Abstract– We present the results of a noble gas (He, Ne, Ar) and cosmogenic radionuclide (10Be, 26Al, 36Cl) analysis of two chondritic fragments (#A100, L4 and #25, H5) found in the Almahata Sitta strewn field in Sudan. We confirm their earlier attribution to the same fall as the ureilites dominating the strewn field, based on the following findings: (1) both chondrite samples indicate a preatmospheric radius of approximately 300 g cm?2, consistent with the preatmospheric size of asteroid 2008 TC3 that produced the Almahata Sitta strewn field; (2) both have, within error, a 21Ne/26Al‐based cosmic ray exposure age of approximately 20 Ma, identical to the reported ages of Almahata Sitta ureilites; (3) both exhibit hints of ureilitic Ar in the trapped component. We discuss a possible earlier irradiation phase for the two fragments of approximately 10–20 Ma, visible only in cosmogenic 38Ar. We also discuss the approximately 3.8 Ga (4He) and approximately 4.6 Ga (40Ar) gas retention ages, measured in both chondritic fragments. These imply that the two chondrite fragments were incorporated into the ureilite host early in solar system evolution, and that the parent asteroid from which 2008 TC3 is derived has not experienced a large break‐up event in the last 3.8 Ga. 相似文献
In this paper, we present a fast streamline-based numerical method for the two-phase flow equations in high-rate flooding scenarios for incompressible fluids in heterogeneous and anisotropic porous media. A fractional flow formulation is adopted and a discontinuous Galerkin method (DG) is employed to solve the pressure equation. Capillary effects can be neglected in high-rate flooding scenarios. This allows us to present an improved streamline approach in combination with the one-dimensional front tracking method to solve the transport equation. To handle the high computational costs of the DG approximation, domain decomposition is applied combined with an algebraic multigrid preconditioner to solve the linear system. Special care at the interior interfaces is required and the streamline tracer has to include a dynamic communication strategy. The method is validated in various two- and three-dimensional tests, where comparisons of the solutions in terms of approximation of flow front propagation with standard fully implicit finite-volume methods are provided. 相似文献
Abstract Evidence from radio polarization measurements is reviewed that indicates that most galactic magnetic field structures fall into one of two categories: axisymmetric spiral and bisymmetric spiral. The resultant challenges to dynamo theorists is stated. Estimates of the magnetic field strengths based on equipartition of field and cosmic ray energies are given, but deviations from equipartition are inferred. Possible goals for future research are suggested. 相似文献
The combined U-series/electron spin resonance (ESR) dating method was applied to nine teeth from two Early Pleistocene archaeological sites located in the Orce area (Guadix-Baza Basin, Southern Spain): Fuente Nueva-3 (FN-3) and Barranco León (BL). The combination of biostratigraphy and magnetostratigraphy places both sites between the Olduvai and Jaramillo subchrons (1.78–1.07 Ma).Our results highlight the difficulty of dating such old sites and point out the limits of the combined U-series/ESR dating method based on the US model. We identified several sources of uncertainties that may lead to inaccurate age estimates. Seven samples could not be dated because the dental tissues had (230Th/234U) activity ratios higher than equilibrium, indicating that uranium had probably leached from these tissues. It was however possible to calculate numerical estimates for two of the teeth, both from FN-3. One yielded a Middle Pleistocene age that seems to be strongly underestimated; the other provided an age of 1.19 ± 0.21 Ma, in agreement with data obtained from independent methods. The latter result gives encouragement that there are samples that can be used for routine dating of old sites. 相似文献
Cadmium is a highly volatile element and its abundance in meteorites may help better understand volatility‐controlled processes in the solar nebula and on meteorite parent bodies. The large thermal neutron capture cross section of 113Cd suggests that Cd isotopes might be well suited to quantify neutron fluences in extraterrestrial materials. The aims of this study were (1) to evaluate the range and magnitude of Cd concentrations in magmatic iron meteorites, and (2) to assess the potential of Cd isotopes as a neutron dosimeter for iron meteorites. Our new Cd concentration data determined by isotope dilution demonstrate that Cd concentrations in iron meteorites are significantly lower than in some previous studies. In contrast to large systematic variations in the concentration of moderately volatile elements like Ga and Ge, there is neither systematic variation in Cd concentration amongst troilites, nor amongst metal phases of different iron meteorite groups. Instead, Cd is strongly depleted in all iron meteorite groups, implying that the parent bodies accreted well above the condensation temperature of Cd (i.e., ≈650 K) and thus incorporated only minimal amounts of highly volatile elements. No Cd isotope anomalies were found, whereas Pt and W isotope anomalies for the same iron meteorite samples indicate a significant fluence of epithermal and higher energetic neutrons. This observation demonstrates that owing to the high Fe concentrations in iron meteorites, neutron capture mainly occurs at epithermal and higher energies. The combined Cd‐Pt‐W isotope results from this study thus demonstrate that the relative magnitude of neutron capture‐induced isotope anomalies is strongly affected by the chemical composition of the irradiated material. The resulting low fluence of thermal neutrons in iron meteorites and their very low Cd concentrations make Cd isotopes unsuitable as a neutron dosimeter for iron meteorites. 相似文献
Some of the distinct noble gas “components” in meteorites represent a record of processes during and even before solar system
formation. This record is difficult to interpret. Often, one of the major problems is to recognize whether a certain noble
gas elemental and isotopic pattern has been established in a presolar epoch, later in the solar accretion disk, during meteorite
parent body formation or finally as a result of metamorphism on a parent body. It would also appear that noble gases are a
preferred tool to deduce the types of matter from which the Earth and other planets accreted—if the respective parent materials
are present in our extraterrestrial sample collections at all. However, also this issue is unsettled. Noble gas isotopes originating
from the decay of radioactive precursors allow us to study the early and later degassing history of terrestrial planets, although
the interpretation often remains model-dependent. This contribution briefly reviews some of the fundamental aspects of the
noble gas record in meteorites and planets. 相似文献
We use a lattice vibrational technique to derive thermophysical and thermochemical properties of the pure elements aluminum
and iron in pressure–temperature space. This semi-empirical technique is based on either the Mie–Grüneisen–Debye (MGD) approach
or an extension of Kieffer’s model to incorporate details of the phonon spectrum. It includes treatment of intrinsic anharmonicity,
electronic effects based on the free electron gas model, and magnetic effects based on the Calphad approach. We show that
Keane’s equation of state for the static lattice is better suitable to represent thermodynamic data for aluminum from 1 bar
to pressures in the multi-megabar region relative to Vinet’s universal and the Birch–Murnaghan equation of state. It appears
that the MGD and Mie–Grüneisen–Kieffer approach produce similar results, but that the last one better represents heat capacity
below room temperature. For iron we show that the high temperature behavior of thermal expansivity can be explained within
the Calphad approach by a pressure-dependent Curie temperature with a slope between –1 and 0 K/GPa. 相似文献
The complex microstructure of kelyphitic rims around garnet in lower crustal garnet granulite xenoliths from the Bakony–Balaton
Highland Volcanic Field, Central Pannonian Basin has been studied in order to identify controls on garnet breakdown. Symplectites
comprised of a vermicular intergrowth of submicron sized anorthite, orthopyroxene and spinel replace garnet at a sharp reaction
front. Based on element distribution maps the transformation of garnet to symplectite is isochemical. Phase diagram calculations
indicate that this reaction was induced by a pressure decrease and/or a temperature increase. In site-specific TEM foils prepared
by focused ion beam technique and oriented parallel and perpendicular to the reaction front 200 nm wide rods of anorthite
and 20 nm wide rods of spinel are identified. The rods are oriented approximately perpendicular to the replacement front and
are embedded in an orthopyroxene matrix. The regular spacing of the symplectite phases along the reaction front suggests that
their growth is controlled by diffusion. The kinetics of symplectite formation has been modelled based on irreversible thermodynamics.
During interaction of the xenolith with the host basalt the microstructure and chemistry of the An–Opx–Spl symplectite was
significantly modified and it was partially replaced by an olivine bearing symplectite. In contrast to primary symplectite
formation, these processes were metasomatic in nature including addition of sodium, titanium and some trace elements from
the basaltic melt and can clearly be discerned from the garnet breakdown. Based on these observations it is inferred that
symplectite formation took place within the deep crust during the extension of the Pannonian Basin between 15 and 30 km depth
at high temperature (850–1,050°C) prior to the volcanic transport to the surface. 相似文献
The Canadian Shield is a large area of exposed bedrock that covers a significant portion of Canada. The focus of this research is on the Canadian Shield in northeastern regions of the Canadian Prairie provinces, the Northwest Territories, and northwestern Ontario. Observations of cloud-to-ground lightning flashes show a distinct reduction in lightning density when crossing the southern boundary of the Canadian Shield. Statistical tests were used to determine whether there was a statistically significant change between the average cloud-to-ground lightning density gradient near the Canadian Shield and away from it. Most regions had a statistically significant change at the 95% confidence level across the boundary. However, regions that contained large lakes or significant topography did not, suggesting that the large open water and rough topography have a greater effect than the Canadian Shield.
Three reasons for the distinct change across the Canadian Shield were explored: less lightning is detected, less lightning occurs with the same number of thunderstorms, and less lightning occurs because fewer thunderstorms occur. Some evidence suggested that less lightning is detected because the Canadian Shield has lower electrical conductivity. The low conductivity distorts the lightning waveform and contributes to higher errors and lower detection efficiency. There is also some evidence that a sharp change in land cover from lusher, higher transpiring forest to sparser lower transpiring forest could be reducing the instability causing fewer, weaker thunderstorms. 相似文献