Mössbauer study of synthetic Mg0.22Fe0.78SiO3 clinopyroxene     

S. G. Eeckhout  E. De Grave  C. A. McCammon  R. Vochten 《Mineralogy and Petrology》2001,73(4):235-245

Summary  Transmission M?ssbauer spectra of synthetic Ca-free P2₁/c Mg_0.22Fe_0.78SiO₃ clinopyroxene were collected at temperatures in the range 4.2 to 745 K and in an external magnetic field of 60 kOe at 180 K. The magnetic order-disorder transition temperature was determined by M?ssbauer thermoscanning to be 21 ± 3 K. Above this temperature, all M?ssbauer spectra consist of a superposition of two doublets, respectively produced by Fe²⁺ ions at an almost regular octahedral M1 site and at a more distorted octahedral M2 site. The temperature variation of the Fe²⁺ center shifts were analyzed using the Debye model for the lattice vibrations. The characteristic M?ssbauer temperatures were found to be 356 K ± 35 K for M1 and 333 K ± 25 K for M2. From the external field (60 kOe) M?ssbauer spectrum recorded at 180 K, the principal component V _zz of the electric field gradient (EFG) was determined to be positive for both sites but precise values for the magnitudes of the asymmetry parameters η of the EFG could not be determined. The temperature variations of the M1 and M2 quadrupole splittings ΔE _Q(T ) are consistent with the higher distortion of the M2 octahedra. Using the crystal-field model to interpret ΔE _Q(T ), the energy gaps δ₁ and δ₂ of the first excited electronic states within the ⁵D orbital term were estimated to be 410 ± 50 cm⁻¹ and 730 ± 50 cm⁻¹ for M1, and δ₁ = 1050 ± 75 cm⁻¹ for M2. Received May 29, 2000;/revised version accepted July 13, 2001  相似文献   

Prediction of seasonal water-table fluctuations in La Pampa and Buenos Aires, Argentina     

Raúl Tanco  Eduardo Kruse 《Hydrogeology Journal》2001,9(4):339-347

The fluctuation of the water table east of La Pampa province and northwest of Buenos Aires province, Argentina, influences agricultural production in the region because it is closely related to the alternation of dry and wet periods. Sea-surface temperature (SST) anomalies have been used as predictors to forecast atmospheric variables in different regions of the world. The objective of this work is to present a simple model to forecast seasonal rainfall using SST distribution in the Pacific Ocean as a predictor. Once the relationship between precipitation and water-table fluctuations was established, a methodology for the prediction of water-table fluctuations was developed. A good agreement between observed and predicted water-table fluctuations was found when estimating water-table fluctuations in the summer and autumn seasons. Electronic Publication  相似文献   

Grain size reduction by dynamic recrystallization: can it result in major rheological weakening?     

J. De Bresser  J. Ter Heege  C. Spiers 《International Journal of Earth Sciences》2001,90(1):28-45

It is widely believed that grain size reduction by dynamic recrystallization can lead to major rheological weakening and associated strain localization by bringing about a switch from grain size insensitive dislocation creep to grain size sensitive diffusion creep. Recently, however, we advanced the hypothesis that, rather than a switch, dynamic recrystallization leads to a balance between grain size reduction and grain growth processes set up in the neighborhood of the boundary between the dislocation creep field and the diffusion creep field. In this paper, we compare the predictions implied by our hypothesis with those of other models for dynamic recrystallization. We also evaluate the full range of models against experimental data on a variety of materials. We conclude that a temperature dependence of the relationship between recrystallized grain size and flow stress cannot be neglected a priori. This should be taken into account when estimating natural flow stresses using experimentally calibrated recrystallized grain size piezometers. We also demonstrate experimental support for the field boundary hypothesis. This support implies that significant weakening by grain size reduction in localized shear zones is possible only if caused by a process other than dynamic recrystallization (such as syntectonic reaction or cataclasis) or if grain growth is inhibited.  相似文献   

Potential Tsunami Hazard Associated with the Kerepehi Fault,Firth of Thames,New Zealand     

Chick  L. M.  De Lange  W. P.  Healy  T. R. 《Natural Hazards》2001,24(3):309-318

Geophysical data have identified four submarine segments of the Kerepehi Fault, roughly bisecting a back-arc rift (Hauraki Rift). These segments have been traced through the shallow waters of the Firth of Thames, which lies at the southern end of the Hauraki Gulf, New Zealand. No historical or paleotsunami data are available to assess the tsunami hazard of these fault segments.Analysis of the fault geometry, combined with paleoseismic data for three further terrestrial segments of the Fault, suggest Most Credible Earthquake (MCE) moment magnitudes of 6.5–7.1. Due to the presence of thick deposits of soft sediment, and thesemi-confined nature of the Firth, the MCE events are considered capable of generating tsunami or tsunami-like waves. Two numerical models (finite element and finite difference), and an empirical method proposed by Abe (1995), were used to predict maximum tsunami wave heights. The numerical models also modelled the tsunami propagation.The MCE events were found not to represent a major threat to the large metropolitan centre of Auckland City (New Zealand's largest population centre). However, the waves were a threat to small coastal communities around the Firth, including the township of Thames, and 35,000 ha of low-lying land along the southern shores of the Firth of Thames.The Abe method was found to provide a quick and useful method of assessing the regional tsunami height. However, for sources in water depths < 25 m the Abe method predicted heights 2–4 times larger than the numerical models. Since the numerical models were not intended for simulating tsunami generation in such shallow water, the Abe results are probably a good guide to the maximum wave heights.  相似文献   

New constraints on the pyroclastic eruptive history of the Campanian volcanic Plain (Italy)   总被引：2，自引：0，他引：2  

B. De Vivo  G. Rolandi  P. B. Gans  A. Calvert  W. A. Bohrson  F. J. Spera  H. E. Belkin 《Mineralogy and Petrology》2001,73(1-3):47-65

Summary The ∼ 150 km³ (DRE) trachytic Campanian Ignimbrite, which is situated north-west of Naples, Italy, is one of the largest eruptions in the Mediterranean region in the last 200 ky. Despite centuries of investigation, the age and eruptive history of the Campanian Ignimbrite is still debated, as is the chronology of other significant volcanic events of the Campanian Plain within the last 200–300 ky. New ⁴⁰Ar/³⁹Ar geochronology defines the age of the Campanian Ignimbrite at 39.28 ± 0.11 ka, about 2 ky older than the previous best estimate. Based on the distribution of the Campanian Ignimbrite and associated uppermost proximal lithic and polyclastic breccias, we suggest that the Campanian Ignimbrite magma was emitted from fissures activated along neotectonic Apennine faults rather than from ring fractures defining a Campi Flegrei caldera. Significantly, new volcanological, geochronological, and geochemical data distinguish previously unrecognized ignimbrite deposits in the Campanian Plain, accurately dated between 157 and 205 ka. These ages, coupled with a xenocrystic sanidine component > 315 ka, extend the volcanic history of this region by over 200 ky. Recent work also identifies a pyroclastic deposit, dated at 18.0 ka, outside of the topographic Campi Flegrei basin, expanding the spatial distribution of post-Campanian Ignimbrite deposits. These new discoveries emphasize the importance of continued investigation of the ages, distribution, volumes, and eruption dynamics of volcanic events associated with the Campanian Plain. Such information is critical for accurate assessment of the volcanic hazards associated with potentially large-volume explosive eruptions in close proximity to the densely populated Neapolitan region. Received August 1, 2000; accepted November 2, 2000  相似文献   

Numerical model simulations of boundary-layer dynamics during winter conditions     

D. Melas  T. Persson  H. De Bruin  S.-E. Gryning  E. Batchvarova  C. Zerefos 《Theoretical and Applied Climatology》2001,70(1-4):105-116

Summary  A mesoscale numerical model, incorporating a land-surface scheme based on Deardorffs’ approach, is used to study the diurnal variation of the boundary layer structure and surface fluxes during four consecutive days with air temperatures well below zero, snow covered ground and changing synoptic forcing. Model results are evaluated against in-situ measurements performed during the WINTEX field campaign held in Sodankyl?, Northern Finland in March 1997. The results show that the land-surface parameterization employed in the mesoscale model is not able to reproduce the magnitude of the daytime sensible heat fluxes and especially the pronounced maximum observed in the afternoon. Additional model simulations indicate that this drawback is to a large extent removed by the implementation of a shading factor in the original Deardorff scheme. The shading factor, as discussed in Gryning et al. (2001), accounts for the fact that in areas with sparse vegetation and low solar angles, both typical for the northern boreal forests in wintertime, absorption of direct solar radiation is due to an apparent vegetation cover which is much greater than the actual one (defined as the portion of the ground covered by vegetation projected vertically). Moreover, the observed asymmetry in the diurnal variation of the sensible heat flux indicates that there might be a significant heat storage in the vegetation. The implementation of an objective heat storage scheme in the mesoscale model explains part of the observed diurnal variation of the sensible heat flux. Received November 12, 1999 Revised October 4, 2000  相似文献   

International Association of Geomorphologists Newsletter No. 18 (3/2001)     

Paul Williams  Selma Simes De Castro  Janet Hooke  Wang Ying  Vít Vilímek  Karl‐Heinz Schmidt  Anto Raukas  Tsuguo Sunamura  Algimantas Cesnulevicios  Irasema Alcntara‐Ayala  James P. Terry  Milos Stankoviansky  Carol P. Harden 《地球表面变化过程与地形》2001,26(12):1353-1360

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远源沙尘暴对城市生态环境影响的初步研究     

胡克  吴东辉  杨德明  杨吉龙  陈姗 《吉林大学学报(地球科学版)》2001,31(2):176-179

200 0年 4月 5日～ 7日袭击我国西北地区北部、华北、东北南部、黄淮地区的扬沙或沙尘暴波及到长春市 ,带来了大量的降尘。据长春市西部 4个采样点测定 ,4月 7日伴随降雪的降尘量最高可达 13.0 g/m2 ,市区总降尘量超过 30 0 0t。泥雪样品在室温条件下自然融化后 ,经实验室 48h细菌培养 ,其细菌含量为每毫升 8.9～ 10 .5万个 ,每平方米泥雪的细菌侵入量达 6 .4亿个左右。扬沙或沙尘暴对城市造成了巨大的环境危害 ,而伴随着大量微生物的侵入 ,势必引起城市微生态环境的改变 ,威胁到城市的生态安全。  相似文献   

正热电离质谱同时测定硝酸盐中氮和氧同位素组成   总被引：2，自引：0，他引：2       下载免费PDF全文

尹德忠  肖应凯  逯海 《盐湖研究》2001,9(3):17-22

初步研究了一种硝酸盐中氮和氧同位素质谱测定的新方法。该法利用石墨的非还原热离子发射特性 ,建立了以测定 Cs2 NO2 + 离子的正热电离质谱测定硝酸盐中氮和氧同位素组成的过程 ,对方法的可行性、特点和存在的问题进行了讨论 ,并对几种测定氮和氧同位素的方法进行了比较。运用正热电离质谱法对几种不同厂家生产的硝酸盐试剂的氮和氧同位素组成进行了测定 ,发现氮和氧同位素组成存在较大的差异  相似文献   

Caracas, Venezuela, Site Effect Determination with Microtremors   总被引：1，自引：0，他引：1  

A.-M. Duval  S. Vidal  J.-P. Méneroud  A. Singer  F. De Santis  C. Ramos  G. Romero  R. Rodriguez  A. Pernia  N. Reyes  C. Griman 《Pure and Applied Geophysics》2001,158(12):2513-2523

—?Caracas 1967 earthquake caused heavy damage to multi-story buildings. In 1995, 184 microtremor measurement points were performed over the city. The measurement grid was more or less dense and covered the main part of the alluvial basin as well as surrounding rock basement. For each point, the horizontal record spectrum was divided by the vertical one (H/V ratio). Subsequently, the strongest value (Ao) of this ratio in a given frequency band was kept, as well as the frequency (Fo) where it occurred. Spatial interpolations of Ao and Fo were performed among all points of Palos Grandes district. A map was plotted representing a single surface where Ao is relief and Fo is represented by grey gradation. An alluvion thickness (H) map can be compared with this result. Damaged buildings are located on the same map. Fo decreases until 0.6?Hz when alluvion thickness (H) increases. Fo values fit with frequencies previously predicted from computation and with 1967 earthquake observations. Moreover, interpolation surfaces show that amplification (Ao) of H/V on microtremor is quite low above rock but is high on the south part of the basin. The maximum occurs over the non-urbanized zone. However the other area, where Ao is higher than 5, roughly corresponds to the location of the four collapsed buildings. Furthermore, the estimated natural frequency of these buildings was around Fo. Graphics showing H, Fo and Ao evolution through the basin were composed. Links between all these values are noticeable. Fo is claimed to be very similar to the resonance frequency of soil. As for Ao, it could be a fairly relevant sign of damage seriousness. Microtremor technique is an economic tool and it allows measurement grid as dense as desired. H/V ratio processing followed by interpolation of maximum values provides precise and useful information about expected site effect.  相似文献