The Anti-Tail of Comet C/1995 O1 (Hale–Bopp) in 1997/1998     

Boehnhardt  Hermann 《Earth, Moon, and Planets》2003,93(1):19-35

The large-scale post-perihelion evolution of the dust tail in Comet C/1995 O1 (Hale–Bopp) was monitored with Schmidt telescope exposures at the European Southern Observatory (ESO) La Silla in Chile. In early October 1997 signatures of a peculiar dust-tail appeared as straight, but diffuse extensions in the northern and southern coma hemisphere. With the approach of the Earth to the orbital plane of the comet a needle-like anti-tail with a similar, but much longer counterpart in the region of the normal dust tail formed. In early January 1998, i.e., close to the plane crossing, the anti-tail pointed towards position angle 190° and was at least 0.4°long, its counter part extended over more than 5° into oppositedirection. During February and March 1998 the position angles of both features increased by more than 40$°– while their appearance became shorter, wider and more diffuse again. Thepeculiar dust tail was last observed in April 1998.Modeling of the dust tail dynamics shows that the anti-tail andpeculiar tail phenomenon observed is formed by a neck-line structureof heavy dust grains released around perihelion passage. Two scenariiare introduced to explain the extension of the anti-tail feature:(1) grains ejected about 20 days post-perihelion with a initial velocity of 170 m/s in radial direction towards the Sun and(2) grains released about 60–80 days before perihelion at a speed of70m/s into the orbital plane and against the motion directionof the comet. The out-of-plane velocity component of the dust wasless than 25 m/s. Both scenarios require dust of < 0.001to be involved, i.e., grains of at least mm-size. The rather symmetric shading of the neck-line structure before and after passage of theEarth through the orbital plane of the comet in early January 1998 supports the idea that dust of both scenarii may have contributed to the phenomenon.  相似文献   

Computing costs for FEM analysis of foundation engineering problems and possible ways of increasing efficiency     

Hermann Schad 《国际地质力学数值与分析法杂志》1985,9(3):261-275

The application of finite element methods (FEM) is often restricted by the available computer capacity and the computing costs. The purpose of this paper, therefore, is, to present test results and to discuss possible ways of achieving a substantial reduction in computing times. Dividing the numerical analysis into the five levels–hardware, software, element-mesh and element-type, iteration technique (approximation of non-linearity) and constitutive model–one can show that the computing time can be reduced by about 20 per cent at every level. In particular, the use of the modified Newton-Raphson method for approximation of non-linearity and the constitutive model–apart from the theoretical concept (elastoplastic, hypoelastic etc.)—must be formulated in a manner which does not produce substantial changes in the coefficients of the material matrix caused by small stress changes. On the whole, computing time can be reduced to about 10 per cent of present computing costs. Thus, it would even be economically viable to solve three-dimensional non-linear problems by FEM.  相似文献   

Effects of mantle upwelling in a compressional setting: the Atlas Mountains of Morocco   总被引：1，自引：0，他引：1  

Antonio Teixell  Puy Ayarza  Hermann Zeyen  Manel Fernàndez  María-Luisa Arboleya 《地学学报》2005,17(5):456-461

We discuss the implications of a lithospheric model of the Moroccan Atlas Mountains based on topography, heat flow, gravity and geoid anomalies, taking into account the regional geology. The NW African cratonic lithosphere, some 160–180 km thick, thins to c. 80 km beneath the Atlas fold-thrust belts, in contrast with the shortening regime prevailing there since the early Cenozoic. This fact explains several geological and geophysical features as high topography with modest tectonic shortening, the occurrence of alkaline magmatism contemporaneous to compression, the absence of large crustal roots to support elevation, the scarce development of foreland basins, and a marked geoid high. The modelled lithosphere thinning is related to a thermal upwelling constrained between the Iberia–Africa convergent plate boundary and the Saharan craton.  相似文献   

The importance of defining chemical potentials,substitution mechanisms and solubility in trace element diffusion studies: the case of Zr and Hf in olivine     

Michael?C.?JollandsEmail author  Hugh?St.?C.?O’Neill  J?rg?Hermann 《Contributions to Mineralogy and Petrology》2014,168(3):1055

The diffusion, substitution mechanism and solubility limits of Zr and Hf in synthetic forsterite (Mg₂SiO₄) and San Carlos olivine (Mg_0.9Fe_0.1)₂SiO₄ have been investigated between 1,200 and 1,500 °C as a function of the chemical potentials of the components in the system MgO(FeO)–SiO₂–ZrO₂(HfO₂). The effect of oxygen fugacity and crystallographic orientation were also investigated. The solubilities of Zr in forsterite are highest and diffusion fastest when the coexisting three-phase source assemblage includes ZrSiO₄ (zircon) or HfSiO₄ (hafnon), and lower and slower, respectively, when the source assemblage includes MgO (periclase). This indicates that Zr and Hf substitute on the octahedral sites in olivine, charge balanced by magnesium vacancies. Diffusion is anisotropic, with rates along the crystal axes increasing in the order a < b < c. The generalized diffusion relationship as a function of chemical activity (as \(a_{{{\text{SiO}}_{2} }}\)), orientation and temperature is: \(logD_{\text{Zr}} = \frac{1}{4}loga_{{{\text{SiO}}_{2} }} + logD_{0} - \left( {\frac{{368 \pm 17\;{\text{kJ}}\;{\text{mol}}^{ - 1} }}{{2.303\;{\text{RT}}}}} \right)\) where the values of log D ₀ are ?3.8(±0.5), ?3.4(±0.5) and ?3.1(±0.5) along the a, b and c axes, respectively. Most experiments were conducted in air (fO₂ = 10^?0.68 bars), but one at fO₂ = 10^?11.2 bars at 1,400 °C shows no resolvable effect of oxygen fugacity on Zr diffusion. Hf is slightly more soluble in olivine than Zr, but diffuses slightly slower. Diffusivities of Zr in experiments in San Carlos olivine at 1,400 °C, fO₂ = 10^?6.6 bars are similar to those in forsterite at the same conditions, showing that the controls on diffusivities are adequately captured by the simple system (nominally iron-free) experiments. Diffusivities are in good agreement with those measured by Spandler and O’Neill (Contrib Miner Petrol 159:791–818, 2010) in San Carlos olivine using silicate melt as the source at 1,300 °C, and fall within the range of most measurements of Fe–Mg inter-diffusion in olivine at this temperature. Forsterite–melt partitioning experiments in the CaO–MgO–Al₂O₃–SiO₂–ZrO₂/HfO₂ show that the interface concentrations from the diffusion experiments represent true equilibrium solubilities. Another test of internal consistency is that the ratios of the interface concentrations between experiments buffered by Mg₂SiO₄ + Mg₂Si₂O₆ + ZrSiO₄ or Mg₂SiO₄ + ZrSiO₄ + ZrO₂ (high silica activity) to those buffered by Mg₂SiO₄ + MgO + ZrO₂ (low silica activity) agree well with the ratios calculated from thermodynamic data. This study highlights the importance of buffering chemical potentials in diffusion experiments to provide constraints on the interface diffusant concentrations and hence validate the assumption of interface equilibrium.  相似文献   

Amphibole and phlogopite in “hybrid” metasomatic bands monitor trace element transfer at the interface between felsic and ultramafic rocks (Eastern Alps,Italy)     

Marta Marocchi  Jörg Hermann  Peter Tropper  Giuseppe Maria Bargossi  Volkmar Mair 《Lithos》2010,117(1-4):135-148

Ultramafic rocks in contact to gneisses in the Mt. Hochwart HP mélange (Eastern Italian Alps) preserve a series of metasomatic mineral zones. A phlogopitite with minor tremolite and accessory zircon and apatite forms close to the gneiss (Zone 1). Zone 2 consists of tremolite, phlogopite and anthophyllite followed by Zone 3 with anthophyllite plus minor chlorite and talc. Zone 3 grades into an amphibole–garnet peridotite lens. This reaction zone has been generated by infiltration of hydrous fluids at T of 660–700 °C and P < 1.2 GPa, which occurred during exhumation of coupled continental crust and mantle peridotites.The reaction zone between a trace element-rich (gneiss) and a trace element depleted reservoir (peridotite) allows assessment of local trace element mobility in aqueous fluids. We present the results of in situ LA-ICP-MS trace element analysis of minerals from the reaction zone. Phlogopite is the main host for Large Ion Lithophile Elements (LILE) and contributes significantly to the Li, Ti, Nb, Ta, Pb and Sc budget. Anthophyllite is the main host for Li whereas all other trace elements including Rare Earth Elements (REE) are preferentially incorporated into tremolite. Combined with the abundance of these minerals over the contact zone, the mineral trace element data suggests that the LILE and REE were mobile on a small scale of a few centimetres only. Limited mobility of Ta, which is generally regarded as barely mobile in fluids, is documented in elevated contents of Ta in anthophyllite coupled with low Nb/Ta. The high Li content in minerals throughout the reaction zone suggests that Li was the most mobile element.The studied metasomatic zones mirror geochemical processes occurring in subduction zones at the slab–mantle interface. Phlogopite crystallization at the slab–mantle interface is an efficient mechanism to filter LILE from the aqueous fluid. Thus, such reaction zones, forming at temperatures < 660–700 °C, likely prevents that the typical slab signature with enriched LILE is transported by aqueous fluids over long distances in the mantle wedge. However, if coupled to the downgoing slab, phlogopite- and tremolite-rich rocks from such reaction zones might be able to act as carriers of trace elements and water into deeper parts of the subduction zone.  相似文献   

Fingerprinting a multistage metamorphic fluid–rock history: Evidence from grain scale Sr,O and C isotopic and trace element variations in high-grade marbles from East Antarctica     

M. Satish-Kumar  J. Hermann  T. Miyamoto  Y. Osanai 《Lithos》2010,114(1-2):217-228

Granulite grade marble layers interlayered with metapelitic granulites from Lützow Holm Bay, East Antarctica, provide insight into fluid–rock interactions during burial to and exhumation from lower crustal levels. Sub-millimeter scale strontium, oxygen and carbon isotope variations along with LA-ICPMS trace element geochemistry and mineral chemistry of texturally characterized carbonates and associated minerals helped to reconstruct the multistage metamorphic fluid history.Fluid–rock interaction dating back to prograde metamorphism are still preserved in consistently low oxygen and high strontium isotope compositions (δ¹⁸O = 12‰; ⁸⁷Sr/⁸⁶Sr_(550Ma) = 0.7248) within a massif dolomitic marble layer that escaped significant later metasomatism. In most marbles, total re-crystallization and isotopic resetting occurred in the presence of “externally derived” hyper-saline fluids that circulated along the carbonate layers during the early stages of prograde metamorphism. This leads to a trend of increased radiogenic Sr in marbles towards the value of associated metapelitic rocks that have ⁸⁷Sr/⁸⁶Sr_(550Ma) of 0.764.LA-ICPMS studies on trace elements in carbonate and associated silicate minerals at different textural settings, distinguished using cathodoluminescence microscopy, revealed multiple metasomatic events during retrograde metamorphism. Trace element contents of Ba, Sr, Pb and U gave compelling evidence for metasomatic alteration that postdate the exsolution of carbonate at ~ 600 ºC, which can be correlated with the fluids released from the crystallization of anatectic melts and pegmatites. Subsequently, meteoric fluid infiltration occurred at a shallower level of the crust and caused extreme oxygen isotopic heterogeneity (δ¹⁸O = 14.7 ~ ? 4.9‰) and imprinted high concentration of fluid mobile elements. Taken together our results emphasize the importance of integrating textural and chemical heterogeneities to reveal the multiple episodes of fluid–rock interaction processes in a dynamic continental crust, which has major implications on migration of fluids and material and help in formulating models on the geodynamic evolution of crust.  相似文献   

基于SWARM双星观测的场向电流的经度变化          下载免费PDF全文

王慧  郑志超  Hermann Luehr 《地球物理学报》2019,62(2):447-461

本文利用SWARM A和C双星高精度的矢量磁场数据研究了不同季节高纬地区场向电流(FACs)随地磁经度和地方时的变化情况.研究发现:在南北半球,FACs存在明显的经度变化,南半球FACs的变化强度大约是北半球的1.2～3.2倍.利用潮汐谱分析法我们发现FACs中占主导的非迁移潮汐分量为DW2和D0.在春秋和夏季半球,DW2波更为明显.D0波可用太阳光照的经度变化来解释,向阳侧靠近磁极的经度带比远离磁极的经度带有更强的太阳光照射.DW2波则与地磁场强度和地磁倾角等因素有关.全球电离层与热层模型计算的FACs中D0波占主导,且中性风和对流电场对D0波的贡献几乎相当.  相似文献