Dynamics of phosphorus in sediments of a naturally acidic lake

The mechanisms which controls the fixation and/or release of P in sediment of an extremely acidic lake（pH = 2.0 to 3.0） and its response to the influence of eutrophic urban waste water were investigated.The results,in the chemical composition,in the mineralogy of the sediment and in the material as obtained from sediment traps,show that the lake sediments are mainly volcanic material reflecting volcanic features of the basin.The sedimentation rate calculated for the lake(2.5×10^-2 mg m^-2 day^-1) was higher than that observed in other similar glacial lakes in both Andean Patagonia and also elsewhere in the world.The Total Phosphorus concentration in sediments was higher than figures reported by other authors for mining acid lakes,and the main fraction of P was found associated with organic matter.There was no control by Fe or Al on P,because both are in solution at pH < 3.0.It was concluded that changes in the natural input of nutrients（derivatives of Copahue volcano fluid,the discharge of sewage,or basin run-off） are responsible for a high concentration of SRP and N-NH₄⁺ in the lake.Laboratory experiments showed that sediments have no ability to retain phosphorus and a continuous release of P from the sediments into the water column was observed.The assays where the pH was artificially increased showed that the P still remains in solution until at least pH 7.0.It was concluded that changes in the natural input of nutrients due to:1) the volcanic fluids,2) the increase in sewage charges,or 3) surface runoff upstream,maintain a high trophic state with high concentrations of dissolved P and N-NH₄⁺,although the threshold of neutral pH in the lake is exceeded.This study will enable a better understanding about of the mechanism of release/fixation of phosphorus in acidic sediments in order to assist in making decisions regarding the conservation and management of this natural environment.     

ELASTIC ANALYSIS OF BURIED STRUCTURES SUBJECT TO THREE-DIMENSIONAL SURFACE LOADING

A method is presented which may be used to compute the displacements, strains and moments (both in-plane and transverse) in buried structures such as pipelines and culverts subjected to longitudinal bending. This type of bending can occur if a surface loading such as a vehicular loading or an embankment loading is applied to the soil above the pipe or culvert. Fourier transforms are used to reduce the three-dimensional problem to one involving only two spatial directions, thereby reducing the data preparation and computation time. Conventional finite element analysis is used to approximate the field quantities in the transformed two-dimensional plane. Two Fourier integral element types have been developed which have many applications in geotechnical engineering.     

The Recrystallization Front of the Ronda Peridotite: Evidence for Melting and Thermal Erosion of Subcontinental Lithospheric Mantle beneath the Alboran Basin

Evidence for a major heating event accompanied by decompressionwas recently reported from crustal rocks drilled in the Alboranbasin. The metamorphic evolution recorded by these rocks impliescomplete removal of lithospheric mantle during the Cenozoic,a process that is confirmed by geophysical modelling indicatingthin lithosphere beneath the Alboran domain. In this region,the Ronda lherzolite massif (Betic Cordillera, southern Spain)provides a unique opportunity for the observation of mantleprocesses associated with lithospheric thinning. A strikingfeature of the Ronda peridotite is a narrow recrystallizationfront, which has been ascribed to kilometre-scale porous meltflow through the massif. The front separates the spinel tectonitedomain, interpreted as old, veined lithospheric mantle, fromthe granular domain where the lithospheric microstructures,mineralogical assemblages and geochemical signatures were obliteratedby grain growth coeval with pervasive infiltration of basalticmelts. On the basis of trace-element abundances in peridotitescollected over a distance of 12 km along the recrystallizationfront, our study confirms that the front is a relatively sharp(     

Age and Petrogenesis of Two Late Archean Magmatic Suites, Northwestern Superior Province, Canada: Zircon U-Pb and Lu-Hf Isotopic Relations

The paper presents U–Pb ages for zircon, titanite, andmonazite, and Hf isotopic data for zircon, from the rocks oftwo magmatic suites occurring mostly in the Archean Uchi Subprovinceand partly in the neighbouring Berens River and English Riversubprovinces of the northwestern Superior Province, Ontario.These data, together with observations on the morphologies and,where evident, the inheritance of the zircon crystals, constrainthe nature of the sources of the magmas and provide a recordof various crustal processes in their evolution. The older of the two magmatic suites formed at 2744–2740Ma along segments of a common arc system. The suite consistsof (1) several trondhjemitic to granodioritic plutons, with_Hf values of 6•1, intruded into older crust and possiblyformed from magma produced by partial melting of subducted,juvenile oceanic crust; (2) an assemblage of dacitic pyroclasticvolcanic rocks, with _Hf values of 3•2–4•0, associatedwith tholeiitic basalts and probably derived from magma meltedfrom arc mantle; and (3) a bimodal assemblage of tholeiiticbasalts, rhyolites, and porphyries, also with _Hf values of 6•1,associated with a volcanogenic massive sulphide deposit andapparently formed by differentiation of mantle-derived basalticmelts at shallow levels in an extensional back-arc setting. The second magmatic suite, formed between 2702 and 2693 Ma,comprises late orogenic plutons and batholiths of dioritic todominantly granodioritic composition. The characteristics ofthese intrusions are consistent with a process combining meltingof a metasomatized mantle source and subsequent fractional crystallizationof the derived magmas at shallow depths. However, most of thestudied occurrences show evidence of crustal contamination throughvarious combinations of assimilation of lower-crustal material,assimilation of underthrust sedimentary rocks, and contaminationby wall rock materials during the latest stages in the emplacementof the plutons. The involvement of crustal material is indicatedby the presence of zircon xenocrysts and by _Hf values rangingfrom 1•4 to 4•4. Only one intrusion, with an _Hf valueof 5•0 and no xenocrystic zircon, appears to have escapedwidespread contamination, perhaps because the ascent of itsmagma was facilitated by a crustal-scale fracture system.     

Petrogenesis of Mafic Garnet Granulite in the Lower Crust of the Kohistan Paleo-arc Complex (Northern Pakistan): Implications for Intra-crustal Differentiation of Island Arcs and Generation of Continental Crust

We report the results of a geochemical study of the Jijal andSarangar complexes, which constitute the lower crust of theMesozoic Kohistan paleo-island arc (Northern Pakistan). TheJijal complex is composed of basal peridotites topped by a gabbroicsection made up of mafic garnet granulite with minor lensesof garnet hornblendite and granite, grading up-section to hornblendegabbronorite. The Sarangar complex is composed of metagabbro.The Sarangar gabbro and Jijal hornblende gabbronorite have melt-like,light rare earth element (LREE)-enriched REE patterns similarto those of island arc basalts. Together with the Jijal garnetgranulite, they define negative covariations of La_N, Yb_N and(La/Sm)_N with Eu^* [Eu^* = 2 x Eu_N/(Sm_N + Gd_N), where N indicateschondrite normalized], and positive covariations of (Yb/Gd)_Nwith Eu^*. REE modeling indicates that these covariations cannotbe accounted for by high-pressure crystal fractionation of hydrousprimitive or derivative andesites. They are consistent withformation of the garnet granulites as plagioclase–garnetassemblages with variable trapped melt fractions via eitherhigh-pressure crystallization of primitive island arc basaltsor dehydration-melting of hornblende gabbronorite, providedthat the amount of segregated or restitic garnet was low (<5wt %). Field, petrographic, geochemical and experimental evidenceis more consistent with formation of the Jijal garnet granuliteby dehydration-melting of Jijal hornblende gabbronorite. Similarly,the Jijal garnet-bearing hornblendite lenses were probably generatedby coeval dehydration-melting of hornblendites. Melting modelsand geochronological data point to intrusive leucogranites inthe overlying metaplutonic complex as the melts generated bydehydration-melting of the plutonic protoliths of the Jijalgarnet-bearing restites. Consistent with the metamorphic evolutionof the Kohistan lower arc crust, dehydration-melting occurredat the mature stage of this island arc when shallower hornblende-bearingplutonic rocks were buried to depths exceeding 25–30 kmand heated to temperatures above c. 900°C. Available experimentaldata on dehydration-melting of amphibolitic sources imply thatthickening of oceanic arcs to depths >30 km (equivalent toc. 1·0 GPa), together with the hot geotherms now postulatedfor lower island arc crust, should cause dehydration-meltingof amphibole-bearing plutonic rocks generating dense garnetgranulitic roots in island arcs. Dehydration-melting of hornblende-bearingplutonic rocks may, hence, be a common intracrustal chemicaland physical differentiation process in island arcs and a naturalconsequence of their maturation, leading to the addition ofgranitic partial melts to the middle–upper arc crust andformation of dense, unstable garnet granulite roots in the lowerarc crust. Addition of LREE-enriched granitic melts producedby this process to the middle–upper island arc crust maydrive its basaltic composition toward that of andesite, affordinga plausible solution to the ‘arc paradox’ of formationof andesitic continental-like crust in island arc settings. KEY WORDS: island arc crust; Kohistan complex; Jijal complex; amphibole dehydration-melting; garnet granulite; continental crustal growth     

Tempestite facies models for the epicontinental Triassic carbonates of the Betic Cordillera (southern Spain)

This study focuses on storm deposits in the Muschelkalk facies of the Betic Cordillera (southern Spain) and interprets their deposition mechanisms. Three types of storm deposit are distinguished: (i) pot/gutter casts; (ii) tempestite beds; and (iii) storm‐winnowed deposits. Each deposit provides information about the carbonate platform environment in which it was deposited. The tempestite models proposed are: (i) the bypass‐zone tempestite model, occurring in a muddy ramp at the epicontinental basin margin. This model is characterized by potholes and gutters that form in a shoreline bypass‐zone during storms; (ii) the gradient‐current tempestite model in which frequent tempestite beds are related to storm gradient currents. Thickness and grain size decrease towards the deep distal ramp; and (iii) the winnowed deposit tempestite model whereby storm deposits are winnowed and deposited in the same environment with only short lateral transport having occurred. This model evokes more restricted and shallower conditions, lagoons or inland seas. The distribution of all these deposits in the stratigraphic sections studied corroborate the eustatic third‐order cycle identified, although the different features of the storm deposits and their positions in each section indicate a subsidence varying in time and space. In the transgressive stage, the margins of the epicontinental basin were a well‐developed ramp with potholes and gutters. In contrast, during the high sea‐level stage, storm deposits generated tempestite beds or storm‐winnowed deposits in the different areas. The epicontinental carbonate platform with ramp edges evolved into a complex depositional system of coastal and shallow‐marine environments with lagoons and restricted inland seas. Thus, the epicontinental platform underwent substantial change from the Late Anisian to the Late Ladinian and this is reflected in its storm deposits.     

防撞梁在超高车辆撞击铁路桥梁中的防撞性能研究

作为交通运输工程中不可或缺的一部分,铁路桥梁在生命线及基础设施系统中发挥着重要的作用,一旦被超高车辆撞击,将会造成难以估计的生命及财产损失。提出4个不同配置（横梁、支撑）防撞梁,作为一种缓冲机制安装于桥梁可能发生撞击的主梁底部区域,并通过1:5缩尺模型对防撞梁的防撞性能及工作原理进行研究,并提出基于数值模拟的简化分析方法,实验结果表明:4个防撞梁对于降低桥梁在冲击荷载下的动力响应均有很好效果,能够有效降低桥梁结构在冲击荷载下的动力响应,塑性变形折减至50%,变形能力较好的防撞梁防撞性能更好,此外,基于数值模拟的简化分析方法是有效的。