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51.
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53.
Early carbonate cements in the Yanchang Formation sandstones are composed mainly of calcite with relatively heavier carbon isotope (their δ^18O values range from -0.3‰- -0.1‰) and lighter oxygen isotope (their δ^18O values range from -22.1‰- -19.5‰). Generally, they are closely related to the direct precipitation of oversaturated calcium carbonate from alkaline lake water. This kind of cementation plays an important role in enhancing the anti-compaction ability of sandstones, preserving intragranular volume and providing the mass basis for later disso- lution caused by acidic fluid flow to produce secondary porosity. Ferriferous calcites are characterized by relatively light carbon isotope with δ^13C values ranging from -8.02‰ to -3.23‰, and lighter oxygen isotope with δ^18O values ranging from -22.9‰ to -19.7‰, which is obviously related to the decarboxylation of organic matter during the late period of early diagenesis to the early period of late diagenesis. As the mid-late diagenetic products, ferriferous cal- cites in the study area are considered as the characteristic authigenic minerals for indicating large-scaled hydrocarbon influx and migration within the clastic reservoir. The late ankerite is relatively heavy in carbon isotope with δ^13C values ranging from -1.92‰ to -0.84‰, and shows a wide range of variations in oxygen isotopic composition, with δ^18O values ranging from -20.5‰ to -12.6‰. They are believed to have nothing to do with decarboxylation, but the previously formed marine carbonate rock fragments may serve as the chief carbon source for their precipitation, and the alkaline diagenetic environment at the mid-late stage would promote this process. 相似文献
54.
Hang Li Dahua Li Tangfu Xiao Libin He Zengping Ning Jialong Sun Changsheng Zhu Yan Shuang 《中国地球化学学报》2008,27(1):21-27
The Jinding Pb-Zn deposit in Yunnan Province is the representative of a Cd-enriched area and mining activities lead to the release of Cd into the hypergenic ecosystem, resulting in Cd pollution. The concentrations of Cd vary greatly from one type to another type of rocks in the mining district. In the host rock, Cd concentrations range from 50×10^-6 to 650×10^-6 with an average of 310×10^-6. In primary ores, Cd concentrations range from 14×10^-6 to 2800×10^-6 with an average of 767×10^-6. However, in oxidized ores, Cd concentrations are highest, varying within the range of 110×10^-6 to 8200×10^-6 , averaging 1661×10^-6. It is shown that the oxidized ores are the main carder and environmental source of Cd. Leaching test showed that Pb/Zn ores are easy to oxidize and thereafter release Cd and other harmful elements. These leached elements in the leachate may be precipitated rapidly in the order of Zn〉Pb〉Cd. As for the concentration distribution of Cd in the Bijiang River, it is estimated to be 15.7 μg/L Cd in water, 49.3 mg/L in suspended substances, and 203.7 mg/L in sediments. The average value of Cd in soil from the polluted area is 83.0 mg/kg. Natural weathering of Cd-rich rocks and minerals imposes a potential environmental risk on the aquatic ecosystem of the Bijiang catchment. 相似文献
55.
Extensive organic-matter (OM) rich facies (black shales) occur in the Ordo-Silurian boundary successions in the Yangtze area, South China. To investigate the redox changes of the Yangtze Sea during the Ordo-Silurian transition, two OM sections (Wangjiawan in Yichang, Hubei Province, and Sanjiaguan in Zhangjiajie, Hunan Province) straddling the Ordo-Silurian boundary are studied. The measurements finished in this study include contents of the total organic carbon (TOC), pyrite sulphur, and different species of Fe, including dithionite-extractable Fe (FED), pyrite Fe (FeP), HCl-extractable Fe (FeH), and total Fe (FeT), in black shales, as well as other redox proxies, such as the SIC ratio, the ratio between highly reactive Fe (FeHR = FeD + FeP) and FeT, and the FeP/(FeP + FeH) ratio, known as the degree of pyritization (DOP). In the Wangjiawan section, the Middle Ashgill sediments have high FeHR/FeT ratios (0.20-0.77; avg. 0.45), high DOP values (0.21-0.72; avg. 0.54), and a relatively constant sulfur content independent of the organic carbon content. By the contrast, the mid-early Hirnantian deposits generally have low FeHR/FeT ratios (0.10-0.35; avg. 0.21), low DOP values (0.11- 0.40; avg. 0.28), and SIC values are clustering on the normal marine value (SIC = 0.36). The late Hirnantian and early Rhuddanian deposits, similar to those of the Middle Ashgill deposits, are characterized by high FeHR/FeT ratios (0.32-0.49; avg. 0.41), high DOP values (0.46-0.68; avg. 0.53) and fairly constant sulfur contents. These data suggest the occurrences of marine anoxia on the Yangtze Sea shelf during intervals of the Mid Ashgill, Late Hirnantian and Early Rhuddanian, and ventilated and oxygenated marine conditions during the mid-early Hirnantian time. The mid-early Hirnantian ventilated event was concomitant with the global glacial period, likely resulted from the glacio-eustatic sea-level fall and subsequent circulation of cold, dense oxygenated waters 相似文献
56.
大金山铅锌矿位于泰宁一政和北东东向铜多金属成矿带上,是该成矿带主要的矿种之一,也是闽北地区一个较有价值的矿床。矿床产于早元古代南山组上段,受燕山期北东东向构造控制,属中低温热液充填交代型矿床。 相似文献
57.
内蒙古兴安盟杜尔基岩体位于大兴安岭中南段,岩性主要为中—细粒二长花岗(斑)岩、中—粗粒二长花岗岩、细粒二长花岗岩。岩石化学分析结果属于碱性岩系列,具有高硅、高钾、富碱、过铝、贫铁、镁、钙特征。稀土元素总量平均值为112.43μg/g,球粒陨石标准化曲线整体平缓右倾,具有Eu负异常;微量元素蛛网图解表现为“W”型形态.具有明显的Sr、Ba、Ti负异常,反映了强烈的分离结晶作用存在。4个样品K-Ar全岩法测年结果为100.15-120.29Ma,说明岩体形成于早白垩世。反映杜尔基地区在早自垩世可能处于相对稳定的拉张环境。 相似文献
58.
Elisa Sacchi Gian Maria Zuppi Luca Pizzino Fedora Quattrocchi Salvatore Lombardi 《Aquatic Geochemistry》2008,14(4):301-319
The EC funded Geochemical Seismic Zonation program (EEC GSZ Project 1996–1998) chose Sardinia as a low-seismicity site, in which the relationships between fluid geochemistry and seismo-tectonics
had to be investigated and results compared with outcomes from other selected high-seismicity sites. A first article, examining
the role of fault segmentation and seismic quiescence on the geochemical composition of groundwaters and gases, has already
been presented (Angelone et al. 2005). This article deals with environmental isotopes which, together with selected hydrochemical data, give hints on tectonically-related
fluid circulations. Four water-dominated hydrothermal systems were considered, all located along regional fault systems and
discharging groundwaters belonging to the Na–HCO3 and Na–Cl facies. In the considered systems, groundwater circulation takes place, principally, in the Palaeozoic Crystalline
Basement (PCB), with the exception of the Logudoro system, where hydrological circuits develop in the Mesozoic Carbonate Platform
(MCP). The high CO2 contents, the non-attainment of fluid-rock equilibrium and the large lithological variability prevent the construction of
a unique hydrogeological–geochemical conceptual model. In this case, stable isotopes provide a useful tool to describe the
origin of fluids and their subterranean movements. Stable isotopes of water, integrated with hydrochemical data, indicate
that fluids are derived from three main end members. The dominant component is a relatively recent local meteoric water; the
second one is marine water; and the third one is a fossil freshwater, depleted in heavy isotopes with respect to modern rains.
The latter end member entered the aquifer system in the past, when climatic conditions were greatly different from today.
At least two circulation systems can be recognised, namely a shallow cold system and a deep hydrothermal system, as well as
two distinct hydrological processes: (1) gravity-controlled descent of cold water towards greater depths and (2) convection
linked to a thermal gradient, causing deep fluids to rise up from the hydrothermal reservoir towards the surface. The highly
variable δ13CTDIC values suggest the presence of two distinct CO2 sources, namely, a biogenic one and a thermogenic one. The relation between the isotopic compositions of CO2 and He indicates an increased mantle signature in uprising CO2-rich fluids. 相似文献
59.
R. A. Oktyabrsky A. A. Vrzhosek A. M. Lennikov T. A. Yasnygina S. V. Rasskazov E. Yu. Moskalenko T. A. Velivetskaya 《Russian Journal of Pacific Geology》2008,2(4):325-335
New geochemical data are presented on the magmatic rocks of the Late Jurassic Koksharovka alkaline-ultrabasic massif, which is associated with deposits of vermiculite, apatite, V-bearing titanomagnetite, and placer isoferroplatinum. The REE geochemistry and strontium, oxygen, and carbon isotopic composition of carbonatites and related ijolites and pyroxenites, together with geological observations, point to the magmatic origin of the Koksharovka carbonatites. The origin of associated magmatic rocks is discussed. Trace element modeling of partial melting of mantle sources was conducted to decipher the genesis of the melts of the Koksharovka carbonatites and host titanite-kaersutite pyroxenites. 相似文献
60.
Boron isotope variations in nature: a synthesis 总被引:9,自引:0,他引:9
S. Barth 《International Journal of Earth Sciences》1993,82(4):640-651
The large relative mass difference between the two stable isotopes of boron, 10B and 11B, and the high geochemical reactivity of boron lead to significant isotope fractionation by natural processes. Published 11B values (relative to the NBS SRM-951 standard) span a wide range of 90. The lowest 11B values around — 30 are reported for non-marine evaporite minerals and certain tourmalines. The most 11B-enriched reservoir known to date are brines from Australian salt lakes and the Dead Sea of Israel with 11B values up to +59. Dissolved boron in present-day seawater has a constant world-wide 11B value of + 39.5. In this paper, available 11B data of a variety of natural fluid and solid samples from different geological environments are compiled and some of the most relevant aspects, including possible tracer applications of boron-isotope geochemistry, are summarized.
Résumé La grande différence relative de masse entre les isotopes stables du bore, 10B et 11B, et la grande réactivité geochimique du bore ont pour conséquence un fractionnement isotopique naturel important. Les valeurs de 11B publiées (par rapport au standard NBS SRM-951) varient de 90. Les valeurs de 11B les plus basses (–30) correspondent aux evaporites non-marines et à certaines tourmalines. Le réservoir le plus enrichi en 11B est représenté par les saumures des lacs salés d' Australie et par la Mer Morte en Israël, qui ont des valuers de 11B allent jusqu'à + 59. L'eau de mer a une valeur de 11B mondialement constante de + 39.5. Des valeurs de 11B des solutions naturelles ainsi que des roches et minéraux de différentes origines, publiées jusqu'à présent, sont présentées ici. En outre quelques aspects importants concernant la géochimie des isotopes du bore y compris quelques applications sont exposés.相似文献