Genesis of zircon and its constraints on interpretation of U-Pb age

Zircon U-Pb dating is the most commonly used method for isotopic geochronology. However, it has been a difficult issue when relating zircon U-Pb ages to metamorphic conditions in complex metamorphic rocks. Much progress has been made in the past decade with respect to the genesis of zircon and its constraints on interpretation of U-Pb age. Three methods have been proposed to link zircon U-Pb age to metamorphic conditions: ( i ) internal structure; ( ii ) trace element feature; (iii) mineral inclusion composition. Magmatic zircon shows typical oscillatory zoning and/or sector zoning, whereas metamorphic zircon has internal structures such as no zoned, weakly zoned, cloudy zoned, sector zoned, planar zoned, and patched zoned ones. Zircons formed in different geological environments generally have characteristic internal structures. Magmatic zircons from different rock types have variable trace element abundances, with a general trend of increasing trace element abundances in zircons from ultramafic through mafic to granitic rocks. Zircons formed under different metamorphic conditions have different trace element characteristics that can be used to relate their formation to metamorphic conditions. It is an effective way to relate zircon growth to certain P-T conditions by studying the trace element partitioning between coexisting metamorphic zircon and garnet in high-grade metamorphic rocks containing both zircon and garnet. Primary mineral inclusions in zircon can also provide unambiguous constraints on its formation conditions. Therefore, interpretation of zircon U-Pb ages can be constrained by its internal structure, trace element composition, mineral inclusion and so on.     

中国传统休闲文化与西方休闲价值观

通过对我国悠久休闲历史文化的介绍,以及对西方休闲理论演进的分析,揭示了人类即将迈入的"休闲时代"以及休闲时代的特征、休闲时代人类的价值观念等。旨在引起社会更多人的关注,提高人们的休闲意识,享受休闲,在休闲中体会生命的价值和意义。     

论弘扬"红军精神" --兼论发展先进文化

“红军精神”是川陕苏区历史留给我们的宝贵精神财富,是重要的苏区文化现象和自己时代精神的精华,是中国先进文化重要的生成因素,今天仍然是推动实践发展的强大精神动力,弘扬“红军精神”要与时俱进,弘扬“红军精神”就是发展先进文化,打造新世纪老区发展的精神文化竞争力。     

河北高寺台超镁铁岩体中锇矿物的锇同位素组成初步研究

报道了河北高寺台超基性岩体中的等轴锇铱矿和六方铱锇矿的Os同位素组成,两样品的187Os/188Os值分别为0.123 582±7和0.123 731±7.数据的解释有赖于年龄的和/或源区性质的制约.若已知矿物源区,可由同位素组成制约其形成时代.如上述矿物的锇来自于原始上地幔,其形成时代可能为500±10 Ma;若其源区为亏损的岩石圈地幔,其形成时代可能年轻到晚中生代;若源区为富集的地幔柱,其形成时代可能老到中元古代.反之,若能限定矿物形成时代,则可制约其源区性质.若上述锇矿物形成于印支期,则很可能源于亏损的岩石圈地幔.若它们形成于中元古代,则其源区很可能来源于富集的地幔柱性质的地幔.根据区域地质情况,我们倾向于这些Os来自富集地幔、高寺台岩体形成于中元古代的解释,但进一步解释有待于新的独立证据.尽管如此,这些数据对今后高寺台岩体的成因或年龄研究有参考价值.     

Test of magnetic susceptibility and grain-size age models of loess

Ages of the stratigraphic boundary MIS1/2 and MIS3/4 of the Yuanbu loess section in Linxia are used as the basis of the nodal control age. The age of MIS1/2 and MIS3/4 are obtained from the latest international research result—the climatic events recorded in the stalagmite in the Hulu Cave in Nanjing, that MIS1/2 is 11.5 kaB.P. and MIS3/4 is 59.8 kaB.P.. The ages of the two climatic events contain three nodal age control models (Model 1: 0 kaB.P.—59.8 kaB.P.; Model 2: 0 kaB.P.—11.5 kaB.P. and 11.5 kaB.P.—59.8 kaB.P.; Model 3: 11.5 kaB.P.—59.8 kaB.P.), which are used as the nodal control age separately. The deposition times of various stratigraphic horizons are calculated by using the magnetic susceptibility age model and grain-size age model, and then compared with each other. In addition, the AMS14C age, OSL age and the ages of YD and H events are compared with the ages of the corresponding horizons calculated by the three models of nodal control ages. From the analyses of lithologic characters and climatic stages it has been found that both the magnetic susceptibility age model and the grain-size age model have some defects. Because the accurate control ages are selected as the nodal points of the glacial period or interglacial period, the stratigraphic deposition times determined by the high resolution of magnetic susceptibility age model and grain-size age model approximate to the actual ages. As for the relative accuracy of the two age models, the magnetic susceptibility age model is more accurate than the grain-size age model.     

Test of magnetic susceptibility and grain-size age models of loess

Ages of the stratigraphic boundary MIS1/2 and MIS3/4 of the Yuanbu loess section in Linxia are used as the basis of the nodal control age. The age of MIS1/2 and MIS3/4 are obtained from the latest international research result—the climatic events recorded in the stalagmite in the Hulu Cave in Nanjing, that MIS1/2 is 11.5 kaB.P. and MIS3/4 is 59.8 kaB.P.. The ages of the two climatic events contain three nodal age control models (Model 1: 0 kaB.P.—59.8 kaB.P.; Model 2: 0 kaB.P.—11.5 kaB.P. and 11.5 kaB.P.—59.8 kaB.P.; Model 3: 11.5 kaB.P.—59.8 kaB.P.), which are used as the nodal control age separately. The deposition times of various stratigraphic horizons are calculated by using the magnetic susceptibility age model and grain-size age model, and then compared with each other. In addition, the AMS14C age, OSL age and the ages of YD and H events are compared with the ages of the corresponding horizons calculated by the three models of nodal control ages. From the analyses of lithologic characters and climatic stages it has been found that both the magnetic susceptibility age model and the grain-size age model have some defects. Because the accurate control ages are selected as the nodal points of the glacial period or interglacial period, the stratigraphic deposition times determined by the high resolution of magnetic susceptibility age model and grain-size age model approximate to the actual ages. As for the relative accuracy of the two age models, the magnetic susceptibility age model is more accurate than the grain-size age model.     

～4.1 Ga xenocrystal zircon from Ordovician volcanic rocks in western part of North Qinlin80rosenic Belt

High-precision U-Pb dating by in situ LA-ICP-MS yields an age of 4079±5 Ma for a xenocrystal zircon from Ordovician volcanics of the Caotangou Group in western part of the North Qinling Orogenic Belt. As a result, the North Qinling Orogenic Belt becomes one of a few localities in the world that contain Hadean age records （4276±6 Ma and 4404±8 Ma detrital zircons from Jack Hill of the Yilgarn craton, 4016 Ma Acasta gneisses of the Wopmay Orogeny and Burang quartzite with detrital zircon of 4103 Ma in Tibet）. It is also the first report of the Hadean age in Phanerozoic volcanics. The finding of the 4.1 Ga xenocrystal zircon provides not only the geochronological record of the oldest crustal materials in China, but also the condition for further search for rocks forming in the region during the early time of the Earth＇s evolution. Thirty-six zircon U-Pb dates from the Ordovician volcanic rocks are subgrouped into seven generations that represent different tectono-magmatic events in the North Qinling Orogenic Belt. Among them, two periods of 0.9--1.5 Ga and 0.4--0.5 Ga are consistent with Mesoproterozoic and Early Paleozoic orogenies, respectively.     

Single grain Rb-Sr dating of euhedral and cataclastic pyrite from the Qiyugou gold deposit in western Henan, central China

The Rb-Sr isotopic dating of pyrite mineral from ore deposits can directly provide mineralization age. However, many geological factors may affect the Rb-Sr isotopic system,which baffles application of this method. Employing ultra-low procedural blank Rb-Sr method,we have dated pyrites separated from the No. 4 breccia pipe of the Qiyugou gold deposit,western Henan Province. Single grains of euhedral pyrite crystal with few microcracks yield an isochron age of 126 ± 11 Ma, which represents time of the main mineralization stage of the deposit. Pyrite grains of cataclastic type show nevertheless scattered Rb-Sr isotopic composition and no reasonable isochron can be defined. Crystal morphology and mineral inclusion studies reveal that Rb and Sr of pyrite mineral probably are preserved mainly in biotite, K-feldspar, and sericite mineral inclusions. The dating results likely suggest that cataclastic pyrite is not suitable for the Rb-Sr dating due to modification of the Rb-Sr isotopic system by later hydrothermal activity of fluid.     

信息时代我国公共图书馆信息咨询服务研究

随着网络信息技术和信息市场的飞速发展,我国的信息咨询服务业也逐渐发展了起来,信息咨询工作已经成为现代图书馆的一项核心职能。但由于各种原因的影响,我国公共图书馆的信息咨询服务工作仍不尽如人意。通过分析信息时代我国公共图书馆信息咨询工作的现状和不足,提出了提高信息咨询服务工作水平的对策。     

Analysis of effective service time with age dependent interruptions and its application to optimal rollback policy for database management

A stochastic model is developed describing a service system subject to inhomogeneous Poisson interruptions with age dependent interruption periods. By studying the probabilistic flow of the underlying multivariate Markov process, the Laplace transform of the effective service time is explicitly obtained. For general renewal interruptions, only the expected effective service time is derived. As an application, an optimal checkpoint policy is examined for database management. It is shown that an optimal policy maximizing the ergodic availability of the database is to implement a checkpoint as soon as the cumulative uptime of the database reaches a prespecified constantk ^*. A computational procedure is then developed for findingk ^* and numerical results are exhibited.This work was supported in part by the National Science Foundation under Grant No. ECS-8600992 and by the IBM Program of Support for Education in the Management of Information Systems.