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1.
The groundwaters of the Great Artesian Basin (Australia) have been previously shown to be accumulating in-situ production helium for groundwaters ages < 50 kyr and an external helium flux equivalent to whole crustal production for groundwater ages > 100 kyr [1,2]. New helium isotope measurements show that the observed in-situ production helium (3He/4He 1.6 × 10−8) is isotopically distinct from the crustal degassing helium flux (3He/4He 6.6 × 10−8). Furthermore, the crustal degassing helium isotope ratio is marginally in excess of the whole crustal production ratio (3He/4He= 3.5 × 10−8) and the production ratio in a variety of continental rock types. This suggests that the upper limit on volatile transport across the mantle-crust boundary beneath the (relatively) stable and “complacent” Australian continent can be characterized by a “conductive-diffusive” helium/heat flux ratio of 2.6 × 1064He atoms mW−1 s−1 which is two orders of magnitude less than the “intrusive-volcanic” ratio of 2.9 × 1084He atoms mW−1 s−1 measured at the Galapagos [16]. These results constrain the transcrustal mantle degassing fluxes of4He and40Ar to be much less than the mid-ocean ridge degassing fluxes; which are much less than the degassing of4He and40Ar from continental crust. Thus, the degassing of the Earth's interior is dominated by magmatic processes but the dominant fluxes of4He and40Ar to the atmosphere must come from the continental crust.  相似文献   

2.
Precipitation collected in continuously open containers for about a year at seven sites around the United States was analyzed for10Be,90Sr,210Pb and238U. Based on these data and long-term precipitation,90Sr and210Pb delivery patterns, the stratospheric, tropospheric and recycled10Be components in the collections were estimated and the global10Be production rate was assessed. Single station production rate estimates range from 0.52 × 106 atoms cm−2 yr−1 to 2.64 × 106 atoms cm−2 yr−1. The mean value is 1.21 × 106 atoms cm−2 yr−1 with a standard error of 0.26 × 106 atoms cm−2 yr−1.  相似文献   

3.
Beryllium isotopes (10Be and9Be) have been measured in suspended particles of < 1 mm size collected by mid-water sediment traps deployed in the eastern Pacific at MANOP sites H (6°32′N, 92°50′W, water depth 3600 m) and M (8°50′N, 104°00′W, 3100 m). For comparison, surface sediments from box cores taken from the two sites were also studied. The concentrations of10Be and9Be in sediment-trap particles are about an order of magnitude smaller than those in the bottom sediments which contain about 8 × 109 and 6 × 1016 atoms g−1 of10Be and9Be, respectively. The sediment trap samples collected from 50 m off the bottom showed significant (26–63%) contributions from resuspended bottom sediments. The10Be/9Be ratio in trap samples varies from 3 to 20 × 10−8. The variation may partly result from varied proportion of authigenic/detrital material. The fluxes of both isotopes exhibit a very strong seasonality. The fluxes of10Be into the traps at about 1500 m are estimated as 9 × 105 and 4 × 105 atoms cm−2 a−1 at sites H and M respectively. These values are to be compared with the fluxes into the sediments of 4–5 × 105 atoms cm−2 a−1 at both locations. Good correlations exist between10Be,9Be and27Al indicating that the primary carrier phase(s) for the beryllium isotopes in the water column may be aluminosilicates.  相似文献   

4.
Fluid flow from pore pressure measurements off La Palma, Canary Islands   总被引：1，自引：0，他引：1  
In situ subseafloor pore pressure results from the western flank of the island of La Palma, Canary Islands, are presented. The data obtained with a Pop Up Pore Pressure Instrument (PUPPI) provide constraints on the fluid circulation and its causes in a very special context: The sediment piles near an intraplate oceanic island built on the continental rise of the Northwest African Margin. The ambient pore pressures estimated from 2 to 4 days long record are negative in almost all cases with values, at depths of a few meters below sea floor, usually on the order of −10 to −70 Pa. Excess pore pressures develop only in the distal most areas. The permeabilities and compressibilities obtained respectively from the decay of the insertion pressures and the amplitude of the tidally induced pore pressure variations range between 2.5×10−18 and 6.6×10−16 m2 and, 6.2×10−9 and 1.5×10−7 Pa−1. According to these permeabilities fluid flow is estimated to be mostly downward and usually on the range between 0 and −0.3 mm y−1. However, from the excess pore pressure profile a complex pattern of fluid circulation is inferred where horizontal fluid motion cannot be neglected. Horizontal flow is probably controlled by significant contrasts in the permeability of the different layers. The prevailing downward fluid flow is abnormal for a classical passive margin. We thus interpret these results as the superposition to the loss of fluids by sediment compaction (on the continental rise), of a large-scale flow system stimulated by thermal buoyancy (100 km wide) related to the volcanic activity on the island of La Palma.  相似文献   

5.
Profiles of velocity turbulence in Monterey Canyon, made with a recently developed expendable probe, show the existence of a very turbulent bottom boundary layer. The turbulent flow is up to 170 m thick and has peak microscale shears of 1 m s−1 per meter. The rate of dissipation of kinetic energy, based on the observed shear variance, averaged over the depth of the turbulent boundary layer ranged from 70 to 500 × 10−6W m−3. Temperature measurements indicate that the flow was up canyon at a time of low tide. The upper bound for the vertical eddy viscosity is estimated to be17 × 10−4m2s−1 and for the vertical eddy diffusivity is estimated to be 15 × 10−4m2s−1. The large vertical scale and the intensity of the observed boundary layer suggest that the flow in Monterey Canyon may be important for the renewal and circulation of water over the continental shelf in the bay area.  相似文献   

6.
Bacterioplankton productivity, numbers, and cell specific activity were studied in nearshore waters of the southeastern U.S. continental shelf during seasons of maximum freshwater discharge. In April 1984, coastal waters were stratified from normal spring discharge and typical northeastward wind stress. In April 1985, shelf waters were vertically homogeneous due to below normal runoff and southwestward wind stress. In 1984, nearshore bacterial productivity ranged from 7.0 to 14.7 × 106 cells l−1 h−1 and midshelf rates were 40–50% less. In 1985, nearshore productivity ranged from 0.9 to 2.4 × 106 cells 1−1 h−1, and productivity was extremely patchy over the entire shelf. The cell-specific activity (thymidine incorporation per cell) suggests that although productivity was high in 1984, only a fraction of the bacterioplankton was actively growing or incorporating thymidine (0.9–2.9 × 10−21 mol cell−1 h−1). In 1985, a higher percentage of cells appeared to be active and incorporating thymidine (5–13 × 10−21mol cell−1h−1) even though productivity was low. Hydrographic conditions along the southeastern coastline may have had a significant impact on the overall community structure and carbon flow through the microbial food web. When coastal waters were stratified in 1984, bacterial biomass was a significant percentage (35–320%) of the phytoplankton biomass. During vertically homogeneous conditions of 1985, bacterial production and biomass were a small percentage (2–13%) of the phytoplankton production and biomass across the shelf. The interannual variation in the microbial food web was attributed to the interannual variability of the southeastern U.S. hydrology due to changes in freshwater discharge and wind direction and intensity. The ecological implications of these results extend to the potential impact of seasonal microbial food webs on nearshore allochothonous and autochothonous organics before removal from the southeastern U.S. coastline.  相似文献   

7.
Gas concentrations and isotopic compositions of water have been measured in hydrothermal waters from 13°N on the East Pacific Rise. In the most Mg-depleted samples ( 5 × 10−3 moles/kg) the gas concentrations are: 3–4.5 × 10−5 cm3 STP/kg helium, 0.62–1.24 cm3 STP/kg CH4, 10.80–16.71 × 10−3 moles/kg CO2. The samples contain large quantities (95–126 cm3/kg) of H2 and some carbon monoxide (0.26–0.36 cm3/kg) which result from reaction with the titanium sampling bottles. δ13C in methane and CO2 (−16.6 to −19.5 and −4.1 to −5.5 respectively) indicate temperatures between 475 and 550°C, whereas δ13CCO is compatible with formation by reduction of CO2 on Ti at 350°C close to the sampling temperature.3He/4He are very homogeneous at (7.5 ± 0.1)RA(3He/4He = 1.0 × 10−5) and very similar to already published data as well as CH4/3He ratios between 1.4 and 2.1 × 106.18O and D in water show enrichments from 0.39 to 0.69‰ and from 0.62 to 1.49‰ respectively. These values correspond to W/R ratios of 0.4–7. The distinct18O enrichments indicate that the isotopic composition of the oceans is not completely buffered by the hydrothermal circulations. The3He-enthalpy relationship is discussed in terms of both hydrothermal heat flux and3He mantle flux.  相似文献   

8.
The 1975 sub-terminal activity was characterised by low effusion rates (0.3–0.5 m3 s−1) and the formation of a compound lava field composed of many thousands of flow units. Several boccas were active simultaneously and effusion rates from individual boccas varied from about 10−4 to 0.25 m3s−1. The morphology of lava flows was determined by effusion rate (E): aa flows with well-developed channels and levees formed when E > 2 × 10−3 m3 s−1, small pahoehoe flows formed when 2 × 10−3 m3 s−1 >E > 5 > 10−4 m3 s−1 and pahoehoe toes formed when E < 5 × 10−4 m3 s−1. There was very little variation with time in the effusion temperature, composition or phenocryst content of the lava.New boccas were commonly formed at the fronts of mature lava flows which had either ceased to flow or were moving slowly. These secondary boccas developed when fluid lava in the interior of mature aa flows either found a weakness in the flow front or was exposed by avalanching of the moving flow front. The resulting release of fluid lava was accompanied by either partial drainage of the mature flow or by the formation of a lava tube in the parent flow. The temperature of the lava forming the new bocca decreased with increasing distance from the source bocca (0.035°C m−1). It is demonstrated from the rate of temperature decrease and from theoretical considerations that many of the Etna lavas still contained a substantial proportion of uncooled material in their interior as they came to rest. The formation of secondary boccas is postulated to be one reason why direct measurements of effusion rates tend, in general, to overestimate the total effusion rates of sub-terminal Etna lava fields.  相似文献   

9.
We have installed a laser strainmeter system in a deep tunnel about 1,000 m below the ground surface at Kamioka, Gifu, Japan. The system consists of three types of independent interferometers: (1) an EW linear strainmeter of the Michelson type with unequal arms, (2) an NS-EW differential strainmeter of the Michelson type with equal arms and (3) a NS absolute strainmeter of the Fabry–Perot type. These are configured in L-shaped vacuum pipes, each of which has a length of 100 m. (1) and (2) are highly sensitive (order of 10−13 strain) and have wide dynamical range (10−13–10−6). Observations with strainmeters (1) and (2) started on June 11, 2003. (3) is a new device for absolute-length measurements of the order of 10−9 of a long-baseline (100 m) Fabry–Perot cavity by the use of phase-modulated light. This third strainmeter will be ready for operation before the end of 2004. The laser source of strainmeters (1) and (2) is a frequency-doubled YAG laser with a wavelength of 532 nm. The laser frequency is locked onto an iodine absorption line and a stability of 2 × 10−13 is attained. The light paths of the laser strainmeter system are enclosed in SUS304 stainless steel pipes. The inside pressure is kept to be 10−4 Pa. Consequently, quantitative measurement of crustal strains of the order of 10−13 can be attained by employing the laser strainmeter system of (1) and (2) at Kamioka. This resolving power corresponds to that of a superconducting gravimeter. Using the laser strainmeter system, we expect to determine parameters related to fluid core resonance, core modes and core undertone as well as other geodynamic signals such as slow strain changes caused by silent earthquakes or slow earthquakes.  相似文献   

10.
Within the scope of the licensing procedure for 12 interim storage facilities for spent nuclear fuel at nuclear power plants (NPP) and one for the dismantling of a NPP in Germany, site-specific expert reports were carried out to determine seismo-engineering design parameters. Our work was based on the German nuclear safety standard KTA 2201 part 1 “Design of Nuclear Power Plants against Seismic Events” which requires the derivation of the design earthquake in terms of (macroseismic) intensity. The intensity of the particular design earthquake was evaluated by the deterministic method stipulated in KTA 2201 part 1. According to the current state-of-the-art of science and technology supplementary to the deterministic approach, probabilistic evaluations were done setting a probability of exceedance for the occurrence of the design earthquake of 10−5 year−1. Geological development and neotectonic conditions of the site surrounding areas were included in the evaluations. For 13 interim storage facility sites, soil-dependent design spectra and corresponding strong-motion duration values were determined.For sites with low seismic hazard site-specific design spectra from the literature were used. In the case of sites with higher seismic hazard (design earthquakeVII 1/2 MSK) strong-motion registrations representative for the site were selected from world wide data and subsequently evaluated to derive site-specific design spectra.  相似文献   

11.
Water samples were collected from Baffin Bay and surrounding areas in order to evaluate this region as a potential source of Nd from old continental material to Atlantic water. The isotopic data ranged from εNd(0) = −9.0 to −26 with most of the data around εNd(0) = −20 compared with values of North Atlantic Deep Water (NADW) with εNd(0) = −13.5. The concentration of Nd in Baffin Bay waters was as high as 6 × 10−12 g/g compared with 2.5 × 10−12 g/g for NADW. The combination of low εNd and high Nd concentration indicates that Baffin Bay may be a significant source of Nd from very old crustal material. A simple box model was used to evaluate the contribution to the Nd budget of NADW and it was concluded that a substantial fraction of the Nd from ancient crustal sources that is required to maintain the isotopic composition of NADW could be supplied by Baffin Bay outflow.  相似文献   

12.
Fluid inclusion leachates obtained from vug and vein quartz samples from an Archean (3.23 Ga) Fe-oxide hydrothermal deposit in the west-central part of the Barberton greenstone belt, South Africa, were analyzed by ion chromatography for chloride, bromide, and iodide. The deposit, known as the ironstone pods, formed by seafloor hydrothermal activity and fluid discharge. Quartz is dominated by type I liquid-vapor, aqueous inclusions with a bimodal salinity distribution (0–0.25 MCl− and 0.9–1.8 MCl−). Bulk analytical salinities range from 0.45 to 0.99 MCl− represent averages of type I inclusions. Bulk fluid inclusion bromide and iodide concentrations are 1.44–3.32 mM and 0.01–0.12 mM, respectively. For comparison, modern seawater has halogen contents of 590 mM chloride, 0.9 mM bromide, and 0.5 μM total iodine. In the fluids from the ironstone pods, bromide and iodide are enriched relative to chloride, when compared with modern seawater.Approximate Br−Cl− and I−Cl− ratios of 3.2 Ga Barberton seawater are 2.5 × 10−3 and 40 × 10−6, respectively. Dispersion to higher values was caused principally by reaction with organic sediments whose trends are similar to those seen for modern vent fluids at unsedimented and sedimented ridges, relative to modern seawater. These halide ratios are greater than those of modern seawater, suggesting a change in the halide ratios of seawater over geological time. The analytical data are consistent with a model in which marine organic sedimentation has fractionated bromine and iodine out of seawater relative to chloride, thereby causing the halide ratios of seawater to decrease from high early and mid-Archean values towards their present day values.  相似文献   

13.
Pressure–volume–temperature relations have been measured to 32 GPa and 2073 K for natural magnesite (Mg0.975Fe0.015Mn0.006Ca0.004CO3) using synchrotron X-ray diffraction with a multianvil apparatus at the SPring-8 facility. A least-squares fit of the room-temperature compression data to a third-order Birch–Murnaghan equation of state (EOS) yielded K0 = 97.1 ± 0.5 GPa and K′ = 5.44 ± 0.07, with fixed V0 = 279.55 ± 0.02 Å3. Further analysis of the high-temperature compression data yielded the temperature derivative of the bulk modulus (∂KT/∂T)P = −0.013 ± 0.001 GPa/K and zero-pressure thermal expansion α = a0 + a1T with a0 = 4.03 (7) × 10−5 K−1 and a1 = 0.49 (10) × 10−8 K−2. The Anderson–Grüneisen parameter is estimated to be δT = 3.3. The analysis of axial compressibility and thermal expansivity indicates that the c-axis is over three times more compressible (KTc = 47 ± 1 GPa) than the a-axis (KTc = 157 ± 1 GPa), whereas the thermal expansion of the c-axis (a0 = 6.8 (2) × 10−5 K−1 and a1 = 2.2 (4) × 10−8 K−2) is greater than that of the a-axis (a0 = 2.7 (4) × 10−5 K−1 and a1 = −0.2 (2) × 10−8 K−2). The present thermal EOS enables us to accurately calculate the density of magnesite to the deep mantle conditions. Decarbonation of a subducting oceanic crust containing 2 wt.% magnesite would result in a 0.6% density reduction at 30 GPa and 1273 K. Using the new EOS parameters we performed thermodynamic calculations for magnesite decarbonation reactions at pressures to 20 GPa. We also estimated stability of magnesite-bearing assemblages in the lower mantle.  相似文献   

14.
Propagation of electromagnetic (EM) waves from an earthquake focus in the conductive Earth has been investigated using 1/1,000,000 scaling models taking earth-ionosphere and ocean-Moho plane parallel-plate waveguides into account. Microwaves at a frequency, ωm, a million times higher than that of seismic EM signal (SEMS), ω, were generated at the model focus. They are propagated in a salt solution modeling the earth's crust and reflected by ocean, fault planes, ionosphere and Moho plane all made by aluminum. Distribution of EM power was mapped by scanning a detector antenna over the model Earth's surface. The skin depth, δ, calculated by the exact skin depth equation, 1/δ=ω(μ/2)1/2 [(1+(1/ωρ)2)1/2 −1]1/2 where dielectric constant, and permeability, μ are the same but resistivity, ρ, 10−6 times smaller than that of Earth, gave 10−6 times small skin depth validating the model scaling index. Images for evanescent and wave-ripple standing waves disturbed by normal, strike-slip and dip-slip conductive fault planes have been obtained using an aluminum plate. The co-circular contour map above the epicenter due to evanescence was pushed to the north east direction from the epicenter by the presence of ocean for the Loma Prieta earthquake, while to north direction for the Kobe earthquake. The intensity of EM ULF emissions for the Loma Prieta earthquake is discussed quantitatively.  相似文献   

15.
Oxygen self diffusion rates were determined in quartz samples exchanged with18O-enriched CO2 between 745 and 900°C and various pressures, and the diffusion profiles were measured using an ion microprobe. The activation energy (Q) and preexponential factor (D0) at P(CO2) = P(tot) = 100 bar, for diffusion parallel to the c-axis are 159 ( ± 13) kJ/g atom and 2.10 (+0.75/ −0.55) × 10−8 cm2/s. This rate is approximately 100 times slower than that obtained from hydrothermal experiments and 100 times faster than a previous 1-bar quartz-O2 exchange experiment. The oxygen diffusion rate measured at 0.6 bar, 888°C, and at 900°C in vacuum is in agreement with the previous 1-bar exchange experiments with18O2. The effect of higher CO2 pressures is small. At 900°C, the diffusion rate exchanged with CO2 is = 2.35 × 10−15 cm2/s at 100 bar, 2.24 × 10−15 cm2/s at 3.45 kbar and 8.13 × 10−15 cm2/s at 7.2 kbar.There is probably a diffusing species, other than oxygen, that enhances the oxygen diffusion rate in these quartz-CO2 systems, relative to that occurring at very low pressures or in a vacuum. The effect of this diffusing species, however, is not as strong as that associated with H2O. Preserved oxygen isotope fractionations between coexisting minerals in a slowly cooled, high-grade metamorphic terrane will vary depending upon whether a water-rich phase was present or not. Closure temperatures will be approximately 100°C higher in rocks where no water-rich phase was present during cooling. The measured fractionations between coexisting minerals in metamorphic rocks may potentially be used as a sensor of water presence during retrogression.  相似文献   

16.
We analyze observations from eight GPS campaigns carried out between 1997 and 2005 on a network of 13 sites in the Suez–Sinai area, where separation between the African and the Arabian plates takes place. This is the key area to understand if and in which way Sinai behaves like a sub-plate of the African plate and the role played by seismic and geodetic (long-term) deformation release.Our analysis shows that, on average, the Suez–Sinai area motion, in terms of ITRF00 velocities, matches the African plate motion defined by the NNR-NUVEL-1A model.The horizontal principal strain rate axes estimated separately in the Gulf of Suez area and in the northern Sinai vary from compression across the Gulf (−2.2 ± 1.2) × 10−8 year−1 to NE extension (1.0 ± 1.5) × 10−8 year−1 in the North, showing the presence of two distinct domains, so that in our opinion Sinai cannot be considered simply a unique rigid block.The analysis of GPS baseline length variations shows short-term deformations across the Gulf of Suez, reaching up a maximum value of more than 1 cm in 8 years.Since current geodynamical models do not predict significant tectonic deformation in this area, we work under the hypothesis that a contribute may be expected by post-seismic relaxation effects. Under this hypothesis, we compare the baselines length variations with the post-seismic relaxation field associated with five major local earthquakes occurred in the area, testing two different viscoelastic models. Our results show that the detected short-term deformations are better modeled for viscosity values of 1018 Pa s in the lower crust and 1020 Pa s in the asthenosphere. However, since the modeled post-seismic effect results modest and a certain amount of the detected deformation is not accounted for, we think that an improved modeling should take into account the lateral heterogeneities of crust and upper mantle structures.  相似文献   

17.
All previous accounts of the spiral patterns at the Martian poles emphasize that the north polar spiral is centered about the geographic pole, whereas that of the south polar region is off-set by about 4°. This paper demonstrates that the patterns near both poles are centered on topographic highs rather than the spin poles themselves. This is circumstantial evidence in favour of the relatively unexplored mechanism of radial outflow of viscous rock by gravity spreading.The hypothesis developed here is that the spiral patterns are essentially due to crevasse patterns formed perpendicular to flow lines which are perturbed by Coriolis forces. In order to account for a crevasse pattern that has a form concave to the east the angular deflection of an hypothetical ice flow emanating from the topographic high centered about the geographical north pole, must be about 40° or 0.7 radians in a westward direction at 85°N latitude.The polar cap rock has previously been assumed to consist mainly of either frozen carbondioxide or water ice. Corresponding viscosities (at 190 K) allow for the occurrence of radial outflow or gravity driven tectonics at a maximum rate of 1 cm a−1, but the flow pattern remains unaffected by Coriolis forces.The spiral patterns of the Martian poles can be explained if the flowing mass has an occasional effective kinematic viscosity as low as about 7 × 106 m2 s−1, because gravity tectonics will then be deflected by Coriolis forces resulting in appropriately curved flowlines. A tensile fracture pattern, resembling an anticlockwise spiral pattern perpendicular to the clockwise deflected flowlines may subsequently form by local brittle failure.The occasional kinematic viscosity 7 × 106 m2 s−1 would cause flow rates of 0.2 m s−1 along the slopes of the topographic highs. This velocity and the corresponding viscosity is tentatively thought to be possible when thermal and pressure runaway occurs in the polar layered deposits. This would mean glacier surges on the Martian poles are two orders of magnitude faster than those hitherto observed on Earth.  相似文献   

18.
K lauea Volcano, Hawai‘i, currently hosts the longest running SO2 emission-rate data set on the planet, starting with initial surveys done in 1975 by Stoiber and his colleagues. The 17.5-year record of summit emissions, starting in 1979, shows the effects of summit and east rift eruptive processes, which define seven distinctly different periods of SO2 release. Summit emissions jumped nearly 40% with the onset (3 January 1983) of the Pu‘u ‘ ‘ -K paianaha eruption on the east rift zone (ERZ). Summit SO2 emissions from K lauea showed a strong positive correlation with short-period, shallow, caldera events, rather than with long-period seismicity as in more silicious systems. This correlation suggests a maturation process in the summit magma-transport system from 1986 through 1993. During a steady-state throughput-equilibrium interval of the summit magma reservoir, integration of summit-caldera and ERZ SO2 emissions reveals an undegassed volume rate of effusion of 2.1×105 m3/d. This value corroborates the volume-rate determined by geophysical methods, demonstrating that, for K lauea, SO2 emission rates can be used to monitor effusion rate, supporting and supplementing other, more established geophysical methods. For the 17.5 years of continuous emission rate records at K lauea, the volcano has released 9.7×106 t (metric tonnes) of SO2, 1.7×106 t from the summit and 8.0×106 t from the east rift zone. On an annual basis, the average SO2 release from K lauea is 4.6×105 t/y, compared to the global annual volcanic emission rate of 1.2×107 t/y.  相似文献   

19.
LaCe ages are reported for two sets of Finnish pegmatites, Lövböle and Mustikkamäki, and for an Amiˆtsoq gneiss, Greenland. When λβ138La value (2.29 × 10−12 yr−1) obtained by radioactivity measurement [1] is used for the chronological calculation, the LaCe ages (2129, 2325, 3271 Myr) evaluated for these rocks are 18–35% older than the SmNd ages for the same samples. To make the LaCe age fit to the SmNd age for the same sample, a new value of (2.77 ± 0.21) × 10−12 yr−1 is evaluated for λβ138La. In this calculation, the LaCe and SmNd ages reported for a Bushveld gabbro [2] have been also taken into account together with those for the Lövböle pegmatite and the Mustikkamäki pegmatite, while the Amiˆtsoq gneiss (GGU110999) has been omitted because of the complicated thermal history of this sample.  相似文献   

20.
Permanent magnetism and conventional dynamo theory are possible but problematic explanations for the magnitude of the Mercurian magnetic field. A new model is proposed in which thermoelectric currents driven by temperature differences at a bumpy core-mantle boundary are responsible for the (unobserved) toroidal field, and the helicity of convective motions in a thin outer core (thickness 102 km) induces the observed poloidal field from the toroidal field. The observed field of 3 × 10−7 T can be reproduced provided the electrical conductivity of Mercury's semiconducting mantle approaches 103 Ω−1 m−1. This model may be testable by future missions to Mercury because it predicts a more complicated field geometry than conventional dynamo theories. However, it is argued that polar wander may cause the core-mantle topography to migrate so that some aspects of the rotational symmetry may be reflected in the observed field.  相似文献   

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