地震荷载下长短桩复合地基的群桩效应系数分析

长短桩复合地基中,地震发生时,短桩在降低长桩所受剪力和弯矩影响中起到重要作用。本研究对弹性地基中的钢管桩基础实行了静态有限元分析,以及动态离心模型试验,通过桩径、桩长、桩间距等参数进行剪切波速,长桩剪力弯矩的对比分析,得到一种新的水平向群桩效应系数计算方法,并通过对模型建筑物的模拟,验证了该方法的有效性与实用性。结果表明,静态有限元数值模拟分析和离心模型试验结果有较好拟合,不同震级作用下通过该方法均能给出较为合理的系数。本文结果可为长短桩复合桩基的抗震性能提供重要参考,实际工程中,通过调整桩径,桩长,短桩数量等参数,可以提出更为合理的设计施工方案。     

金伯利岩:地球深部探测的重要探针

金伯利岩是一种偏碱性的超基性岩,来源于地幔深部,富含挥发份和钾质,属于火成岩类,金伯利岩中主要含有镁铝榴石、金刚石、橄榄石、铬铁矿、铬透辉石、镁钛铁矿,锆石、碳硅石等造岩矿物.同时金伯利岩也被认为是含金刚石最主要的岩石.本文通过文献调研方法,野外现场表明金伯利岩中含有深源包裹体;全球金伯利岩主要分布在俄罗斯、博茨瓦纳、加拿大、安哥拉、南非、刚果民主和纳米比亚;中国金伯利岩主要分布在华北地台,在山东、辽宁、吉林、山西、河南和新疆等地.这些金伯利岩常常与深大断裂甚至地幔深部地质作用关系密切,常出现标志性矿物橄榄石、石榴石、高铬磁铁矿,伴有烃类或氢气.但含金刚石金伯利岩主要沿郯庐断裂带分布,如辽宁瓦房店、山东蒙阴等地.从时代上看,以往认为的早古生代的金伯利岩,更可能都是在早期形成于华北地台之岩石圈底部,而在中生代白垩纪时期才在大规模岩石圈拆沉的地质背景下的以快速上升的,尤其是那些含金刚石的金伯利岩岩管更是快速上升的典型代表,其标型矿物是镁铝榴石、高铬磁铁矿、钙钛矿等.国内辽宁瓦房店含金刚石金伯利岩产于郯庐断裂带东侧,有着与同期金伯利岩相同的岩石矿物学特征,其中的以50号岩管为代表的金刚石矿床是我国重要的战略矿产.金伯利岩及其中的金刚石带来众多直接的深部地幔信息,中国瓦房店、蒙阴一带的金刚石来自上地幔,而一些含硼蓝色金刚石则来自下地幔,不同层圈的金刚石携带不同的标志矿物,以橄榄石为例:来自上地幔金刚石携带的橄榄石为橄榄石;过渡带金刚石携带的主要为瓦兹利石和林伍德石;下地幔的金刚石则为布里奇曼石,它们是深部探测的重要探针.     

地表“矿物膜”中锐钛矿-β-胡萝卜素协同增强日光响应特性研究

为探究"矿物膜"中半导体矿物与光合色素间的日光响应协同作用,对采自安徽潜山市郊的"矿物膜"样品进行了矿物组成和色素组成分析。同步辐射X射线粉末衍射测定表明,样品富含锐钛矿、赤铁矿等半导体矿物;拉曼光谱证实其中存在地表广泛分布的光合色素:β-胡萝卜素。在此基础上开展锐钛矿-β-胡萝卜素协同增强日光响应模拟实验。合成锐钛矿电极并对其进行系统的矿物学表征,紫外-可见漫反射吸收谱计算其禁带宽度约为3.07 eV;莫特-肖特基计算得到的平带电位约为-0.16 V,载流子浓度约为3.25×1026 cm-3;光电化学测试结果显示其具有一定可见光光电转化能力。经β-胡萝卜素敏化处理后,锐钛矿电极平均光电流密度提升了400%,在425~550 nm间光吸收值提高,该波段位于日光辐射能量集中的波长范围内,且与β-胡萝卜素吸收范围吻合,表明二者间存在日光吸收及光电响应协同增强作用。     

深层碎屑岩自生矿物成因机理及其对储层物性的影响——以渤海歧南断阶带侏罗系为例

自生矿物特征和成因机理对深层碎屑岩储层物性具有重要影响。以渤海海域歧南断阶带侏罗系为例,通过岩心、薄片、扫描电镜、电子探针、同位素、包裹体、X衍射分析等技术手段,对研究区侏罗系深层碎屑岩储层自生矿物的类型、特征、成因机理及对优质储层发育的控制作用进行研究。结果表明：研究区侏罗系碎屑岩属于中孔-低渗储层,非均质性强;主要自生矿物类型为硅质矿物、碳酸盐矿物、黏土矿物3类;早期形成的硅质石英衬垫和碳酸盐胶结物控制了孔隙的发育和演化,沿颗粒表面分布的早期硅质石英衬垫有效抑制了后期石英加大生长,并增强了岩石的抗压能力,有利于原生孔隙保存;早期碳酸盐胶结物增强了储层抗压实能力,并为后期储层遭受溶蚀形成溶蚀孔提供了物质基础,有利于高孔隙储层形成;黏土矿物控制了储层渗透率差异,储层渗透率与自生高岭石体积分数呈正相关性,较高渗透率储层分布于侏罗系中段高岭石富集带内。研究区侏罗系碎屑岩发育原生孔隙体积分数高、微晶石英衬垫发育的Ⅰ类有利储层和粒间溶蚀孔隙较发育、具显著表生成岩作用特点的Ⅱ类有利储层,二者孔隙演化存在明显差异。Ⅰ类有利储层主要受早—中成岩期微晶石英衬垫抗压实作用控制,浅层和深层均可发育高孔隙储层;Ⅱ类有利储层主要受表生期风化淋滤作用控制,可在风化壳附近形成优质储层,整体上Ⅰ类储层物性优于Ⅱ类。     

豫西地区山西组—太原组页岩孔隙结构及其影响因素——以ZXY1井为例

页岩孔隙结构是储层评价的关键,对页岩气的勘探、开发以及页岩储层改造等都具有十分重要的意义。本文以ZXY1井太原组—山西组页岩为例,利用低温氮吸—脱附实验、X衍射、扫描电镜等实验手段研究页岩孔隙结构及其影响因素。结果表明,太原组—山西组页岩孔隙复杂,发育有机孔、微裂缝和多种无机孔;孔隙孔径主要为2~50 nm,以介孔为主,其次为微孔;孔容和比表面几乎全部由微孔和介孔提供。有机质、黏土矿物及其组合对孔隙结构影响分析表明,孔容与TOC含量、孔容与黏土矿物含量、比表面积与TOC含量、比表面积与黏土矿物含量均具有较为显著的正线性相关关系;TOC含量和黏土矿物含量数值乘积与孔容、比表面积表现为非常显著的正线性相关关系;TOC含量和黏土矿物含量双参数回归孔容与比表面积,相关系数高,误差较小。综合分析认为,有机质和黏土矿物对孔隙结构的影响并非简单正线性相关关系,而是复杂的正向关系。     

Sedimentary processes controlling ultralong cells of littoral transport: Placer formation and termination of the Orange sand highway in southern Angola

This study focuses on the causes, modalities and obstacles of sediment transfer in the longest cell of littoral sand drift documented on Earth so far. Sand derived from the Orange River is dragged by swell waves and persistent southerly winds to accumulate in four successive dunefields in coastal Namibia to Angola. All four dunefields are terminated by river valleys, where aeolian sand is flushed back to the ocean; and yet sediment transport continues at sea, tracing an 1800 km long submarine sand highway. Sand drift would extend northward to beyond the Congo if the shelf did not become progressively narrower in southern Angola, where drifting sand is funnelled towards oceanic depths via canyon heads connected to river mouths. Garnet–magnetite placers are widespread along this coastal stretch, indicating systematic loss of the low‐density feldspatho‐quartzose fraction to the deep ocean. More than half of Moçamedes Desert sand is derived from the Orange River, and the rest in similar proportions from the Cunene River and from the Swakop and other rivers draining the Damara Orogen in Namibia. The Orange fingerprint, characterized by basaltic rock fragments, clinopyroxene grains and bimodal zircon‐age spectra with peaks at ca 0·5 Ga and ca 1·0 Ga, is lost abruptly at Namibe, and beach sands further north have abundant feldspar, amphibole‐epidote suites and unimodal zircon‐age spectra with a peak at ca 2·0 Ga, documenting local provenance from Palaeoproterozoic basement. Along with this oblique‐rifted continental margin, beach placers are dominated by Fe–Ti–Cr oxides with more monazite than garnet and thus have a geochemical signature sharply different from beach placers found all the way along the Orange littoral cell. High‐resolution mineralogical studies allow us to trace sediment dispersal over distances of thousands of kilometres, providing essential information for the correct reconstruction of ‘source to sink’ relationships in hydrocarbon exploration and to predict the long‐term impact of man‐made infrastructures on coastal sediment budgets.     

Understanding heavy mineral enrichment using a three‐dimensional numerical model

Layered deposits of relatively light and heavy minerals can be found in many aquatic environments. Quantification of the physical processes which lead to the fine‐scale layering of these deposits is often limited with flumes or in situ field experiments. Therefore, the following research questions were addressed: (i) how can selective grain entrainment be numerically simulated and quantified; (ii) how does a mixed bed turn into a fully layered bed; and (iii) is there any relation between heavy mineral content and bed stability? Herein, a three‐dimensional numerical model was used as an alternative measure to study the fine‐scale process of density segregation during transport. The three‐dimensional model simulates particle transport in water by combining a turbulence‐resolving large eddy simulation with a discrete element model prescribing the motion of individual grains. The granular bed of 0·004 m in height consisted of 200 000 spherical particles (D50 = 500 μ m). Five suites of experiments were designed in which the concentration ratio of heavy (5000 kg m⁻³) to light particles (i.e. 2560 kg m⁻³) was increased from 6%, 15%, 35%, 60% to 80%. All beds were tested for 10 sec at a predefined flow speed of 0·3 m sec⁻¹. Analysis of the particle behaviour in the interior of the beds showed that the lighter particles segregated from the heavy particles with increasing time. The latter accumulated at the bottom of the domain, forming a layer, whereas the lighter particles were transported over the layer forming sweeps. Particles below the heavy particle layer indicated that the layer was able to armour the particles below. Consequentially, enrichment of heavy minerals in a layer is controlled by the segregation of a heavy mineral fraction from the light counterpart, which enhances current understanding of heavy mineral placer formation.     

Comparisons among the Oortsog,Dulaan, and Nomgon mafic-ultramafic intrusions in central Mongolia and Ni-Cu deposits in NW China: implications for economic Ni-Cu-PGE ore exploration in central Mongolia

Although there are many mafic-ultramafic intrusions in the western and central regions of Mongolia, Central Asian Orogenic Belt (CAOB), no economic-grade Ni-Cu deposits have yet been discovered. To understand the economic Ni-Cu deposit potential of the intrusions in central Mongolia, the parental magma affinity and sulfide saturation of the Oortsog, Dulaan, and Nomgon Ni-Cu mineralized mafic-ultramafic intrusions are studied. These three intrusions are predominantly gabbroic in composition, while the Oortsog and Dulaan intrusions also contain small proportions of peridotites. The parental magmas of the Oortsog and Dulaan intrusions are tholeiitic, as indicated by their Cr-spinel and clinopyroxene compositions, whereas the parental magma of the Nomgon intrusions is likely calc-alkaline. The compositions of Cr-spinel and clinopyroxene, combined with the presence of significant Nb-Ta depletions, indicate that these rocks were most likely derived from modified mantle sources. Both the Oortsog and Nomgon intrusions form two clusters in terms of their olivine composition, suggesting that multiple magma surges were involved during their emplacement. The relatively low Fo values and Ni contents in olivine from the three intrusions compared to those from Ni-Cu deposits in NW China, as well as those in the Voisey’s Bay deposit in Canada, indicate that the three intrusions were crystallized from relatively evolved magmas. The Cu/Zr ratios of rocks of the Oortsog, Dulaan, and Nomgon intrusions are higher than 1, suggesting that these rocks contain cumulus sulfide. This, coupled with the presence of rounded sulfide inclusions in olivine of the Oortsog and Dulaan intrusions, suggests that sulfide saturation occurred before or during olivine crystallization. The distribution patterns of platinum group elements (PGEs) of the Dulaan and Oortsog intrusions record slight Rh, Pt, and Pd (PPGE) enrichment relative to Os, Ir, and Rh (IPGE). Furthermore, the Ni/Cu ratios of sulfide-bearing rocks from the Oortsog intrusion vary from 1.8 to 3.8, which are consistent with those of the Ni-Cu sulfide deposits in NW China. In contrast, the Ni/Cu ratios of sulfide-bearing rocks from the Nomgon intrusion are extremely low (0.03 to 0.07). This, together with the significant enrichment in PPGE relative to IPGE, suggests that these sulfides of the Nomgon intrusion were segregated from a magma that was extremely enriched in Cu and PPGE but depleted in Ni and IPGE. The characteristics of the chalcophile elements in these intrusions are attributed to the fact that the derivation of the Nomgon magma was significantly different from that of the Dulaan and Oortsog parental magmas. Overall, although the parental magmas of the intrusions in central Mongolia are more evolved than those in NW China, they are comparable in terms of the sizes of their intrusions, constituent minerals, and mineral chemistry. These similarities suggest that the intrusions in central Mongolia have economic Ni-Cu sulfide potential. Furthermore, intrusions similar to the Nomgon intrusion may feature PGE mineralization potential.     

The sum of component concentrations as a quality indicator in X-ray electron probe microanalysis of minerals

X-ray electron probe microanalysis is a complete mineral analysis technique. Therefore, the sum of obtained concentrations can be used as an indicator of the quality of analysis. There are two kinds of errors of the sum of concentrations. The first is related to the deviation of the sum from the basic value and can be called a difference error. The second is the scattering of results around the mean value and can be called an averaging error. The difference error characterizes the reproducibility of the measurements and equals 0.41% at the current precision level. The averaging error characterizes the repeatability of the measurements and equals 0.23%. The difference error can be used as a quality indicator directly during analysis, while the averaging error can be used after analysis of a batch of specimens. According to the modified three-sigma rule, the acceptable sum values of separate analyses are in the range 98.8-100.8%. This range is obviously valid only for an ideal model with a basic sum value of 100%. In real analyses, the basic value deviates from 100% because of the presence of elements which are not detected in the analysis and elements with a variable valence. According to the three-sigma rule, the acceptable scattering of the sum of concentrations around the average value is ± 0.7%.     

K–Ar dating of authigenic minerals in siliciclastic sequences: an example from the south Sanfranciscana Basin (Western Minas Gerais,Brazil)

K–Ar dating was applied on authigenic potassic minerals which are abundant in sandstones from the south of the Sanfranciscana Basin, Western Minas Gerais State, central Brazil. The Quintinos Member fluvial sandstones (Três Barras Formation, Areado Group) contain significant amounts of authigenic K‐feldspar as microcrystals of adularia and sanidine habits. The ages of these microcrystals cluster into three groups: 106.1 ± 2.2, 89.9 ± 1.9 and 88.8 ± 1.8 Ma (from Albian to Coniacian). The older age of 106.1 ± 2.2 Ma was obtained from the coarse fraction analysed (10–20 µm) that can contain a mixture of detrital potassic minerals (K‐feldspar, muscovite, biotite and illite) and different authigenesis of K‐feldspar (overgrowths and microcrystals). Thus, only the younger ages were interpreted as precipitation of K‐feldspar microcrystals during the Late Cretaceous into the Quintinos Member sandstones. Moreover, these ages can document the formation of microcrystals within a few million years after deposition of the sandstones. The ages of authigenic illite from the Capacete Formation epiclastic sandstones (Mata da Corda Group) range from 88.5 ± 1.9 to 71.5 ± 1.9 Ma (Coniacian–Campanian). These results suggest the timing of the illitization event in these sandstones as well as a synchrony with K‐feldspar authigenesis in the Quintinos Member sandstones. These results are well constrained and are in agreement with stratigraphic, biostratigraphic and radiometric ages previously reported for the Sanfranciscana Basin. Copyright © 2014 John Wiley & Sons, Ltd.