Negative Ce Anomaly in the Indian Banded Iron Formations: Evidence for the Emergence of Oxygenated Deep-Sea at 2.9-2.7 Ga     

Yasuhiro KATO  Takashi KANO  Keitaro KUNUGIZA 《Resource Geology》2002,52(2):101-110

Abstract: Major and rare earth element contents are reported for Late Archean banded iron formations (BIFs) in the Bababudan Group of the Dharwar Craton, South India. The BIFs are mostly composed of SiO₂ (average1ρ = 54.88.1 wt%) and Fe₂O₃* (44.38.2 wt%). The Al₂O₃ and TiO₂ contents are remarkably low, suggesting that detrital components were starved during the BIF deposition. The BIFs have a LREE-enriched pattern with a relatively high (La/Yb)_N (6.644.07). Total REE concentrations (RE) vary from 5.2 to 65.3 ppm. The REE patterns are characterized by the presence of a very large negative Ce anomaly (Ce/Ce*: 0.13-0.83) and a positive Eu anomaly (Eu/Eu*: 0.96-2.45). The Eu/Eu* decreases and (La/Yb)_N increases with a increase of RE. These correlations of REE indices are similar to those of modern hydrothermal iron-rich sediments near a mid-ocean ridge (MOR). Greenstones associated with the BIFs have MORB-like geochemical features. These geochemical and geological lines of evidence indicate that the depositional site of the BIFs was remote from a continent and/or island arc and that the BIFs were in situ hydrothermal sediments near a MOR. A striking negative Ce anomaly in the BIFs indicates that oxygenated deep-sea environments emerged at 2.9-2.7 Ga. The existence of contemporaneous Mn deposits in the Dharwar Craton supports this assertion. Our scenario of oxygen in the Earth's surface of the Late Archean is different from long-held notion that the atmosphere and ocean were persistently anoxic throughout the Archean.  相似文献   

Une méthode de quantification de la dégradation d'un escarpement de faille au cours des cycles climatiques du Quaternaire : la faille de Jobourg (Nord Cotentin,France)Quantification of a fault-scarp degradation through Quaternary: the Jobourg fault (North Cotentin,France)     

Marianne Font  Jean-Louis Lagarde  Daniel Amorese  Jean-Pierre Coutard  Jean-Claude Ozouf 《Comptes Rendus Geoscience》2002,334(3):171-178

The degradation of the Jobourg fault-scarp occurred by cryoclastic processes in a periglacial environment during a part of Quaternary time. An attempt of quantification indicates a bulk scarp erosion of about 39 m³ m^?2, while the head accumulated at the bottom of the fault scarp only represents 4.6 m³ m^?2. To cite this article: M. Font et al., C. R. Geoscience 334 (2002) 171–178.  相似文献   

昆仑山口西8.1级地震地表破裂的类型与性质   总被引：12，自引：0，他引：12  

王赞军  党光明  张瑞斌  张启胜  都昌庭 《高原地震》2002,14(1):17-25

通过对昆仑山口西8．1级地震地表破裂全带的考察认为：本次地震形成的地表破裂带长达350－400km，由地震裂缝，鼓梁（包）、塌陷、陡坡等基本形态组成，属构造性破裂，并具有明显的继承性和新生性。  相似文献   

Torsional vibrations of elastic foundation on saturated media     

Longzhu Chen  Guocai Wang   《Soil Dynamics and Earthquake Engineering》2002,22(3)

A comprehensive analytical solution is developed to examine the torsional vibration of an elastic foundation on a semi-infinite saturated elastic medium for the first time. First, the governing equations of saturated media are solved by use of Hankel transform techniques. Then, based on the assumption that the contact between the foundation and the half-space is perfectly bonded, this dynamic mixed boundary-value problem can lead to dual integral equations, which are further reduced to the standard Fredholm integral equations of the second kind and solved by numerical procedures. Numerical examples are given at the end of the paper. The numerical results indicate that the response of the elastic foundation strongly depends on the material and geometrical properties of both the saturated soil-foundation system and the load acting on the foundation. In most of the cases, the dynamic behavior of an elastic foundation on saturated media significantly differs from that of a rigid plate bearing on the elastic half-space.  相似文献   

Hydrogeochemistry of the Simav geothermal field, western Anatolia, Turkey     

Ünsal Gemici  Gültekin Tarcan 《Journal of Volcanology and Geothermal Research》2002,116(3-4)

Thermal waters hosted by Menderes metamorphic rocks emerge along fault lineaments in the Simav geothermal area. Thermal springs and drilled wells are located in the Eynal, Çitgöl and Na a locations, which are part of the Simav geothermal field. Studies were carried out to obtain the main chemical and physical characteristics of thermal waters. These waters are used for heating of residences and greenhouses and for balneological purposes. Bottom temperatures of the drilled wells reach 163°C with total dissolved solids around 2225 mg/kg. Surface temperatures of thermal springs vary between 51°C and 90°C. All the thermal waters belong to Na–HCO₃–SO₄ facies. The cold groundwaters are Ca–Mg–HCO₃ type. Dissolution of host rock and ion-exchange reactions in the reservoir of the geothermal system shift the Ca–Mg–HCO₃ type cold groundwaters to the Na–HCO₃–SO₄ type thermal waters. Thermal waters are oversaturated at discharge temperatures for aragonite, calcite, quartz, chalcedony, magnesite and dolomite minerals giving rise to a carbonate-rich scale. Gypsum and anhydrite minerals are undersaturated with all of the thermal waters. Boiling during ascent of the thermal fluids produces steam and liquid waters resulting in an increase of the concentrations of the constituents in discharge waters. Steam fraction, y, of the thermal waters of which temperatures are above 100°C is between 0.075 and 0.119. Reservoir pH is much lower than pH measured in the liquid phase separated at atmospheric conditions, since the latter experienced heavy loss of acid gases, mainly CO₂. Assessment of the various empirical chemical geothermometers and geochemical modelling suggest that reservoir temperatures vary between 175°C and 200°C.  相似文献   

Chemical equilibria of thermal waters for the application of geothermometers from the Guanzhong basin, China     

Zheng Xilai  Halldor Armannsson  Li Yongle  Qiu Hanxue 《Journal of Volcanology and Geothermal Research》2002,113(1-2)

In this study, representative samples from thermal wells and springs were chemically analyzed and geothermometers were used to calculate the deep temperatures of geothermal reservoirs on the basis of water–mineral equilibrium. In some cases, however, the chemical components are not in equilibrium with the minerals in the reservoir. Therefore, log(Q/K) diagrams are used to study the chemical equilibrium for the minerals that are likely to participate. The Na–K–Mg triangular diagram is also applied to evaluate the equilibrium of water with reservoir rocks. Standard curves at the reference temperatures are prepared to reveal which type of silica geothermometer is appropriate for the specified condition. This study shows that water samples from geothermal wells W9 and W12 are in equilibrium with the selective minerals, and chalcedony may control the fluid–silica equilibrium. It is estimated that there is an exploitable low-temperature reservoir with possible temperatures of 80–90°C in the Guanzhong basin.  相似文献   

Stable isotope values of Costa Rican surface waters   总被引：3，自引：0，他引：3  

Matthew S. Lachniet  William P. Patterson 《Journal of Hydrology》2002,260(1-4):135-150

Stable isotope data of surface waters from the humid tropics in general, and Costa Rica in particular, are scarce. To improve our understanding of the spatial distribution of stable isotopes in surface waters, we measured δ¹⁸O and δD in river and lake (n=63) and precipitation (n=3) samples from Costa Rica. We also present data from the IAEA/WMO isotopes in precipitation network as context for our study. Surface water isotope values do not strongly correlate with elevation, stream head elevation, stream length, distance from Caribbean Sea, or estimated mean annual precipitation for the country as a whole. However, the data show distinct regional trends. The δ¹⁸O and δD values downwind of mountain ranges are inversely related to the altitude of the ranges the air masses traverse. In the lee of the high Talamanca Range, δ¹⁸O values are 6–8‰ lower, while in the lee of the lower Tilarán Range δ¹⁸O values are 2–3‰ lower than upwind sites along the Caribbean Slope. An altitude effect of −1.4‰ δ¹⁸O/km is present on the Pacific slope of southern Costa Rica, equivalent to a temperature effect of −0.3‰/°C. The Nicoya and Osa Peninsulas have higher values than upwind sites, suggesting input of Pacific-sourced moisture, evaporative enrichment, or decreased condensation temperatures. Elevated and increasing d-excess values inland along the Nicaragua Trough suggest a recycled component may be an important contributor to the water budget. These data provide preliminary stable isotope information for Costa Rica, and will benefit paleoclimatic research in the region. More detailed studies would be beneficial to our understanding of the controls on stable isotope composition of tropical waters.  相似文献   

Interpretation of charging on fracture or frictional slip surface of rocks     

Akihiro Takeuchi  Hiroyuki Nagahama 《Physics of the Earth and Planetary Interiors》2002,130(3-4):285-291

Fracturing and frictional sliding of quartz and granite under dry condition generates fractoluminescence, charged particle emission and electromagnetic radiation. Various kinds of experiments indicate that surface charge density on fracture or frictional slip surface of quartz and granite is 10⁻⁴ to 10⁻² C/m² which is larger than bound charges induced by the disappearance of piezoelectricity due to the release of stress. Hole and electron trapping centers, which is found in semiconductor devices with the Si–SiO₂ system, are causes of surface charging on fracture or frictional slip surface of quartz crystal. The quantity of the surface charge is enough to cause corona discharge that can generate earthquake lights. The mechanism considering the hole and electron trapping centers has a probability to explain why non-piezoelectric minerals or rocks generate electromagnetic phenomena. It can be one of origins of seismo-electromagnetic phenomena (SEP).  相似文献   

山东海阳断裂东石兰沟段晚更新世以来地表断错特征与最大潜在地震估计   总被引：7，自引：2，他引：7       下载免费PDF全文

周本刚  冉勇康  环文林  冉洪流 《地震地质》2002,24(2):159-166

海阳断裂是胶东半岛NE向牟平 -即墨断裂带东部一条规模较大的断裂 ,尽管晚更新世以来该断裂的地表断错活动总体上已基本停息 ,但东石兰沟段在晚更新世晚期以来仍有断错地表的活动。最后一次断错地表的活动发生在距今 3 7～ 1 2万年 ,但接近 1 2万年。地表破裂长度约6 5km ,活动段长度 8km。地表断错以走滑活动为主 ,可见最大倾滑位移 0 2m ;根据断层擦痕侧伏角推测最大水平位移 1 13m。最后一次断错地表的活动若以距今 1 2万年计算 ,则最大平均倾滑速率为 0 0 17mm/a ;最大平均右旋走滑速率为 0 0 94mm/a。野外观测到该活动段的断错活动表现为突发断错 ,根据地震地表破裂参数、活动段长度与地震的关系 ,估计其最大潜在地震为 6 级  相似文献   

克孜尔水库大坝的面波检测     

赵纯青  尹力峰  唐丽华  刘景元  马新龙 《内陆地震》2002,16(1):53-57

1999年 3月 15日克孜尔地震之后对克孜尔水库坝体出现的裂缝进行了面波勘探。首先介绍了面波勘探法的基本原理 ,然后通过典型剖面检测与资料处理找出异常区、带并进行解释。试验表明面波勘探法适用于克孜尔水库大坝监测  相似文献