A rare earth element-rich carbonatite dyke at Bayan Obo, Inner Mongolia, North China   总被引：8，自引：0，他引：8  

X.-M. Yang  X.-Y. Yang  Y.-F. Zheng  M. J. Le Bas 《Mineralogy and Petrology》2003,78(1-2):93-110

Summary ?A carbonatite dyke, extremely enriched in rare earth elements (REE), is reported from Bayan Obo, Inner Mongolia, North China. The REE content in the dyke varies from 1 wt% to up to 20 wt%. The light REEs are enriched and highly fractionated relative to the heavy REEs, and there is no Eu anomaly. Although carbon isotope δ¹³C (PDB) values of the carbonatites (−7.3 to −4.7‰) are within the range of normal mantle (−5±2‰), oxygen isotope δ¹⁸O (SMOW) (11.9 to 17.7‰) ratios apparently are higher than those of the mantle (5.7±1.0‰), indicating varying degrees of exchange with hydrothermal fluids during or after magmatic crystallization. The carbonatite is the result of partial melting followed by fractional crystallization. Primary carbonatite melt was formed by less than 1% partial melting of enriched mantle, leaving a garnet-bearing residue. The melt then rose to a crustal magma chamber and underwent fractional crystallization, producing further REE enrichment. The REE and trace element distribution patterns of the carbonatites are similar to those of fine-grained dolomite marble, the ore-host rock of the Bayan Obo REE–Nb–Fe giant mineral deposit. This fact may indicate a petrogenetic link between the dykes described here and the Bayan Obo mineral deposit. Received November 1, 2001; revised version accepted June 16, 2002  相似文献   

The Pitcairn hotspot in the South Pacific: distribution and composition of submarine volcanic sequences     

R. Hekinian  J. L. Chemine  J. Dubois  P. Stoffers  S. Scott  C. Guivel  D. Garbe-Schnberg  C. Devey  B. Bourdon  K. Lackschewitz  G. McMurtry  E. Le Drezen 《Journal of Volcanology and Geothermal Research》2003,121(3-4):219-245

Multibeam bathymetry and bottom imaging (Simrad EM12D) studies on an area of about 9500 km² were conducted over the Pitcairn hotspot near 25°10′S, 129° 20′W. In addition, 15 dives with the Nautile submersible enabled us to obtain ground-true observations and to sample volcanic structures on the ancient ocean crust of the Farallon Plate at 3500–4300 m depths. More than 100 submarine volcanoes overprint the ancient crust and are divided according to their size into large (>2000 m in height), intermediate (500–2000 m high) and small (<500 m high) edifices. The interpretation of seafloor backscatter imagery accompanied by submersible observations and sampling enabled us to infer that the total volume of submarine lava erupted during hotspot activity is about 5900 km³ within a radius of about 110 km. The most recent volcanic activities occur on both small and large edifices composed of a great variety of lava flows. These flows vary in composition, following a succession from picritic basalt to alkali basalt, trachybasalt, trachy-andesite and to trachyte. Their large range of SiO₂ (48–62%), Na₂O+K₂O (2–11%), Ba (300–1300 ppm), MgO (1–11%), Nb (19–130 ppm), Ni (4–400 ppm) and rare earth elements suggests that crystal–liquid fractionation from basanite and/or picritic melt sources was a major process. The variation in composition between the least evolved basaltic rocks and the other more evolved silicic lava is marked by a difference in their flow morphology (pillow, giant tubes, tabular to blocky flows). The lava composition and field observation indicate that several magmatic pulses giving rise to cyclic eruptions are responsible for the construction of the edifices. The two larger edifices (>2000 m high) show more extensive eruptive events and a wider range in compositional variability than the smaller (<500 m high) ones. Several (five) submersible transects made along the slope of one of the largest edifices (Bounty) enabled us to observe at least nine successive eruptive cycles progressing from pillow and giant tubular basalt to tabular/blocky trachy-andesite and trachyte flows. Pyroclasts and hyaloclastites are often found with these eruptive sequences. The smaller edifices, forming individualized cones, are built mainly of evolved silicic (SiO₂>53%) flows consisting essentially of alternating sequences of trachy-andesite and trachyte. The distribution and composition of the small edifices suggest that they are the result of sub-crustal forceful magma injection and channeling supplied from reservoirs associated with the large volcanoes.  相似文献   

Long-term thermal evolution and effect of low power heating in an underground quarry     

Catherine Crouzeix  Jean-Louis Le Mouël  Frédéric Perrier  Patrick Richon  Pierre Morat 《Comptes Rendus Geoscience》2003,335(4):345-354

Vertical profiles of temperature have been monitored in the atmosphere of an underground quarry since June 2001. A long-term temperature increase of the order of 0.1 °C per year is observed. Three heating experiments with 100-W sources, performed to study plume dynamics, can also be used to investigate the effect of visits on this increase. Ten days after the 24-h heatings, the initial state is not restored. Artificial contributions to the observed long-term temperature trend, triggered by internal heat release, thus cannot be ruled out. This experiment defines strict criteria for the underground monitoring of global warming or the preservation of painted caves. To cite this article: C. Crouzeix et al., C. R. Geoscience 335 (2003).  相似文献   

陕西旬邑新近系化学地层与环境变迁   总被引：7，自引：0，他引：7  

薛祥煦  岳乐平  张云翔  柳小明  王建其 《地层学杂志》2002,26(2):81-86

陕西旬邑下墙剖面化学组成的系统测试资料、年均温度与年均降水量的定量计算揭示出 :该区红土堆积时的气候总体上比第四纪黄土堆积时的要暖湿 ,在暖湿背景下 ,早期相对较凉干 ,中期最暖湿 ,晚期较暖湿。进入第四纪后 ,气候突然变冷变干。如更进一步细分 ,新近系红土段自下向上存在着由相对凉干—相对暖湿的四个百万年级气候旋回 ,其中第二、三旋回之间 ,即第 17与第 16层组之间 ,有气候较明显降温变干现象 ,它有可能主要是北极冰盖形成的影响所致。进入第四纪的突然明显变冷变干可能主要受控于当时青藏高原剧烈隆升作用。剖面上化学组成的变化在地层划分上有重要意义 ,从旬邑剖面的地化资料出发 ,并参考岩性、磁化率等将第四系底划在第 13层组顶 ,上新统底划在第 17层组与第 18层组之间  相似文献   

Sediment concentration in interrill flow: interactions between soil surface conditions,vegetation and rainfall     

O. Cerdan  Y. Le Bissonnais  V. Souchre  P. Martin  V. Lecomte 《地球表面变化过程与地形》2002,27(2):193-205

A database composed of 673 natural rainfall events with sediment concentration measurements at the field or plot scale was analysed. Measurements were conducted on similar soil type (loess soils prone to sealing phenomenon) to apprehend the variability and complexity involved in interrill erosion processes attributable to soil surface conditions. The effects of the dominant controlling factors are not described by means of equations; rather, we established a classification of potential sediment concentration domain according to combination of the dominant parameters. Thereby, significant differences and evolution trends of mean sediment concentration between the different parameter categories are identified. Further, when parameter influences interact, it allows us to discern the relative effects of factors according to their respective degree of expression. It was shown that crop cover had a major influence on mean sediment concentration, particularly when soil surface roughness is low and when maximum 6‐min intensity of rainfall events exceeds 10 mm h^?1: mean sediment concentration decreases from 8·93 g l^?1 for 0–20 per cent of coverage to 0·97 g l^?1 for 21–60 per cent of coverage. The established classification also indicates that the increase of the maximum 6‐min intensity of the rainfall factor leads to a linear increase of mean sediment concentration for crop cover over 21 per cent (e.g. from 2·96 g l^?1 to 14·44 g l^?1 for the 1–5 cm roughness class) and to an exponential increase for low crop cover (e.g. from 3·92 g l^?1 to 58·76 g l^?1 for the 1–5 cm roughness class). The implication of this work may bring perspective for erosion prediction modelling and give references for the development of interrill erosion equation. Copyright © 2002 John Wiley & Sons, Ltd.  相似文献   

Determination of Zr, Hf, Th and U by Isotope Dilution and Inductively Coupled Plasma-Quadrupole Mass Spectrometry After Concomitant Separation Using Extraction Chromatography     

Brieuc Le Fèvre  Christian Pin 《Geostandards and Geoanalytical Research》2002,26(2):161-170

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From economic globalisation to globalising economic processes: towards post-structural political economies     

Wendy LarnerRichard Le Heron 《Geoforum》2002,33(4):415-419

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Relating Radar Remote Sensing of Biomass to Modelling of Forest Carbon Budgets   总被引：1，自引：0，他引：1  

Thuy Le Toan  Shaun Quegan  Ian Woodward  Mark Lomas  Nicolas Delbart  Ghislain Picard 《Climatic change》2004,67(2):379-402

This paper addresses the use of radar remote sensing to map forest above-ground biomass, and discusses the use of biomass maps to test a dynamic vegetation model that identifies carbon sources and sinks and predicts their variation over time. For current radar satellite data, only the biomass of young/sparse forests or regrowth after disturbances can be recovered. An example from central Siberia illustrates that biomass can be measured by radar at a continental scale, and that a significant proportion of the Siberian forests have biomass values less than 50 tonnes/ha. Comparison between the radar map and calculations by the Sheffield Dynamic Global Vegetation Model (SDGVM) indicates that the model considerably overestimates biomass; under-representation of managed areas, disturbed areas and areas of low site quality in the model are suggested reasons for this effect. A case study carried out at the Büdingen plantation forest in Germany supports the argument that inadequate representations of site quality and forest management may cause model overestimates of biomass. Comparison of the calculated biomass of stands planted after 1990 with biomass estimates by radar allows identification of forest stands where the growth conditions assumed by the model are not valid. This allows a quality check on model calculations of carbon fluxes: only calculations for stands where there is good agreement between the data and the model predictions should be accepted. Although the paper only uses the SDGVM model, similar effects are likely in other dynamic vegetation models, and the results show that model calculations attempting to quantify the role of forests as carbon sources or sinks could be qualified and potentially improved by exploiting remotely sensed measurements of biomass.  相似文献   

地面沉降对工程沉降监测数据的影响及分析   总被引：1，自引：0，他引：1  

周金龙  乐艳莉 《上海国土资源》2004,(3):57-61

该文通过对工程沉降监测中的作业方法及其监测数据进行分析，由此得出在工程沉降监测中，地面沉降的影响因素不容忽视。  相似文献   

Hydraulic behavior of tsunami backflows: insights from their modern and ancient deposits     

J. P. Le Roux  G. Vargas 《Environmental Geology》2005,49(1):65-75

Tsunamis are unpredictable, catastrophic events, and so present enormous difficulties for direct studies in the field or laboratory. However, their sedimentary deposits yield evidence of a wide variety of hydrodynamic conditions caused by flow transformations on a spatial and temporal scale. Tsunami deposits ranging from the Miocene to modern times identified at different localities along the Chilean coast are described to provide a database of their characteristics. Among the typical features associated with tsunami deposits are well-rounded megaclasts eroded from coastal alluvial fans or beaches by very dense, competent flows. Sand injections from the base of these flows into the substrate indicate very high dynamic pressures, whereas basal shear carpets suggest hyperconcentrated, highly sheared flows. Turbulence develops in front of advancing debris flows, as indicated by megaflutes at the base of scoured channels.  相似文献