Fluid-mobile trace element constraints on the role of slab melting and implications for Archaean crustal growth models   总被引：11，自引：0，他引：11  

Balz S. Kamber  Anthony Ewart  Kenneth D. Collerson  Michael C. Bruce  Graeme D. McDonald 《Contributions to Mineralogy and Petrology》2002,144(1):38-56

We use published and new trace element data to identify element ratios which discriminate between arc magmas from the supra-subduction zone mantle wedge and those formed by direct melting of subducted crust (i.e. adakites). The clearest distinction is obtained with those element ratios which are strongly fractionated during refertilisation of the depleted mantle wedge, ultimately reflecting slab dehydration. Hence, adakites have significantly lower Pb/Nd and B/Be but higher Nb/Ta than typical arc magmas and continental crust as a whole. Although Li and Be are also overenriched in continental crust, behaviour of Li/Yb and Be/Nd is more complex and these ratios do not provide unique signatures of slab melting. Archaean tonalite-trondhjemite-granodiorites (TTGs) strongly resemble ordinary mantle wedge-derived arc magmas in terms of fluid-mobile trace element content, implying that they did not form by slab melting but that they originated from mantle which was hydrated and enriched in elements lost from slabs during prograde dehydration. We suggest that Archaean TTGs formed by extensive fractional crystallisation from a mafic precursor. It is widely claimed that the time between the creation and subduction of oceanic lithosphere was significantly shorter in the Archaean (i.e. 20 Ma) than it is today. This difference was seen as an attractive explanation for the presumed preponderance of adakitic magmas during the first half of Earth's history. However, when we consider the effects of a higher potential mantle temperature on the thickness of oceanic crust, it follows that the mean age of oceanic lithosphere has remained virtually constant. Formation of adakites has therefore always depended on local plate geometry and not on potential mantle temperature.  相似文献   

数值预报产品在夏季持续高温预报中的释用   总被引：6，自引：0，他引：6  

汪秀清  杨雪艳  石大明 《气象科技》2004,32(Z1):47-49

利用 1999～ 2 0 0 2年 6～ 8月 96～ 192h日本数值预报产品 85 0hPa气温与吉林省的日平均气温、阶段性高温及极端最高气温 (长春市 )资料 ,通过线性分析 ,找出了日本数值预报产品的 85 0hPa气温与吉林省地面日平均气温、阶段性高温及极端最高气温的对应关系 :吉林省 6～ 8月日平均气温在 96～ 192h的 85 0hPa日本数值预报长春站日平均值上加 6 6～ 4 1℃ ,6～ 8月的极端最高气温在相应的日本数值预报产品上加 9 3～ 13 0℃。  相似文献   

Volcanic resurfacing and the early terrestrial crust: Zircon U-Pb and REE constraints from the Isua Greenstone Belt, southern West Greenland     

Balz Samuel Kamber  Martin J. Whitehouse  Stephen Moorbath 《Earth and Planetary Science Letters》2005,240(2):276-290

The oldest known bona fide succession of clastic metasediments occurs in the Isua Greenstone Belt, SW Greenland and consists of a variety of mica schists and rare metaconglomerates. The metasediments are in direct contact with a felsic metavolcanic lithology that has previously been dated to 3.71 Ga. Based on trace element geochemical data for > 30 metasediments, we selected the six samples with highest Zr concentrations for zircon extraction. These samples all yielded very few or no zircon. Those extracted from mica schists yielded ion probe U/Pb ages between 3.70 and 3.71 Ga. One metaconglomerate sample yielded just a single zircon of 3.74 Ga age.The mica schist hosted zircons have U/Pb ages, Th / U ratios, REE patterns and Eu anomalies indistinguishable from zircon in the adjacent 3.71 Ga felsic metavolcanic unit. Trace element modelling requires the bulk of material in the metasediments to be derived from variably weathered mafic lithologies but some metasediments contain substantial contribution from more evolved source lithologies. The paucity of zircon in the mica schists is thus explained by incorporation of material from largely zircon-free volcanic lithologies. The absence of older zircon in the mica schists and the preponderance of mafic source material imply intense, mainly basaltic resurfacing of the early Earth. The implications of this process are discussed.Thermal considerations suggest that horizontal growth of Hadean crust by addition of mafic-ultramafic lavas must have triggered self-reorganisation of the protocrust by remelting. Reworking of Hadean crust may have been aided by burial of hydrated (weathered) metabasalt due to semi-continuous addition of new voluminous basalt outpourings. This process causes a bias towards eruption of Zr-saturated partial melts at the surface with O-isotope compositions potentially different from the mantle. The oldest zircons hosted in sediments would have been buried to substantial depth or formed in plutons that crystallised at some depth, from which it took hundreds of millions of years for them to be exhumed and incorporated into much younger sediments.  相似文献   

Variations of Hydrodynamics and Submarine Groundwater Discharge in the Yellow River Estuary Under the Influence of the Water-Sediment Regulation Scheme   总被引：1，自引：0，他引：1  

Dong Xia  Zhigang Yu  Bochao Xu  Maosheng Gao  Tiezhu Mi  Xueyan Jiang  Peng Yao 《Estuaries and Coasts》2016,39(2):333-343

The “Water-Sediment Regulation Scheme” (WSRS) is critically important to the hydrologic evaluation of the Yellow River estuary since a huge pulse of water and sediment are delivered into the sea during a short period. We used the natural geochemical tracers radium (²²³Ra, ²²⁴Ra, ²²⁶Ra) and radon (²²²Rn) isotopes as well as other hydrological parameters to investigate the mixing variations and submarine groundwater discharge (SGD) in the Yellow River estuary under the influence of the 2013 WSRS. Dramatically elevated radium and radon isotopic activities were observed during this WSRS compared with activities measured during a non-WSRS period. Radium “water ages” indicated that the offshore transport rate nearly tripled when the river discharge increased from 400 to 3400 m³/s. We calculated the SGD flux in the Yellow River estuary based on a radium mass balance model as well as radium and radon time-series models. The SGD flux was estimated at 0.02~0.20 m/day during a non-WSRS period and 0.67~1.22 m/day during the 2013 WSRS period. The results also indicate that large river discharge tends to lead more intense SGD along the river channel direction with a large amount of fresh SGD.  相似文献   

4种蒸发波导模型的对比与分析     

刘立行  李煜斌  高志球  毕雪岩  朱光明 《气象科学》2019,39(1):78-92

蒸发波导模型常用来计算海上蒸发波导高度。为了认识当前不同蒸发波导模型之间的差异和方法,本文选取了目前使用广泛的4种蒸发波导模型(即P-J模型、Babin模型、NPS模型和伪折射率模型)进行对比和分析。本文首先探讨了在理想情况下它们对气象要素的敏感性,随后并利用我国南海近海大气层观测试验数据对比了这4种模型的蒸发波导高度计算结果。分析表明:相对湿度、风速和气—海温差的变化对4种模型的计算都有着较大的影响,特别是在不稳定层结状况下,4种模型计算得到的蒸发波导高度都随着相对湿度的增大而降低、随着风速的增大而增高。Babin模型和NPS模型计算的波导高度较为一致,伪折射率模型与前两种模型的计算结果存在差异,而P-J模型与其他3种模型存在较明显的偏差。基于南海气象数据的计算结果表明,不同蒸发波导模型在该海域蒸发波导的模拟结果略有不同,但4种模型计算得到的波导高度日变化变化趋势较为一致,波导高度极低值常出现在早晨,而极高值常出现在傍晚。  相似文献   

多重压力对重点生态功能区农户生计资产可得性的影响——以甘南黄河水源补给区为例   总被引：1，自引：1，他引：0  

高志玉  赵雪雁  兰海霞  石育中 《地理研究》2020,39(5):1152-1165

重点生态功能区主要承担着提供持续、稳定生态服务的功能,农户作为维护其主体功能的最基础单元和该区最主要的经济活动主体,面临着自然、社会、政策等多重压力,其交互作用不仅加剧了农户的生计脆弱性,更影响到该区主体功能的发挥,当前急需辨明多重压力对农户生计的影响,以便寻求有效的生计脆弱性减缓对策。本文以甘南黄河水源补给区为例,运用入户调查数据与有序Logistic回归模型等方法在识别农户遭受关键压力的基础上,进一步探讨生计压力的交互作用对生计资产可得性的影响。结果显示：① 甘南黄河水源补给区大多数农户遭受自然+经济型多重压力,其中,重点保护区、恢复治理区及经济示范区农户遭受的多重压力分别以自然+经济型、自然+经济型、经济+社会型为主。② 在多重压力的冲击下,重点保护区与恢复治理区农户除了社会资产可得性变化处于稳定状态,其余各类资产可得性变化几乎处于降低态势,而经济示范区农户的金融资产与社会资产的可得性变化处于稳定状态,其余各类资产可得性变化均处于降低态势。③ 婚嫁支出高与农牧产品价格下降、牲畜患病与人畜饮用水困难、子女学费支出高+婚嫁支出高+农牧产品价格下降+养老无保障、建房开支高与生态政策、子女就业困难与农业病虫害等压力的交互作用分别对自然资产、物质资产、金融资产、社会资产、人力资产等的可得性变化影响最强烈。  相似文献   

贫困山区脱贫农户的生计可持续性及生计干预——以陇南山区为例   总被引：6，自引：3，他引：3  

赵雪雁  刘江华  王伟军  兰海霞  马平易  杜昱璇 《地理科学进展》2020,39(6):982-995

增强贫困山区脱贫农户的生计可持续性不仅是新时期农村扶贫的现实需求,更是推动乡村振兴的客观要求。论文从生计资本、生计策略及生计环境出发,建立了脱贫农户的生计可持续性评价指标体系,利用陇南山区脱贫农户的入户调查资料,评估贫困山区脱贫农户的生计可持续性、识别脱贫农户的生计障碍。研究发现:① 从川坝河谷区、半山区到高山区,从早期脱贫到后期脱贫,脱贫农户的生计可持续性依次降低,且务工型与农工互补型脱贫农户的生计可持续性强于其他生计方式农户;② 陇南山区生计不可持续脱贫农户比重达28.83%,高山区、传统务农型及后期脱贫户中生计不可持续农户比重较高,而川坝河谷区、农工互补型与务工型及早期脱贫户中该比重较低;③ 生计不可持续脱贫户均面临着多元生计障碍,其中近2/3的农户面临多维资本—环境阻滞型与多重要素阻滞型障碍;④ 针对生计不可持续脱贫农户面临的多元生计障碍,需分类实施多维生计干预。  相似文献   

甘南高原乡村社会固有脆弱性及其影响因素   总被引：2，自引：1，他引：1  

李花  赵雪雁  王伟军  李巍 《地理科学》2020,40(5):804-813

开展社会固有脆弱性研究不仅有助于了解区域社会系统的可持续发展状态,更有助于寻求提高社会系统适应能力的恰当策略。通过构建乡村社会固有脆弱性分析框架和评价指标体系,并以甘南高原为研究区,在分析乡村社会固有脆弱性特征的基础上识别了其关键影响因子。结果表明：① 甘南高原乡村社会固有脆弱性呈“梭型”分异,其中,高、中、低脆弱性乡镇分别占18.95%、47.37%、33.68%。② 社会固有脆弱性存在明显的区域差异性,从农区-半农半牧区-牧区、高山峡谷区-山地丘陵区-山原区,社会固有脆弱性趋于增加;同时,随着少数民族聚居度降低、经济发展水平升高,社会固有脆弱性趋于降低。③ 社会固有脆弱性在空间上呈“北高南低”的不均衡分布,敏感性呈“中间高四周低”的集聚分布,适应能力呈相对均衡的“碎片化”分布。④ 气温、生育结构、民族结构、住房安全性、社会弱势群体比重、经济条件、信息可得性和参保比例是影响社会固有脆弱性的关键因子。最后,提出相应的对策建议。  相似文献   

北极薄冰-煎饼冰区域中不同重力波驱动形成海流的敏感性分析     

张雪妍  戴海瑨  赵军  殷何卿 《海洋学报(英文版)》2020,39(3):123-129

A coupled ocean-ice-wave model is used to study ice-edge jet and eddy genesis during surface gravity wave dissipation in a frazil-pancake ice zone. With observational data from the Beaufort Sea, possible wave dissipation processes are evaluated using sensitivity experiments. As wave energy dissipated, energy was transferred into ice floe through radiation stress. Later, energy was in turn transferred into current through ocean-ice interfacial stress. Since most of the wave energy is dissipated at the ice edge, ice-edge jets, which contained strong horizontal shear, appeared both in the ice zone and the ocean. Meanwhile, the wave propagation direction determines the velocity partition in the along-ice-edge and cross-ice-edge directions, which in turn determines the strength of the along-ice-edge jet and cross-ice-edge velocity. The momentum applied in the along-ice-edge(cross-ice-edge)direction increased(decreased) with larger incident angle, which is favorable condition for producing stronger mesoscale eddies, vice versa. The dissipation rate increases(decreases) with larger(smaller) wavenumber, which enhances(reduces) the jet strength and the strength of the mesoscale eddy. The strong along-ice-edge jet may extend to a deep layer(> 200 m). If the water depth is too shallow(e.g., 80 m), the jet may be largely dampened by bottom drag, and no visible mesoscale eddies are found. The results suggest that the bathymetry and incident wavenumber(magnitude and propagation direction) are important for wave-driven current and mesoscale eddy genesis.  相似文献   

山普拉墓地出土眼珠的结构工艺研究          下载免费PDF全文

张雪雁  郭金龙  成倩  王博  雷勇 《CT理论与应用研究》2021,30(2):161-169

新疆和田地区的山普拉墓地位于古丝绸之路南线的洛浦县城西南14公里处.20世纪80年代,考古人员对山普拉墓地进行多次发掘,其中玻璃珠类共计发现1369颗.本文主要以X射线CT技术为基础,对其中具有典型性代表的4颗蜻蜓眼玻璃珠从玻璃眼珠的基体、眼部装饰、气孔和珠孔等方面进行了深入分析,从而推测了最晚公元4世纪的历史背景下,...  相似文献