plumes: stable Lower-Mantle superplumes are followed by groups of small upper-Mantle plumes. Both radiative thermal conductivity at the base of the Lower Mantle and a strongly decreasing thermal expansivity of perovskite in the Lower Mantle can help induce partially layered convection with intense shear heating under the transition zone, which creates a low-viscosity zone and allows for the production of secondary Mantle plumes emanating from this zone. Large-scale upwellings in the Lower Mantle, which are induced mainly by both the style of Lower-Mantle viscosity stratification and decrease of thermal expansivity, control position of central upperMantle plumes of each group as well as the upper-Mantle plume-plume interactions.
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How flat is the Lower-Mantle temperature gradient?
Earth and Planetary Science Letters, 2002Co-Authors: Marc Monnereau, David A. YuenAbstract:Abstract The temperature gradient in the Lower Mantle is fundamental in prescribing many transport properties, such as the viscosity, thermal conductivity and electrical conductivity. The adiabatic temperature gradient is commonly employed for estimating these transport properties in the Lower Mantle. We have carried out a series of high-resolution 3-D anelastic compressible convections in a spherical shell with the PREM seismic model as the background density and bulk modulus and the thermal expansivity decreasing with depth. Our purpose was to assess how close under realistic conditions the horizontally averaged thermal gradient would lie to the adiabatic gradient derived from the convection model. These models all have an endothermic phase change at 660 km depth with a Clapeyron slope of around −3 MPa K −1 , uniform internal heating and a viscosity increase of 30 across the phase transition. The global Rayleigh number for basal heating is around 2×10 6 , while an internal heating Rayleigh number as high as 10 8 has been employed. The pattern of convection is generally partially layered with a jump of the geotherm across the phase change of at most 300 K. In all thermally equilibrated situations the geothermal gradients in the Lower Mantle are small, around 0.1 K km −1 , and are subadiabatic. Such a low gradient would produce a high peak in the Lower-Mantle viscosity, if the temperature is substituted into a recently proposed rheological law in the Lower Mantle. Although the endothermic phase transition may only cause partial layering in the present-day Mantle, its presence can exert a profound influence on the state of adiabaticity over the entire Mantle.
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Is the Lower‐Mantle rheology Newtonian today?
Geophysical Research Letters, 1996Co-Authors: Arie P. Van Den Berg, David A. YuenAbstract:The issue concerning the dominant creep mechanism in Mantle convection has been studied numerically with a rheology incorporating both linear and non-linear elements. We have employed a rheology suggested by the recently obtained melting temperature of Lower-Mantle constituents. For an effective Rayleigh number between 105 and 106 this type of strongly temperature and pressure-dependent rheology induces a circulation in the Earth's Mantle, which is characterized by a sluggish type of flow in the Lower Mantle with a, prevailing Newtonian character, while in the upper Mantle much more vigorous flows are developed in a predominantly non-Newtonian rheology.
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Lower-Mantle viscosity constrained by seismicity around deglaciated regions
Nature, 1991Co-Authors: Giorgio Spada, David A. Yuen, Roberto Sabadini, Enzo BoschiAbstract:KNOWLEDGE of the viscosity structure of the Earth's Mantle is important for constraining models of Mantle convection and isostatic rebound. Here we show that seismicity around the margins of deglaciated areas provides a constraint on the viscosity of the Lower Mantle, in addition to those previously proposed1,2. Calculations using a spherical, viscoelastic Earth model show that the present-day magnitude of the stress fields induced in the lithosphere beneath the (now-disappeared) Laurentide and Fennoscandian ice sheets is very sensitive to the value of the Lower-Mantle viscosity. Stresses of ∼100 bar, sufficient to cause seismicity, can still remain in the lithosphere for Lower-Mantle viscosities greater than ∼1022 Pa s; for Lower-Mantle viscosities of ∼1021 Pa s, only a few tens of bars of stress persist in the lithosphere today. This influence of Lower-Mantle viscosity on the state of stress in the lithosphere also has implications for the migration of stress from earthquakes, and hence for earthquake recurrence times.
