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1.
Oxygen isotope measurements of phosphate from fish teeth and bones   总被引：2，自引：0，他引：2  
In situ measurements of lunar surface brightness temperatures made as a part of the Apollo Lunar Surface Experiments Package at the Apollo 15 Hadley Rille landing site are reported. Data derived from 5 thermocouples of the Heat Flow Experiment, which are lying on or just above the surface, are used to examine the thermal properties of the upper 15 cm of the lunar regolith using eclipse and nighttime cool-down temperatures. Application of finite-difference techniques in modeling the lunar soil shows the thermocouple data are best fit by a model consisting of a low-density and low-thermal conductivity surface layer approximately 2 cm thick overlying a region increasing in conductivity and density with depth. Conductivities on the order of 1 × 10?5 W/cm-°K are postulated for the upper layer, with conductivity increasing to the order of 1 × 10?4 W/cm-°K at depths exceeding 20 cm. An increase in mean temperature with depth indicates that the ratio of radiative to conductive transfer at 350°K is 2.7 for at least the upper few centimeters of lunar soil; this value is nearly twice that measured for returned lunar fines. The thermal properties model deduced from Apollo 15 surface temperatures is consistent with earth-based microwave observations if electrical properties measured on returned lunar fines are assumed.  相似文献   

2.
The electrical conductivity of a single crystal of San Carlos olivine (Fo92, 0.16 wt.% Fe2O3) has been measured as a function of temperature and oxygen fugacity (fO2). After heating to 1338°C at fO2 = 10?12 atm., the conductivity at 950°C was 10?5 (ohm-m)?1, almost 3 orders of magnitude less than that measured in air. This decrease is due to the reduction of Fe3+ to Fe2+. Further heating to 1500°C at fO2 = 10?14 atm., decreased the electrical conductivity at 950°C to 10?6 (ohm-m)?1. When recovered at room temperature, the speciment had Fo96 composition and contained small, opaque blebs distributed throughout the crystal. Derivations of temperature profiles for the earth's mantle from conductivity-depth models must take account of the important role played by iron oxidation state in the electrical conductivity of olivine.  相似文献   

3.
Mean atomic weight profiles for the lunar mantle have been calculated from velocity-density systematic relations using lunar density and seismic velocity models. Despite large variability among the models, the calculation including Poisson's ratio yields a range of mean atomic weight values between 22 and 23 g mol?1 below 150 km. A similar calculation for the Earth's mantle produces a mean atomic weight of 21.1 ±0.4 g mol?1. This suggests that the Moon cannot be derived directly from the Earth's mantle, or that it has had a differentiation history different from the Earth's. The lunar m's require an Fe mole fraction between 0.25 and 0.33 for a pure olivine mantle, or between 0.33 and 0.45 for pure pyroxene.The present profiles are 0.5–3.0 g mol?1 higher than those calculated from lunar compositional models based on lunar rock compositions and petrology and assumed lunar histories, indicating inadequacies in either the seismic or compositional models, or in both. The mean atomic weight approach provides a method of comparing the consistency of seismic and compositional models of planetary interiors.  相似文献   

4.
Electromagnetic measurements have demonstrated that the lower continental crust has remarkable electrical anomalies of high conductivity and electrical anisotropy on a global scale (probably with some local exceptions), but their origin is a long-standing and controversial problem. Typical electrical properties of the lower continental crust include: (1) the electrical conductivity is usually 10−4 to 10−1 S/m; (2) the overlying shallow crust and underlying upper mantle are in most cases less conductive; (3) the electrical conductivity is statistically much higher in Phanerozoic than in Precambrian areas; (4) horizontal anisotropy has been resolved in many areas; and (5) in some regions there appear to be correlations between high electrical conductivity and other physical properties such as seismic reflections. The explanation based on conduction by interconnected, highly conductive phases such as fluids, melts, or graphite films in grain boundary zones has various problems in accounting for geophysically resolved electrical conductivity and other chemical and physical properties of the lower crust. The lower continental crust is dominated by mafic granulites (in particular beneath stable regions), with nominally anhydrous clinopyroxene, orthopyroxene, and plagioclase as the main assemblages, and the prevailing temperatures are mostly 700–1,000°C as estimated from xenolith data, surface heat flow, and seismic imaging. Pyroxenes have significantly higher Fe content in the lower crust than in the upper mantle (peridotites), and plagioclase has higher Na content in the lower crust than in the shallow crust (granites). Minerals in the lower continental crust generally contain trace amounts of water as H-related point defects, from less than 100 to more than 1,000 ppm H2O (by weight), with concentrations usually higher than those in the upper mantle. Observations of xenolith granulites captured by volcano-related eruptions indicate that the lower continental crust is characterized by alternating pyroxene-rich and plagioclase-rich layers. Experimental studies on typical lower crustal minerals have shown that their electrical conductivity can be significantly enhanced by the higher contents of Fe (for pyroxenes), Na (for plagioclase), and water (for all minerals) at thermodynamic conditions corresponding to the lower continental crust, e.g., to levels comparable to those measured by geophysical field surveys. Preferred orientation of hydrous plagioclase, e.g., due to ductile flow in the deep crust, and alternating mineral fabrics of pyroxene-rich and plagioclase-rich layers can lead to substantial anisotropy of electrical conductivity. Electrical conductivity properties in many regions of the lower continental crust, especially beneath stable areas, can mostly be accounted for by solid-state conduction due to the major constituents; other special, additional conduction mechanisms due to grain boundary phases are not strictly necessary.  相似文献   

5.
The lunar gravity field can be satisfactorily explained by relatively-small wavelength, density anomalies located near the lunar surface. The exceptions to this are the second degree harmonics and we postulate a dual origin for these terms; they are partly a consequence of the same near-surface anomalies as the remainder of the gravity field and partly a measure of a fossil bulge. This latter contribution represents an equilibrium condition preserved from a time when the synchronously rotating Moon was approximately 25 Earth-radii from the Earth. If this shape was acquired at some time soon after 4.0 · 109 years ago, then the rate of dissipation of tidal energy in the Earth has been much lower in the past then it is now. The implications of this on the evolution of the Earth-Moon system are briefly discussed.  相似文献   

6.
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7.
Electrical conductivity anomalies in the earth   总被引：1，自引：0，他引：1  
Anomalies of short-period geomagnetic variations have been found in various regions over the world. It is known that such anomalies arise from electromagnetic induction within an electrical conductivity anomaly or from local perturbation of induced electric currents by a conductivity anomaly. In order to investigate a regional electric state in the Earth, conductivity anomaly (CA) studies based on anomalous behaviors of geomagnetic variations have been extensively undertaken, as well as studies based on magnetotelluries in which induced currents are directly used.Some of the geomagnetic variation anomalies, however, turned out to be caused by surface conductors, such as sea water and sediments. Anomalies of this sort have been intensively studied and classified into coast, island, peninsula, and strait effects in the case of sea effects. Three-dimensional conduction or channelling of induced electric currents is sometimes observed in the cases of sediments and some crustal conductivity anomalies. However, anomalies of such surface origins often provide some information of the underground conductivity structure.Electrical conductivity anomalies can be classified into two types: anomalies originating in the crust and in the upper mantle. Many of crustal anomalies are well correlated with metamorphic belts, fracture zones, and hydrated layers, and magnetic and gravity anomalies are also often found over the conductivity anomalies. Most of mantle anomalies have been interpreted mainly in terms of high temperature and partial melting, since conductivity anomalies coincide well with anomalies in heat flow and seismic wave velocities.  相似文献   

8.
The geomagnetic lunar daily variation, and the solar daily variation, have been recently analysed by Malin. The response measurements that he obtained are used here to investigate the radial distribution of electrical conductivity in the upper mantle. It is found that these observed response measurements require low conductivity in the mantle, at least as far down as 650 km from the surface. This result is compatible with the conductivity models determined by the quiet-day daily variation, and so justifies the use of these variation fields for geomagnetic deep sounding.  相似文献   

9.
—Methods and the results of estimating the anomalies characterising the density inhomo geneities in the European-Mediterranean upper mantle are described. These anomalies were obtained by subtracting the gravity effect of a crustal density model derived from seismic velocities from the observed gravity field averaging over an area of 1°× 1°. The 3-D density model of the study region comprises two regional layers of varying thickness with lateral variation of average density the sedimentary cover and the crystalline crust. The average densities for model layers were evaluated by using a velocity/density conversion function and taking into account sediment consolidation with depth. Clear correlation between residual gravity anomalies and both velocity heterogeneities and thermal regime data of the upper mantle has been revealed. An agreement of positive anomalies over the Alps, the Adriatic plate and the Calabrian Arc with high velocity domains in the upper mantle and reduced temperatures at the subcrustal layer are caused by lithospheric "roots" and thickened lithosphere below these structures. Gravity residual lows, revealed over the Western Mediterranean Basin and Pannonian Basin, are in correspondence with both low velocities and high temperatures in the upper mantle. These anomalies are the result of the presence of asthenosphere in shallow near-Moho depths below these basins.  相似文献   

10.
A numerical method is used to investigate the effect of topographic and local thermal conductivity anomalies on near-surface heat flow for two-dimensional models. Heat flow associated with a sloping topographic structure is calculated. Also, the effects of a fault structure associated with the sloping topography are considered. Vertical and horizontal heat flow components are calculated alone; the surface of the earth as well as throughout the whole region of interest. The results indicate that surface heat flow is substantially affected by topographic relief and the horizontal heat flow component associated with topographic features can be large. Also, regional heat flow is greatly perturbed by local thermal conductivity anomalies and the effect of topographic features may be considerably modified by the subsurface structure.  相似文献   

11.
Deep in the Earth, the electrical conductivity of geological material is extremely dependent on temperature. The knowledge of temperature is thus essential for any interpretation of magnetotelluric data in projecting lithospheric structural models. The measured values of the terrestrial heat flow, radiogenic heat production and thermal conductivity of rocks allow the extrapolation of surface observations to a greater depth and the calculation of the temperature field within the lithosphere. Various methods of deep temperature calculations are presented and discussed. Characteristic geotherms are proposed for major tectonic provinces of Europe and it is shown that the existing temperatures on the crust-upper mantle boundary may vary in a broad interval of 350–1,000°C. The present work is completed with a survey of the temperature dependence of electrical conductivity for selected crustal and upper mantle rocks within the interval 200–1,000°C. It is shown how the knowledge of the temperature field can be used in the evaluation of the deep electrical conductivity pattern by converting the conductivity-versustemperature data into the conductivity-versus-depth data.  相似文献   

12.
We use thermodynamically self-consistent and hybrid methods to analyze the correlation of important physical parameters (e.g. bulk density, elastic moduli) with bulk Mg# and modal composition in mantle peridotites at upper mantle conditions. Temperature (anharmonic and anelastic), pressure and compositional derivatives for all these parameters are evaluated. The results show that the widely used correlations between Vp/Vs and Mg# in peridotites are strictly valid only for garnet-bearing assemblages at temperatures < 900 °C. The correlation breaks down when: i) spinel is the stable Al-rich phase in the assemblage and ii) when anelastic attenuation of seismic velocities becomes important (T ? 900 °C). This implies that the range of applicability of published Vp/Vs–Mg# correlations for the upper mantle is limited to a depth interval between the spinel–garnet phase transition and the 900 °C isotherm. We use numerical simulations to show that this depth interval is virtually nonexistent in lithospheres thinner than ～ 140 km and can comprise up to ～ 50% of the lithospheric mantle in thick (> 220 km) lithospheric domains. In addition, we show that for most of the upper mantle the expected Δ(Vp/Vs) values associated with compositional variations are smaller than the resolution limit of current seismological methods. All these considerations suggest that the Vp/Vs ratio is not a reliable measure of compositional variations and that for large parts of the upper mantle compositional anomalies cannot be separated from thermal anomalies on the basis of seismological studies only. We further confirm that the only reliable indicator of compositional anomalies in a peridotitic mantle is the ratio of density to shear wave velocities (ρ/Vs). Our results demonstrate that geophysical–petrological models (forward or inverse) that model these two fields (i.e. density and Vs) self-consistently within a robust thermodynamic framework are necessary for characterizing the small-scale thermal and compositional structure of the lithosphere and sublithospheric upper mantle.  相似文献   

13.
The electrical conductivities of natural pyrolitic mantle and MORB materials were measured at high pressure and temperature covering the entire lower mantle conditions up to 133 GPa and 2650 K. In contrast to the previous laboratory-based models, our data demonstrate that the conductivity of pyrolite does not increase monotonically but varies dramatically with depth in the lower mantle; it drops due to high-spin to low-spin transition of iron in both perovskite and ferropericlase in the mid-lower mantle and increases sharply across the perovskite to post-perovskite phase transition at the D″ layer. We also found that the MORB exhibits much higher conductivity than pyrolite. The depth–conductivity profile measured for pyrolite does not match the geomagnetic field data below about 1500-km depth, possibly suggesting the existence of large quantities of subducted MORB crust in the deep lower mantle. The observations of geomagnetic jerks suggest that the electrical conductivity may be laterally heterogeneous in the lowermost mantle with high anomaly underneath Africa and the Pacific, the same regions as large low shear-wave velocity provinces. Such conductivity and shear-wave speed anomalies are also possibly caused by the deep subduction and accumulation of dense MORB crust above the core–mantle boundary.  相似文献   

14.
Apollo 15 and 16 subsatellite measurements of lunar surface magnetic fields by the electron reflection method are summarized. Patches of strong surface fields ranging from less than 14° to tens of degrees in size are found distributed over the lunar surface, but in general no obvious correlation is observed between field anomalies and surface geology. In lunar mare regions a positive statistical correlation is found between the surface field strength and the geologic age of the surface as determined from crater erosion studies. However, there is a lack of correlation of surface field with impact craters in the mare, implying that mare do not have a strong large-scale uniform magnetization as might be expected from an ancient lunar dynamo. This lack of correlation also indicates that mare impact processes do not generate strong magnetization coherent over ～ 10 km scale size. In the lunar highlands fields of >100 nT are found in a region of order 10 km wide and >300 km long centered on and paralleling the long linear rille, Rima Sirsalis. These fields imply that the rille has a strong magnetization (>5 × 10?6 gauss cm3 gm?1 associated with it, either in the form of intrusive, magnetized rock or as a gap in a uniformly magnetic layer of rock. However, a survey of seven lunar farside magnetic anomalies observed by the Apollo 16 subsatellite suggests a correlation with inner ejecta material from large impact basins. The implications of these results for the origin of lunar magnetism are discussed.  相似文献   

15.
The composition, structure and evolution of the moon's interior are narrowly constrained by a large assortment of physical and chemical data. Models of the thermal evolution of the moon that fit the chronology of igneous activity on the lunar surface, the stress history of the lunar lithosphere implied by the presence of mascons, and the surface concentrations of radioactive elements, involve extensive differentiation early in lunar history. This differentiation may be the result of rapid accretion and large-scale melting or of primary chemical layering during accretion; differences in present-day temperatures for these two possibilities are significant only in the inner 1000 km of the moon and may not be resolvable. If the Apollo 15 heat-flow result is representative of the moon, the average uranium concentration in the moon is 0.05–0.08 p.p.m.Density models for the moon, including the effects of temperature and pressure, can be made to satisfy the mass and moment of inertia of the moon and the presence of a low-density crust inferred from seismic refraction studies only if the lunar mantle is chemically or mineralogically inhomogeneous. The upper mantle must exceed the density of the lower mantle at similar conditions by at least 5%. The average mantle density is that of a pyroxenite or olivine pyroxenite, though the density of the upper mantle may exceed 3.5 g/cm3. The density of the lower mantle is less than that of the combined crust and upper mantle at similar temperature and pressure, thus reinforcing arguments for early moon-wide differentiation of both major and minor elements. The suggested density inversion is gravitationally unstable and implies stresses in the mantle 2–5 times those associated with the lunar gravitational field, a difficulty that can be explained or avoided by: (1) adopting lower values for the moment of inertia and/or crustal thickness, or (2) postulating that the strength of the lower mantle increases with depth or with time, either of which is possible for certain combinations of composition and thermal evolution.A small iron-rich core in the moon cannot be excluded by the moon's mass and moment of inertia. If such a core were molten at the time lunar surface rocks acquired remanent magnetization, then thermal-history models with initially cold interiors strongly depleted in radioactive heat sources as a primary accretional feature must be excluded. Further, the presence of ～||pre|40 K in a FeFeS core could significantly alter the thermal evolution and estimated present-day temperatures of the deep lunar interior.  相似文献   

16.
Laboratory Electrical Conductivity Measurement of Mantle Minerals   总被引：4，自引：1，他引：3  
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 100 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.  相似文献   

17.
Introduction Richter and Mckenzie (1978) supposed that there is a small-scale convection system in the mantle. For a long time lots of research provides observational data to infer the possibility of a small-scale convection in the upper mantle. For example, Haxby and Weissel (1986) discussed the relationship between SEASAT map and small-scale convection. Baudry and Kroenke (1991), Maia and Diament (1991) found that the geoid and bathymetry exhibit peaks in the 400~650 km range in the Pa…  相似文献   

18.
Anomalies of electrical conductivity are considered in relation to other geophysical parameters, such as seismic wave velocity, attenuation, seismicity and density, and to tectonic features. In the case of active subduction zones there appears to be a good correlation between low conductivity and the seismic quality factor Q. Beneath western North America, a conductive zone in the uppermost mantle apparently is controlled by the thickness and severity of the low-velocity layer. Anomalies in conductivity beneath rift valleys can be related to regions of intermediate seismic P-wave velocity, typically about 7.0 km/sec, which is suggestive of partial melting of mantle material. Within the continental crust, anomalies in conductivity are not, in general, thermally controlled, but they can show correlations with seismicity, and may indicate intra-plate boundaries.  相似文献   

19.
Recent seismic and magnetotelluric experiments, aimed at better characterizing the shape and state of the subducting slab and continental crust beneath Central Mexico, exposed significant differences with conclusions of previous studies. A new slab geometry is revealed in which the subducting Cocos slab is perfectly flat between 120 to 290?km from the trench, after which it plunges into the asthenosphere at a dip angle of ~65°, in sharp contrast with the previously proposed ~20° dip angle. Seismic tomography studies show negative P-wave velocity anomalies (?2 to ?4%) in the mantle wedge beneath the Mexican Volcanic Belt, and positive anomalies (+2 to +3%) for the subducted Cocos slab. Magnetotelluric experiments exposed a very low-resistivity area (1?C10? ??m) located within the continental crust just below the Mexican Volcanic Arc. Finally, several spots of non-volcanic tremors (NVTs) have been recorded inside the continental crust above the flat-slab segment. While all these experiments provide a better picture of the subduction system beneath Central Mexico, several key processes need further investigation. In this study, we take advantage of these new observations to better constrain the thermal structure beneath Central Mexico. Two different thermal models are computed for a mantle potential temperature (T p) of 1,350 and 1,450°C, respectively. The new thermal structures are then converted into P-wave velocity anomalies and compared with the observed V p anomalies. We found that a T p of 1,450°C produced larger V p anomalies that do not fit the observations. However, using a T p of only 1,350°C, our predicted V p anomalies are positive (+2 to +3%) for the cold slab and negative (?2 to ?4%) in the mantle wedge. These V p estimates are consistent with the observed seismic tomography from P-wave arrivals, and therefore we conclude that a T p of 1,350°C is a better estimate for the mantle potential temperature beneath Central Mexico. The new thermal model, in conjunction with phase diagrams for sediments, hydrated basalt and lithospheric mantle, have been used to estimate the amount and location of fluids released from the subducting Cocos slab. Several dehydration pulses have been identified along the slab interface where most of the fluids stored in sediments and oceanic crust are released into the overlying continental crust above the flat-slab. We found a good correlation between the pattern of these dehydration pulses and the location of NVTs, suggesting that slab dehydration is responsible for triggering the tremors. We suggest that NVT bursts localized above the flat slab segment represent the manifestation of ongoing continental crust hydration and weakening, a process that has been going on since 15?Ma ago when the Cocos slab entered into a flat-slab regime. Such continuous weakening would have reduced the suction forces that kept the slab in a flat regime in the last 15?Ma, allowing the slab to easily roll back. The continuous low-resistivity region recorded beneath the volcanic front in Central Mexico might represent the evidence of slab dehydration and crust weakening over time.  相似文献   

20.
Inferences on the rheology of the mantle based on theoretical and experimental rate equations for steady state creep are discussed and compared with results from geophysical models. The radial increase of viscosity by one to three orders of magnitude across the mantle, required by inversion of postglacial rebound and geodynamic data, is confirmed by microphysical models based on the estimation of continuous and discontinuous changes of creep parameters with depth. The upper mantle (viscosity 1020–1021 Pa s) is likely to show non-Newtonian rheology (power-law creep) for average grain sizes larger than 0.1 mm as an order of magnitude. Given the variability of both grain size and stress conditions, local regions of linear rheology can be present. The rheology of transition zone and lower mantle (viscosity 1022–1024 Pa s) cannot be definitely resolved at present. Estimation of creep parameters leads to possible nonlinear or mixed rheology, if grain sizes are not lower than 0.1 mm and flow conditions can be approximated by a constant strain rate of about 10−15 s−1. This conclusion can be modified by different flow conditions (e.g. a decrease in strain rate or constant viscous dissipation). Furthermore, experiments on fine-grained garnetites and perovskite analogues have shown that diffusion creep is predominant at laboratory conditions. However, the pressure dependence of creep in these phases is unknown, and therefore direct extrapolation to lower mantle conditions is necessarily speculative. Lateral variations of viscosity, largest in the upper and lowermost mantle (up to 2–4 orders of magnitude) are predicted by models based on lateral temperature anomalies derived from seismic tomographic models.  相似文献   

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