摘要:

The effect of pressure on the wetting behavior of liquid Fe‐Ni‐S alloy in a solid silicate matrix was investigated. Experiments were performed on a chondritic starting material in piston‐cylinder and multi‐anvil apparatuses over a pressure range of 2 to 20 GPa, at temperatures below the silicate solidus. The dihedral angle of the quenched liquid iron alloy in contact with the silicate matrix exhibited no change over the pressure interval. The average value of 108° for the dihedral angle is well above the 60° critical value for efficient percolation of iron alloy through a solid matrix, therefore core formation by percolation at upper mantle pressures is not feasible. We conclude that melting of the silicate mantle may be required for efficient planetary core

ISSN:0094-8276    

DOI:10.1029/96GL02817

年代:1996

数据来源: WILEY

摘要:

We examined shear wave splitting inSKSandSphases from 22 teleseisms at 10 temporary stations on a transect across the Northern Apenninic arc. The array, near 43°N, spans from Corsica Island across the Tyrrhenian region and the Apenninic belt to the Adriatic coast. We applied particle motion, covariance matrix decomposition, and cross correlation methods to estimate the polarization direction of the fast split‐shear wave (ϕ) and the delay time between split phases (δt). Most of the analyzed shear waves show clear evidence of splitting. The ϕ in the Adriatic foreland and in the Apennines are approximately parallel to the strike of the mountain belt (NW‐SE). The largest δt correspond to the highest elevations, suggesting that anisotropy is related to the compressional tectonics which built the Apennines, and that this tectonic compression involved at least the entire lithosphere. In the Tyrrhenian area we observe ϕ oriented about E‐W, suggesting a reorientation of the mantle fabric due to astenospheric flow, responsible for the E‐W post‐orogenic extension observed

ISSN:0094-8276    

DOI:10.1029/96GL02519

年代:1996

数据来源: WILEY

摘要:

We report the results of a first‐principles LAPW calculation of the equation‐of‐state, dynamic stability, and infrared‐active transverse optic vibrational mode frequencies of CaSiO3perovskite. A Birch‐Murnaghan fit to the computed energy‐volume relation of the cubic phase yields values of Vo=45.62 ų, Ko=227 GPa, and Ko′=4.29 for the thermally‐corrected equation‐of‐state parameters. These values are in excellent agreement with recent quasi‐hydrostatic compression data to 10 GPa, but significantly differ from values derived from higher pressure non‐hydrostatic compression data. We calculate the volume dependence of the infrared‐active TO mode frequencies using a frozen‐phonon approach. The lowest frequency ferroic mode is predicted to occur near 228 cm−1at ambient pressure and displays classic soft‐mode behavior in the tensile regime, in quantitative agreement with earlier molecular and lattice dynamical calculations, based on empirical potentials. These established a link between the low frequency ferroic mode and the thermally activated crystalline‐amorphous transition in a model CaSiO3perovskite system. Our present calculations also reveal that the static cubic perovskite structure is unstable with respect to small octahedral rotations, corresponding to Brillouin zone edge dynamical instabilities, at ambient pressure and throughout the pressure range of the lower mantle. We speculate that coupling of the low frequency ferroic mode with octahedral tilting modes and strain lower the activation energy for t

ISSN:0094-8276    

DOI:10.1029/96GL02624

年代:1996

数据来源: WILEY

摘要:

We propose a simple approach for treating non‐planar free‐surface boundary conditions that can be easily implemented into an existing finite‐difference code. The approach is based on the three‐dimensional staggered‐grid finite‐difference method (FDM, hereafter). The proposed scheme gives very good results when compared with other numerical methods. Its accuracy depends on how well the topography is matched by the 3D‐FD grid. Based on the proposed technique, we made a qualitative evaluation of the local topography effect on ground motion at KOB‐JMA site, Kobe, Japan. The local topography at JMA station amplifies the horizontal ground motion in the frequency range 1–3 Hz by a factor of less than two, and deamplifies it at frequencies around 5 Hz by

ISSN:0094-8276    

DOI:10.1029/96GL02493

年代:1996

数据来源: WILEY

摘要:

The mantle is most likely to be convecting and the resultant temperature anomalyδT(i.e. the deviation from the horizontally averaged temperature) may be the major cause of seismic velocity anomalies. Convection at high Rayleigh number is characterized by thin thermal boundary layers and plumes. Thus, it is very important to determine their thickness. A simple tomographic inversion of model thermal anomalies caused by mantle convection is presented. It clearly reveals thin plumes and improves the estimate of amplitude anomalies. This could be done by using the adaptive grid method. Our results suggest that our approach has a potential ability of getting a clearer image of mantle convection

ISSN:0094-8276    

DOI:10.1029/96GL02369

年代:1996

数据来源: WILEY

摘要:

Suspension of sediment by subterranean springs is a very common phenomena in the karstic, multibasin Lake Banyoles. The flow of groundwater into the lake introduces a suspension of marly and argilaceous materials. The temperature of this slurry is nearly constant throughout the year, around 19°C. The lake is formed by 6 basins and at the bottom of these basins there are 12 sources where the presence of suspended material signifies entering groundwater (Figure 1). Suspension of bottom lake sediment by groundwater is a feature of Lake Banyoles that has not been described elsewhere. Here we present experimental results which show that the maximum height that the sediment can rise to is affected by the formation of aggregates

ISSN:0094-8276    

DOI:10.1029/96GL02639

年代:1996

数据来源: WILEY

摘要:

Ammonia is the primary gaseous alkaline species in the atmosphere over Europe, neutralising up to 70% of the original acidity in precipitation [Buijsman et al., 1987]. It is directly involved in the conversion of sulphur dioxide and nitrogen oxides into the aerosol phase. Furthermore, ammonium contributes considerably to the nitrogen deposition and soil acidification which causes extensive changes in plant communities in many ecosystems [Moore, 1995; Rohde et al., 1995]. However, in Europe continuous long‐term measurements of atmospheric ammonia or ammonium in precipitation are lacking. Here, we present a continuous, high‐resolution record of ammonium in precipitation for the time period 1780 to 1980 deduced from an ice core recovered from a high‐altitude glacier in the Alps. The ammonium level remained constant from 1780 to 1870 and increased afterwards by a factor of three. This trend shows that ammonia emissions in Europe have substantially increased in the last 100

ISSN:0094-8276    

DOI:10.1029/96GL02615

年代:1996

数据来源: WILEY

摘要:

In this paper we present the first comparative estimations of ionic diffusion rates for sporadic meteor trains near the mesopause made using VHF radar and UV Rayleigh lidar observations. In both cases we initially assumed that the meteor trains dissipate primarily through ambipolar diffusion. For the radar data, the diffusion coefficient within the meteor train was determined from the decay rate of the backscattered power. From the the lidar data we then calculated profiles of the atmospheric temperature and density in the height range at which the meteor echoes were detected. These data were used to estimate the ambipolar diffusion coefficients that would result assuming different species of ions. Our results appear consistent with the notion that short‐lived underdense meteor trains in the height range of 85–95 km decay primarily by ambipolar diffusion. However, the diffusion coefficients obtained from the radar observations were smaller than those found from the lidar data assuming metal meteoric ions. One possible explanation could be that the radar meteor echoes resulted from ionized constituents of the atmosph

ISSN:0094-8276    

DOI:10.1029/96GL02577

年代:1996

数据来源: WILEY

摘要:

The potential impact of increases in atmospheric CO2is a topic of considerable controversy. Even though volcanic emission of CO2may be very small as compared to anthropogenic emissions, evaluation of natural degassing of CO2is important for any model of the geochemical C cycle and evolution of the Earth's atmosphere. We report here the mantle C flux in subduction zones based on He and C isotopes and CO2/³ He ratios of high‐temperature volcanic gases and medium‐ and low‐temperature fumaroles in circum‐Pacific volcanic regions. The calculated volcanic C flux of 3.1 × 1012mol/a from subduction zones is larger than the flux of 1.5 × 1012mol/a from mid‐ocean ridges, while contributions from the mantle in subduction zone is only 0.30 × 1012mol/a, equivalent to about 20% of the C flux in mid‐ocean ridges. Since the estimated mantle C flux in hot spot regions is insignificant, 0.029 × 1012mol/a, we propose that the global mantle C flux is 1.8 × 1012mol/a in total. The flux, if accumulated over 4.5 billion year of geological time, amounts to 8.3 × 1021mol which agrees well with 9 × 1021mol of the present inventory of C at the Earth's surface. This may support a continuous degassing model of C or the idea that subducted C is recycled in

ISSN:0094-8276    

DOI:10.1029/96GL02260

年代:1996

数据来源: WILEY

摘要:

Spectral irradiance measured at two stations at different altitudes (the Zugspitze‐summit at 2964 m a.s.l. and the Fraunhofer‐Institute for Atmospheric Environmental Research at 730 m a.s.l., 8 km apart in horizontal direction) was used to examine the spectral attenuation of the UV‐irradiance by a homogeneous cloud‐layer at an altitude of about 1200 m. Calculating the ratio of the global spectral irradiance beneath a homogenous cloud‐cover (22 October, 1995) to the global spectral irradiance on a cloudless day (24 October, 1995) both measured at the valley station, delivered the spectral transmittance of the cloud. By means of data measured on the Zugspitze on the same two days possible differences in the atmospheric conditions above 2964 m were eliminated and thus the influence of the cloud on the UV‐irradiance at Garmisch‐Partenkirchen was separated. The transmittance of the cloud‐layer was found to be wavelength‐dependent, ranging from 45% in the UVA to 60% in the UVB. Therefore it can be assumed that clouds are not generally “grey” (i.e. that the attenuation is dep

ISSN:0094-8276    

DOI:10.1029/96GL02614

年代:1996

数据来源: WILEY