Electrical conductivity structures of the Earth’s mantle estimated from the magnetotelluric and geomagnetic deep sounding methods generally show increase of conductivity from 10
−4
–10
−2
to 10
0
... S/m with increasing depth to the top of the lower mantle. Although conductivity does not vary significantly in the lower mantle, the possible existence of a highly conductive layer has been proposed at the base of the lower mantle from geophysical modeling. The electrical properties of mantle rocks are controlled by thermodynamic parameters such as pressure, temperature and chemistry of the main constituent minerals. Laboratory electrical conductivity measurements of mantle minerals have been conducted under high pressure and high temperature conditions using solid medium high-pressure apparatus. To distinguish several charge transport mechanisms in mantle minerals, it is necessary to measure the electrical conductivity in a wider temperature range. Although the correspondence of data has not been yet established between each laboratory, an outline tendency of electrical conductivity of the mantle minerals is almost the same. Most of mineral phases forming the Earth’s mantle exhibit semiconductive behavior. Dominant conduction mechanism is small polaron conduction (electron hole hopping between ferrous and ferric iron), if these minerals contain iron. The phase transition olivine to high-pressure phases enhances the conductivity due to structural changes. As a result, electrical conductivity increases in order of olivine, wadsleyite and ringwoodite along the adiabat geotherm. The phase transition to post-spinel at the 660 km discontinuity further can enhance the conductivity. In the lower mantle, the conductivity once might decrease in the middle of the lower mantle due to the iron spin transition and then abruptly increase at the condition of the D″ layer. The impurities in the mantle minerals strongly control the formation, number and mobility of charge carriers. Hydrogen in nominally anhydrous minerals such as olivine and high-pressure polymorphs can enhance the conductivity by the proton conduction. However, proton conduction has lower activation enthalpy compared with small polaron conduction, a contribution of proton conduction becomes smaller at high temperatures, corresponding to the mantle condition. Rather high iron content in mantle minerals largely enhances the conductivity of the mantle. This review focuses on a compilation of fairly new advances in experimental laboratory work together with their explanation.
The electrical conductivity of San Carlos clinopyroxene aggregates with various water contents were measured under Ni–NiO buffer at 1.5 GPa and 600–1200 K in a DIA-type apparatus. The conductivity ...increases with increasing water content in clinopyroxene. Hidden conduction mechanism was detected because of the much smaller iron content in clinopyroxene, which was usually covered by small polaron conduction in other nominally anhydrous minerals. The identified activation enthalpies ranged from 0.70–0.75 eV to 1.23–1.37 eV. Our result reveals that the dominant charge-carrying species in electrical conductivity could change with temperature and water content. At high temperatures relevant to asthenospheric condition, activation enthalpy for the conductivity agrees well with that for the hydrogen self-diffusion. The dominant charge carrier therefore might be M site vacancy. However, contrary to previous view that all hydrogens contribute to increasing conductivity equally, our result shows that only a limited amount (20%–40%) of hydrogen acts as effective charge carrier in clinopyroxene. On the other hand, the activation enthalpy for the conductivity at low temperatures is significantly lower than that for the hydrogen self-diffusion, similar to what has been observed in olivine and orthopyroxene. This type of conduction is probably caused by fast diffusion of specific hydrogen or fast hydrogen grain boundary diffusion. At low temperatures, the proton conduction of clinopyroxene is nearly one order and two orders of magnitude lower than those of olivine and orthopyroxene, respectively, and tends to converge at high temperatures. Using the present data combined with conductivity of olivine and orthopyroxene, a laboratory-based conductivity-depth profile in the uppermost mantle shows that hydrous clinopyroxene cannot account for the high conductive regions observed beneath the ocean floor near Eastern Pacific Rise. The presence of partial melt would be unavoidable.
•Electrical conductivity of mantle Cpx as a function of water content is measured.•Measured conductivity matches well with calculation based on H–D diffusion.•Only limited amount of water acted as effective charge carriers in conductivity.•Conductivity of Cpx is much smaller than those of Opx and Ol at low temperatures.•Hydrous Cpx cannot account for the conductive anomaly near Eastern Pacific Rise.
The high conductivity anomalies observed in the oceanic asthenosphere have shown anisotropic signature parallel to the plate motion. Anisotropic alignment of partial melt has been considered to one ...of probable explanations for the observed anisotropic conductivity structure, but the effect of temperature on the distribution of melt, the composition of melt, and the magnitude of electrical anisotropy for partial molten peridotite is unknown under shear deformation. In this study, the electrical conductivity of partially molten peridotite (KLB-1) under shear deformation was measured at 1 GPa in a DIA type apparatus with a uniaxial deformation facility to provide new constraints on the anisotropic signature in the oceanic asthenosphere. The conductivity measurements were performed simultaneously in two directions of three principal axes: parallel and normal to the shear direction on the shear plane, and perpendicular to the shear plane, by using impedance spectroscopy at temperature ranges of 1483-1548 K. Our results indicate that the total melt fraction, the absolute conductivity values, and the magnitude of electrical anisotropy of partially molten peridotite increase with increasing temperature. Although the Na2O content varies widely at constant temperature in the recovered melt, very small changes of shear-parallel conductivities (σx) before and after shear deformation suggest that the electrical conductivity of partially molten peridotite is mainly controlled by temperature, rather than alkali content in partial melt. Microstructural observations of the recovered samples reveal that the development of conductivity anisotropy was caused by the realignment of melt pockets parallel to the shear direction, which forms two melt-rich regions. Furthermore, we estimate how much melt fraction is partitioned into melt-rich regions by calculating the area ratio of two melt-rich regions to the whole area. Our calculations show that once melt segregation occurs, more than 50% of the total melt fraction will partition into the melt-rich regions, and this proportion will continue to increase with the increase of temperature. This finding suggests that development of electrical anisotropy in partially molten peridotite under shear deformation will increase with increasing temperature, which may provide new constraints on interpretation of high conductivity anomalies observed in the oceanic asthenosphere.
•We report in-situ 3D EC measurements on partial molten peridotite under shear.•Electrical anisotropy of partially molten peridotite increase with temperature.•Melt fraction in the upper mantle may be underestimated in previous studies.
Graphite is considered as one of candidate to explain the high‐conductivity anomalies revealed through magnetotelluric (MT) observations. To investigate the effect of interfacial energy on the ...interconnection of graphite in olivine matrix, we measured the electrical conductivity of polycrystalline San Carlos olivine mixed with 0.8 vol % graphite on the grain boundaries via impedance spectroscopy at 1 GPa and 300–1700 K in a cubic multianvil apparatus. The olivine‐graphite dihedral angle of the recovered sample was also measured to determine interfacial energy between graphite and olivine. The bulk electrical conductivities and large activation enthalpy (∼1.32 eV) of the carbon‐bearing sample were consistent with those of dry polycrystalline olivine. This behavior implies that graphite cannot be interconnected on olivine grain boundaries, which is also supported by the large dihedral angle (98°) of the olivine/graphite system. Impedance spectroscopy measurements were performed at 3 GPa and a temperature of up to 1700 K for carbon‐coated olivine bicrystal samples to investigate the stability of graphite films on the grain boundaries of silicate minerals under upper‐mantle conditions. The conductivities rapidly or slowly dropped as a function of time and graphite film thickness during annealing at the target temperature. This phenomenon exhibits that graphite film on the olivine grain boundary is readily destroyed under upper‐mantle conditions as supported by microstructural observations on the recovered carbon‐coated olivine bicrystal samples. Higher interfacial energy and larger dihedral angle (∼98°) between graphite and olivine would not allow the maintenance of graphite film on olivine grain boundaries. The activation enthalpy for the apparent disconnection rate of a graphite film on olivine grain boundaries is close to that of carbon diffusion in olivine grain boundaries, which suggests that the disconnection of the graphite film is likely to be controlled by carbon grain boundary diffusion. Therefore, graphite is an unlikely candidate to explain the high‐conductivity anomalies revealed by MT surveys in the upper mantle.
Key Points
We report the electrical conductivity of C‐bearing and C‐coated olivine bicrystal systems
Graphite film on olivine grain boundary is not stable under upper mantle conditions
Graphite cannot explain the high‐conductivity anomalies in the lithospheric mantle
We have simultaneously measured thermal conductivity (λ) and thermal diffusivity (κ) for polycrystalline ferropericlase with different Fe contents (Fp3, Fp5, Fp10, Fp20, Fp30 and Fp50) up to 23 GPa ...and 1100 K by a pulse heating method. Experiment results reveals that even small amounts of Fe in ferropericlase can strongly reduce the thermal conductivity by several times at low temperature compared with MgO periclase. With increasing Fe in ferropericlase, both their pressure and temperature dependence decrease. λ of Fe‐bearing ferropericlase is not sensitive to temperature compared with silicates and shows a tendency of first increasing and then decreasing with increasing temperature. A universal equation was derived to estimate λ of ferropericlase with arbitrary composition under mantle conditions. The low λ values of Fe‐rich ferropericlase can explain one possible origin of ultralow velocity zones. This study suggests that Fe content in ferropericlase is an important factor controlling the cooling history of terrestrial planets.
Plain Language Summary
To understand different roles of ferropericlase in heat transport during the evolution of terrestrial planet, pressure, temperature, and compositional dependence of thermal conductivity (λ) of ferropericlase need to be well constrained. We measured thermal conductivity and thermal diffusivity of ferropericlase with different Fe content simultaneously under high pressure and high temperature. The measurement results revealed a dramatic effect of Fe: a small addition of Fe causes a decrease in the absolute value and pressure dependence of ferropericlase by several times compared with end member MgO. Fe substitution also reduces the temperature sensitivity of thermal conductivity of ferropericlase, leading to nonmonotonic temperature dependence. Based on the systematic measurement data, we derive a universal equation as an applicable tool to estimate thermal conductivity of ferropericlase with variable compositions as functions of temperature and pressure. The results suggest that Fe‐poor ferropericlase (XFe < 0.1) is a thermal conductor while Fe‐rich ferropericlase (XFe ≥ 0.3) is likely to work as a thermal insulator under terrestrial planet mantle conditions. The highly composition‐dependent λ of ferropericlase suggests that the crystallization sequence of ferropericlase may influence the magma ocean cooling of terrestrial planets and result in thermal heterogeneity in the deep mantle.
Key Points
We simultaneously measured the thermal conductivity and diffusivity of ferropericlase with different Fe contents by pulse heating method
A dramatic effect of Fe on pressure and temperature dependence of thermal conductivity of ferropericlase was found
The low λ of Fe‐rich ferropericlase at the bottom of the mantle supports patches of Fe‐rich materials as an origin for ultralow velocity zones
Chondrite-normalized abundance of fluorine (F) in the bulk silicate Earth (BSE) is more abundant than the other heavy halogens. Recent studies suggested that the common nominally anhydrous minerals ...(NAMs) in the mantle can contribute to the bulk F content. In this study, the F solubility in bridgmanite was determined by high pressure experiments on the system MgO − SiO2 + MgF2 (±Al2O3 ± H2O) at 2073 K and 25 GPa to understand the major host for F in the lower mantle. Electron microprobe measurements showed that the F concentrations in Al-bearing bridgmanite occasionally exceed 1 wt%, while the F content in bridgmanite for the Al- and H2O-free system is below the detection limit (50 ppm). The F solubility in aluminous bridgmanite is higher than that in wadsleyite. The solubility of F in bridgmanite shows a strong compositional dependence, with a positive correlation between Al3+ and F−. In H2O-free system, the solubility of F in bridgmanite increases with increasing Al content, implying a coupled substitution of F− for O2− balanced by Al3+ for Si4+. Water enhances F solubility in bridgmanite, regardless of Al content, by adding a second substitution mechanism of F in bridgmanite. In water-bearing system, we observe a coupled substitution mechanism involving a coupled incorporation of F and OH to form □Fx, OHy, O6−x−y8− (x+y≤6) octahedrons associated with Si vacancies in a SiO68− in bridgmanite. The F-storage capacity of aluminous bridgmanite (>0.8 wt%) is even higher than those of wadsleyite and ringwoodite. Al-bearing bridgmanite has sufficient capacity to store the amount of F in the lower mantle predicted from the oceanic island basalt source region. Therefore, bridgmanite can be considered as the main host of F in the lower mantle. The large storage capacity of bridgmanite for F must be taken into account when calculating the Earth's budget of halogens or global cycles of halogens in the deeper Earth.
•We investigated fluorine solubility in bridgmanite under lower mantle condition.•Al-bearing bridgmanite can contain high fluorine content exceeding 1 wt%.•Large amounts of water incorporate to bridgmanite under F-saturated condition.•Substitution of F in bridgmanite occurs in Si site.•The lower mantle is a potential budget for fluorine in the Earth's interior.
•Progenitors of the dorsal mesentery and gonad arise from ventro-medial lateral plate mesoderm (LPM).•Ventromedial LPM repetitively undergoes epithelial to mesenchymal transitioning (EMT) during ...embryogenesis.•Transition of BMP4 expression medio-lateral axis to dorso-medial axis plays key roles in the gonad and mesentery formation.•Sonic hedgehog, secreted from endoderm, maintains BMP4 expression to establish dorso-ventral patterning in medial LPM.•Ovarian surface epithelium, a progeny of coelomic epithelium, causes EMT in a strictly regulated manner.
Dorsal mesentery and gonad (ovary and testis) are formed in distinct regions of the body and have different characteristics. Recent studies using chicken embryos showed that progenitors of these two organs are derived from the coelomic lining region, a ventral part of the medial lateral plate mesoderm (M-LPM). Furthermore, both types of progenitors develop in a similar manner, concomitant with morphological changes termed the epithelial-to-mesenchymal transition (EMT). EMT processes in both dorsal mesentery and gonad formation are regulated by BMP signaling. Interestingly, EMT-based morphogenetic events occur repetitively at M-LPM specification before dorsal mesenteric and gonadal formation, at ovary formation later in embryogenesis, and even during adult ovary repair. We review recent findings related to EMT-based morphogenesis and the governing molecular mechanisms, mainly in early dorsal mesenteric and gonadal formation, as well as in their anlages and derivatives.
Reconstituting the ovarian follicle
Recent advances have enabled the generation of oocytes from pluripotent stem cells in vitro. However, these cells require a somatic environment to develop fully as ...reproductive cells. Yoshino
et al.
applied what is known about differentiation processes in vivo to determine a culture condition to differentiate embryonic stem cells into gonadal somatic cell–like cells (see the Perspective by Yang and Ng). When the embryonic stem cell–generated ovarian gonadal tissue was combined with early primordial germ cells or in vitro–derived primordial germ cell–like cells, germ cells developed into viable oocytes within the reconstituted follicles that could be fertilized and result in viable offspring. This system enables an alternative method for mouse gamete production and advances our understanding of mammalian reproduction and development.
Science
, abe0237, this issue p.
eabe0237
; see also abj8347, p.
282
Functional eggs were successfully produced in a mouse ovarian environment reconstituted from pluripotent stem cells.
INTRODUCTION
Germ cells develop in a specific environment in the reproductive organs. Throughout oogenesis, oocytes are encapsulated by somatic cells in follicle structures that provide numerous signals and components essential for key events in oocyte development, such as meiosis and growth. The interaction between the oocyte and the somatic follicular cells is regulated in a stage-dependent manner. Recently, in vitro gametogenesis, reconstitution of germ cell development in culture using pluripotent stem cells, has been developed in mammalian species, including mice and humans. In mice, functional oocytes can be produced from pluripotent stem cell–derived primordial germ cell–like cells (PGCLCs) by reaggregation with embryonic ovarian somatic cells at embryonic day 12.5. Therefore, in vitro gametogenesis is expected to be an innovative means of producing a robust number of oocytes in culture. This should be particularly useful for application to humans and endangered animals. However, the in vitro reconstitution of germ cell development is highly dependent on the somatic cell environment provided by embryonic ovarian tissue, which is difficult to obtain from mammalian species. Here, we provide a model system that reconstitutes the ovarian somatic cell environment using mouse pluripotent stem cells.
RATIONALE
During mouse development, the embryonic ovaries originate from the nascent mesoderm, followed by the intermediate mesoderm and coelomic epithelium at the genital ridge region. For the formation of embryonic ovarian somatic cells from mouse pluripotent stem cells, appropriate signals need to be provided in culture to mimic those embryonic events. Using mouse embryonic stem cells (mESCs) harboring reporter constructs that monitor the expression of key genes for each step, we set out to explore culture conditions for the recreation of the differentiation process. Faithful gene expression and functionality should be conferred in induced embryonic ovarian somatic cells under the appropriate conditions. The functionality of the induced cells should be verified by the ability to support the generation of functional oocytes capable of fertilization and subsequent development.
RESULTS
Based on reporter gene expression, we determined a series of culture conditions that recreate the differentiation process from pluripotent cells to gonadal somatic cells in a stepwise manner. Under these conditions, mESCs differentiated into fetal ovarian somatic cell–like cells (FOSLCs) expressing
Nr5a1
, a representative marker gene of gonadal somatic cells, through the nascent mesoderm, intermediate mesoderm, and coelomic epithelium states. FOSLCs exhibited a transcriptional profile and cellular composition similar to those in embryonic ovarian somatic cells at embryonic day 12.5. When FOSLCs were aggregated with PGCLCs derived from mESCs, the PGCLCs entered meiosis, and subsequent oocyte growth accompanied the development of FOSLC-derived follicles in culture. PGCLC-derived oocytes developing in the FOSLC-derived follicles were capable of fertilization and developed to live offspring. These results demonstrate the reconstitution of functional follicle structures that are fully capable of supporting oocyte production.
CONCLUSION
Our results demonstrate that functional gonadal somatic cells can be induced from mESCs through a faithful differentiation process in culture. The generated material may serve as a useful source to replace embryonic ovarian tissue for in vitro gametogenesis. Furthermore, this system contributes to a better understanding of gonadal somatic cell differentiation and the interactions between oocytes and follicular somatic cells. Because it does not require embryonic gonads, the methodology opens the possibility for application in other mammalian species with fewer ethical and technical concerns. This system will accelerate our understanding of gonadal development and provide an alternative source of gametes for research and reproduction.
Reconstitution of follicle structures, including oocytes, entirely from mouse pluripotent stem cells.
Illustrations on the left show a schematic overview of reconstitution of both FOSLCs and PGCLCs from mESCs. Oocytes in the reconstituted environment gave rise to offspring after fertilization. The right image represents fully grown cumulus-oocyte complexes derived from FOSLCs (red) and PGCLCs (blue).
Oocytes mature in a specialized fluid-filled sac, the ovarian follicle, which provides signals needed for meiosis and germ cell growth. Methods have been developed to generate functional oocytes from pluripotent stem cell–derived primordial germ cell–like cells (PGCLCs) when placed in culture with embryonic ovarian somatic cells. In this study, we developed culture conditions to recreate the stepwise differentiation process from pluripotent cells to fetal ovarian somatic cell–like cells (FOSLCs). When FOSLCs were aggregated with PGCLCs derived from mouse embryonic stem cells, the PGCLCs entered meiosis to generate functional oocytes capable of fertilization and development to live offspring. Generating functional mouse oocytes in a reconstituted ovarian environment provides a method for in vitro oocyte production and follicle generation for a better understanding of mammalian reproduction.
The group IV metals Ti and Zr are isoelectronic and crystallize in a hexagonal closed packed structure at ambient conditions. Herein, the electrical resistivity of Ti and Zr at room temperature with ...increasing pressure up to 20 GPa is reported using a 4‐wire resistivity method. The slope of their resistivity against pressure changes value at various pressures. At 10 GPa, resistivity of Ti increases with pressure whereas Zr resistivity decreases with pressure. The opposite behavior is interpreted to be due to incoherent scattering in Ti versus coherent scattering in Zr, as both undergo solid α → ω transition. By comparing the Zr result with a previous study that combined in situ X‐ray synchrotron with compressional and shear wave velocity measurement, the boundaries of the region of α → ω phase coexistence are constrained in Zr to be 4–8 GPa. The Ti result will be valuable for constraining its boundary when such in situ velocity measurement becomes available for Ti as well.
Herein, the electrical resistivity of Ti and Zr with increasing pressure (P) at room temperature is reported. At 10 GPa, the resistivity of Ti increases with P whereas Zr resistivity decreases with P even though they both belong to the same group (IV).
Adiabatic temperature profile in the mantle Katsura, Tomoo; Yoneda, Akira; Yamazaki, Daisuke ...
Physics of the earth and planetary interiors,
November 2010, 2010-11-00, Letnik:
183, Številka:
1-2
Journal Article
Recenzirano
The temperature at the 410-km discontinuity is re-evaluated by comparing the depth of the discontinuity with the olivine–wadsleyite transition pressure obtained using in situ X-ray diffraction ...experiments by Katsura et al. (2004a) and equation of state (EoS) of MgO by Tange et al. (2009) (Tange scale) and Matsui et al. (2000). The newly estimated temperature is 1830±48K, 70K higher than that by our previous estimation. The EoSes of the major mantle minerals (olivine, wadsleyite, ringwoodite and perovskite) are also recalculated using the Tange scale. The adiabatic temperature gradient is calculated using the thermal expansion coefficient obtained from these EoSes. The adiabatic temperature gradient gradually decreases with increasing depth without a phase transition, and abruptly increases in association with phase transitions. The adiabatic temperature gradients are found to be 04–0.5 and 0.3K/km in the upper and lower parts of the mantle, respectively. The temperatures at a depth of 200km, the bottom of the mantle transition zone, the top of the lower mantle and a depth of 2700km are found to be 1720±40, 2010±40, 1980±40, and 2730±50K. The mantle potential temperature is found to be 1610±35K.
