Strategies towards robust interpretations of in situ zircon Lu-Hf isotope analyses     

《地学前缘(英文版)》2020,11(3):843-853

The combination of U-Pb and Lu-Hf compositions measured in zircon crystals is a remarkably powerful isotopic couplet that provides measures on both the timing of mineral growth and the radiogenic enrichment of the source from which the zircon grew.The U-Pb age documents the timing of zircon crystallization/recrystallization and Hf isotopes inform on the degree to which the host melt was derived from a radiogenic reservoir(e.g.depleted mantle) versus an unradiogenic reservoir(e.g.ancient continental crust),or some mixture of these sources.The ease of generating large quantities of zircon U-Pb and Lu-Hf data has been in large part facilitated by instrument advances.However,the dramatic increase in time constrained zircon Lu-Hf analyses in the Earth science community has brought to the fore the importance of careful data collection and reduction workflows,onto which robust geological interpretations may be based.In this work,we discuss the fundamentals of Lu-Hf isotopes in zircon,which then allows us to provide a robust,accessible,methodology for the assessment of data quality.Additionally,we discuss some novel techniques for:data visualization—that facilitates better transparency of data interpretation;integration of geographic information—that may reveal spatial trends where temporal trends were only apparent before;and some novel statistical evaluation tools—that may provide more rigorous interand intra-sample comparisons.  相似文献   

热扩散驱动的元素分异和同位素分馏:一种不容忽视的硅酸盐成分分异机制     

常翱飞  丁兴 《岩石学报》2020,36(1):99-112

热扩散在地质过程中是否发挥重要作用一直存有争议。本文回顾了热扩散的研究历史和现状,重点总结了热扩散驱动的元素和同位素行为规律,并探讨了温度、硅酸盐组分、压力和氧逸度等因素对热扩散行为的影响。已有的研究表明,稳定热梯度下的硅酸盐热扩散效应类似于结晶分异或AFC过程,可以造成轻、重同位素分别在高温端和低温端富集,而主、微量元素的扩散方向则取决于两端化学势的高低和熔体中的电价平衡。从基性岩浆到酸性岩浆,熔体聚合度增大,黏度增加,热扩散速率明显降低,成网元素的热扩散效应减弱,变网元素则反之;水、氟、氯和硫化氢等挥发组分能增加熔体的非桥氧比例,降低熔体聚合度,因而能显著增强硅酸盐熔体中元素和同位素的热扩散效应。在此基础上,本文提出了当前硅酸盐体系热扩散研究中存在的五个亟需解决的问题,即:1)对不同硅酸盐体系的热扩散规律的研究还不够全面;2)对微量元素的热扩散行为认识不足;3)硅酸盐体系热扩散作用的影响因素及尺度还不够明确;4)热扩散作用的地质与地球化学关键识别标志有待确立;5)硅酸盐体系热扩散作用的理论模型有待建立。尽管硅酸盐体系热扩散的研究还存在诸多不足,但越来越多的证据表明,热扩散是地质过程中不容忽视的一种成分分异机制。这种机制会造成岩浆房或岩浆通道中的元素分异和同位素分馏,可能对于一些成分分异的岩石和矿床的形成具有重要的意义。  相似文献   

Rhyacian and Neoproterozoic magmatic associations of the Gurupi Belt,Brazil: Implications for the tectonic evolution,and regional correlations     

《地学前缘(英文版)》2020,11(6):2243-2269

The Gurupi Belt, in north-northeastern Brazil, is a mobile belt developed in the south-southwestern margin of the São Luís cratonic fragment and crops out as a tectonic and erosional window within the Phanerozoic cover. Field, petrographic, geochemical, geochronological, and Nd isotopic information (new and published) constrain the timing and types of magmatic associations present in the belt and the tectonic settings in which they formed. The Rhyacian was the main period of magmatic activity, which can be grouped into two main stages. (1) ~2185–2130 ​Ma: pre-collisional, juvenile, calc-alkaline magnesian and calcic ferroan granitoid suites, and minor calc-alkaline and tholeiitic mafic plutonism (now amphibolites), formed in intra-oceanic to transitional/continental arcs; and intra- or back-arc volcano-sedimentary basin. (2) ~2125–2070 ​Ma: syn- (two-mica granites) to late-collisional (potassic to shoshonitic granites and quartz-syenite) plutonic suites produced after crustal thickening and melting, with localized migmatization, that intruded during the compressive D₁ deformational phase and concomitantly with greenschist to amphibolite metamorphism. There is a zonation of the Rhyacian episodes, with intra-oceanic stages occurring to the northeast, and the continental arc and collisional phases occurring to the southwest, indicating the presence of an active continental margin to the southwest, and subduction from NE to SW (present-day configuration). This magmatic framework is a continuation to the south of what is described for the São Luís cratonic fragment to the north, and the orogenic scenario is identical to what is observed for the same period in the West African Craton (Eburnean/Birrimian orogen), which additionally supports previous geological correlations. In the Neoproterozoic, a few magmatic occurrences are recognized. An extensional event allowed the intrusion of an anorogenic, nepheline syenite at ca. 730 Ma, which was followed by the intrusion of a crustal, calc-alkaline microtonalite, of uncertain tectonic setting, at 624 ​Ma. Both intrusions underwent greenschist to amphibolite facies metamorphism between 580 Ma and 529 ​Ma. This metamorphic event is probably related to crustal thickening, which produced crustal melting and intrusion of two-mica granites between 595 Ma and 549 ​Ma. The absence of oceanic and arc-related assemblages, along with geophysical information about the basement of the Phanerozoic cover indicates an intracontinental setting for the Neoproterozoic–Early Cambrian evolution of the Gurupi Belt, with rifting and posterior closure of the basin, without oceanization. Rifting and closure correlate in time with the onset of Rodinia breakup and West Gondwana assembly, respectively, but we interpret the events in the Gurupi Belt as having no direct role in these two global supercontinent-related events, but, instead, as being related to orogenic events occurring in the periphery of the West African and Amazonian cratons at that time.  相似文献   

ROCKS & MINERALS DETERMINATION AND ANALYSIS     

《Abstract of Chinese Geological Literature》2014,(3)

正20141456 Chen Juan(Laboratory of High Temperature and High Pressure Study of the Earth’s Interior,Institute of Geochemistry,Chinese Academy of Sciences,Guiyang550002,China);Tang Hongfeng Progress in Analytical Methods of Tungsten Isotope and Experimental Research on Digestion of Whole  相似文献   

Archean (about 2500 Ma) anatexis in eastern North China Block     

《China Geology》2021,4(2):215-229

Two Neoarchean alkaline feldspar-rich granites sourced from partially melted granulite-facies granodioritic orthogneiss have been here recognised in the eastern part of the North China Block (NCB). These poorly foliated granites have previously been assumed to be Mesozoic in age and never dated, and so their significance has not been recognised until now. The first granite (AG1) is a porphyritic syenogranite with megacrystic K-feldspar, and the second (AG2) is a quartz syenite with perthitic megacryst. Zircons from the granites yield LA-ICP-MS U-Pb ages of 2499 ± 10 Ma (AG1), and 2492 ± 28 Ma (AG2), which are slightly younger than the granodioritic orthogneiss that they intrude with a crystallisation U-Pb age of 2537 ± 34 Ma. The younger granites have higher assays for SiO₂ (71.91% for AG1 and 73.22% for AG2) and K₂O (7.52% for AG1 and 8.37% for AG2), and much lower assays for their other major element than the granodioritic orthogneiss. All of the granodioritic orthogneiss and granite samples have similar trace element patterns, with depletion in Th, U, Nb, and Ti and enrichment in Rb, Ba, K, La, Ce, and P. This indicates that the granites are derived from the orthogneiss as partial melts. Although they exhibit a similar REE pattern, the granites have much lower total REE contents (30.97×10⁻⁶ for AG1, and 25.93×10⁻⁶ for AG2), but pronounced positive Eu anomalies (Eu/Eu* = 8.57 for AG1 and 27.04 for AG2). The granodioritic orthogneiss has an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.70144, εNd(t) value of 3.5, and εHf(t) values ranging from −3.2 to +2.9. The orthogneiss is a product of fractional crystallisation from a dioritic magma, which was derived from a mantle source contaminated by melts derived from a felsic slab. By contrast, the AG1 sample has an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.6926 that is considered too low in value, εNd(t) value of 0.3, and εHf(t) values between +0.57 and +3.82; whereas the AG2 sample has an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.70152, εNd(t) value of 1.3, and εHf(t) values between +0.5 and +14.08. These assays indicate that a Sr-Nd-Hf isotopic disequilibrium exists between the granite and granodioritic orthogneiss. The elevated εHf(t) values of the granites can be explained by the involvement of Hf-bearing minerals, such as orthopyroxene, amphibole, and biotite, in anatectic reactions in the granodioritic orthogneiss. Based on the transitional relationship between the granites and granodioritic orthogneiss and the geochemical characteristics mentioned above, it is concluded that the granites are the product of rapid partial-melting of the granodioritic orthogneiss after granulite-facies metamorphism, and their crystallisation age of about 2500 Ma provides the minimum age of the metamorphism. This about 2500 Ma tectonic-metamorphic event in NCB is similar to the other cratons in India, Antarctica, northern and southern Australia, indicating a possible connection between these cratons during the Neoarchean.  相似文献   

“井水不犯河水”水文地质学含义初步探讨   总被引：1，自引：0，他引：1  

刘元章  武强  刘久荣  林沛  邢立亭  刘守强 《地下水》2011,(6):122-127

利用水化学及同位素水文地质学的方法,对密云水库、潮白河及位于潮白河边的2个地下水分层专门监测井进行取样测试,并结合收集相关资料,从水位、水质及氚、氘、氧-18等同位素方面进行分析研究,对中国古语“井水不犯河水”的水文地质学方面的含义进行了一定的微观层次的分析探讨.得出研究点处潮白河水对50 m外的25 m深的浅井井水有...  相似文献   

Defining dynamic pelagic habitats in oceanic waters off eastern Australia     

A.J. Hobday  J.W. YoungC. Moeseneder  J.M. Dambacher 《Deep Sea Research Part II: Topical Studies in Oceanography》2011,58(5):734-745

Although many species in the pelagic ocean are widespread, they are not randomly distributed. These species may have associations with particular water masses or habitats, but to best understand patterns in the ocean, these habitats must be identified. Previous efforts have produced static or seasonal climatologies, which still represent smearing over habitats. The Eastern Tuna and Billfish Longline Fishery (ETBF) targets a range of high trophic level species in oceanic waters off eastern Australia. In this study, dynamic ocean habitats in the region were identified for each month based on cluster analysis of five oceanographic variables averaged at a monthly time scale and a spatial scale of 0.5° for the period 1995-2006. A total of seven persistent habitats were identified off eastern Australia with intra and interannual variation in size and location, indicating the importance of spatial and temporal variation in the dynamics of the region. The degree to which these dynamic habitats were distinguished was tested using (i) stable isotope analysis of top fish predators caught in the region and (ii) estimates of variation in estimated abundance generated from catch data from the fishery. More precise estimates (measured as lower total CV) of isotopic values from swordfish (Xiphias gladius), yellowfin tuna (Thunnus albacares) and albacore (Thunnus alalunga) were obtained for 4 of 6 isotope comparisons using the dynamic habitat groupings, which indicate that stratifying by pelagic habitat improved precision. Dynamic habitats produced more precise abundance estimates for 7 of 8 large pelagic species examined, with an average reduction in total CV of 19% compared to when abundance was estimated based on static habitat stratification. These findings could be used to guide development of effective monitoring strategies that can distinguish patterns due to environmental variation, and in the longer term, climate change.  相似文献   

Vegetation history of the penultimate glacial period (Marine isotope stage 6) at Ioannina,north‐west Greece     

K. H. Roucoux  P. C. Tzedakis  I. T. Lawson  V. Margari 《第四纪科学杂志》2011,26(6):616-626

The vegetational history of the penultimate glacial period, Marine Isotope Stage (MIS) 6 (c. 185–135 ka), has remained relatively unexplored. Here we present a new record from the Ioannina basin, north‐west Greece, which constitutes the highest‐resolution terrestrial pollen record for this interval produced to date. It shows that the vegetation history of MIS 6 in this region can be divided into two parts: an early period (185–155 ka) with pronounced oscillations in tree population extent, and a later period (155–135 ka) with much smaller tree populations and subdued oscillations. This pattern is analogous to the MIS 3/MIS 2 division during the last glacial in the same sequence, although the early part of MIS 6 had larger Pinus populations and fewer temperate trees relative to the equivalent interval in MIS 3. This implies cooler and wetter conditions, which is somewhat counterintuitive given the high summer insolation during MIS 6e, but is in line with other palaeoclimatic evidence from the Mediterranean. Comparison with North Atlantic records suggests that despite the absence of pronounced iceberg discharges during MIS 6, North Atlantic millennial‐scale variability had a significant downstream impact on tree populations in north‐west Greece. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

¹⁵N示踪-离子质谱法测定新生产力的研究   总被引：4，自引：0，他引：4       下载免费PDF全文

焦念志  王荣  黄庆文 《海洋与湖沼》1993,24(1):65-71

本文提出了~(15)N示踪-离子质谱法实测新生产力的方法,并于1991年11月—1992年5月结合胶州湾现场调查的实际样品,对示踪剂量、培养时间、样品贮存、消解剂量和时间、铵离子的扩散吸收,以及方法的空白值、检出限、精密度和准确度等进行了实验研究。结果表明,该法具有较高的精密度和良好的准确度,与经典的气体质谱法相比,还具有进样量少以及可同时分析~(15)N丰度、总氮浓度和水中铵离子浓度等特点,对于含氮量极低、铵氮再生迅速的海洋环境中新生产力的研究,具有特殊的意义。  相似文献   

Study on Water Internal Recycle Process and Mechanism in Typical Mountain Areas of Inland Basins,Northwest China: Progress and Challenge     

Yongjian Ding  Shiqiang Zhang 《地球科学进展》2018,33(7):719-728

The water resource and its change of mountainous area are very important to the oasis economic system and ecosystem in the arid areas of northwest China. Accurately understanding the water transfer and circulation process among vegetation, soil, and atmosphere over different hydrological units in mountainous areas such as snow and ice, cold desert, forest and grassland is the basic scientific issue of water research in cold and arid regions, which is also the basis of water resource delicacy management and regulation. There are many research results on the hydrological function of different land covers in mountain areas, basin hydrological processes, however, there are only very limited studies on the water internal recycle at basin scale. The quantitative study on the mechanism of water internal recycle is still at the starting stage, which faces many challenges. The key project “Study on water internal recycle processes and mechanism in typical mountain areas of inland basins, Northwest China” funded by National Natural Science Foundation of China will select the Aksu River and Shule River Basin, which have better observation basis, as study area. The internal mechanism of moisture transfer and exchange process of different land cover and atmosphere, the internal mechanism of water cycle in the basin, and water transfer paths in atmosphere will be studied through enhancing runoff plot experiments on different land cover, analyzing the mechanism of water vapor transfer and exchange between different land covers in the watershed by isotope tracing on the water vapor flux of vegetation water, soil moisture and atmospheric moisture, improving the algorithms of remote sensing inversion and ground verification on land surface evapotranspiration on different land cover, and analyzing the water vapor flux from reanalysis data, and the coupling modeling of regional climate model and land surface process model. At last, the effect of different land cover in hydrological process of mountain area, and the impact of land cover on downstream oasis will be systematically analyzed.  相似文献