Dating of lower terrace sediments from the Middle Rhine area, Germany

Modern and known-age Pleistocene fluvial sediments were investigated by optical dating of quartz to test the suitability of the approach for dating deposits from the deeply incised Middle Rhine Valley. Samples from modern flood sediments revealed skewed distributions indicating different residual levels of equivalent dose (D_e) within the different aliquots. Nevertheless, a substantial number of aliquots from the modern deposits reflect D_e values close to zero. For the Pleistocene samples, optical ages are in general consistent with age control given by the presence of the Laacher See Tephra and radiocarbon dating. However, some samples overestimate the known age by a few thousand years when using the arithmetic mean. This is apparently explained by including aliquots in the determination of mean D_e where the optical signal was incompletely bleached at deposition. The most difficult issue in this context is identifying a suitable approach that can distinguish between the variability of D_e due to partial bleaching and microdosimetry. However, even when considering these limitations it appears that optical dating will by a quite suitable method to date Pleistocene sediments from such a complex fluvial environment, especially when focusing on a precision scale beyond a few thousand years.     

Investigating the amount of zeroing in modern sediments of River Danube, Austria

Sediments attributed to flooding events of River Danube concerning the bleaching of the optical stimulated luminescence (OSL) signal were investigated. It is demonstrated that the OSL signal in both quartz and feldspar is not completely but differentially bleached in the sediment grains. Partial bleaching of the samples is clearly indicated by the scatter of equivalent dose determined for several individual single aliquots. It is also shown that residual ages in feldspars are significantly higher than those calculated for quartz. It is furthermore demonstrated that analysing measurement-time dependent equivalent dose estimates is not a suitable method to identify partial bleaching in the investigated sediment grains. However, the transport and deposition process of the investigated samples was probably disturbed by an artificial input of sediment, and this case study may thus not be representative of undisturbed high flood events in the past.     

青藏高原新生代火车头山碱性及钙碱性两套火山岩的地球化学特征及其物源讨论

藏北羌塘火车头山新生代火山岩可区分为钙碱性及碱性两个不同的系列。钙碱性火山岩主要岩石组合为玄武岩-安山岩-英安岩,其SiO_2介于49%～70%之间,Al_2O_3>10%,Na_2O/K_2O>1;其中玄武岩具平坦型稀土配分型式,LREE/HREE为1.3～1.8,(La/Yb)_N为2.87～4.45,无明显铕异常,δEu为0.96～1.09;该套岩石的Mg~#与SiO_2相关关系以及La/sm-La等亲岩浆元素与超亲岩浆元素协变关系表明,它们应为幔源岩浆经分离结晶演化的产物,其岩石组合类型以及低的Sm/Yb值(Sm/Yb=1.53～5.35)表明它们的原始岩浆应来源于岩石圈地幔尖晶石二辉橄榄岩的局部熔融。本区碱性火山岩为一套典型的钾质岩石系列,主要岩石组合类型为碱玄岩-碱玄质响岩-响岩,其SiO_2介于44%～59%之间,Al_2O_3>14%,Na_2O/K_2O介于0.47～1.51之间;岩石轻稀土强烈富集,LREE/HREE为13.20～15.76,(La/Yb)_N=50.44～91.99;其岩石组合类型以及Mg~#与SiO_2相关关系以及La/Sm-La协变关系同样表明它们为共源岩浆分离结晶演化的产物;然而,其较高的Sm/Yb值(Sm/ Yb=2.63～13.98)表明它们并非地幔橄榄岩直接局部熔融的产物,岩石弱的负Eu异常(δEu=0.77～0.85)以及Th、U的强烈富集和Nb、Ta的相对亏损,又反映了原始岩浆中有显著的地壳物质的贡献;该套钾质碱性系列岩石在La/Co-Th/Co同分母协变图上呈直线型分布,而在La/Co-Sc/Th异分母协变图上呈显著的双曲线分布,从而表明其源区为二源混合型,是青藏高原特殊的壳幔混合层局部熔融的产物,这些特征是新生代青藏高原壳幔层圈物质交换的重要岩石学证据。     

Geochemistry of the Pliocene basalts erupted along the Malatya-Ovacik fault zone (MOFZ), eastern Anatolia, Turkey: Implications for source characteristics and partial melting processes

The study area is located on the middle sector of the Malatya-Ovacık Fault Zone (MOFZ) in the eastern Anatolia. Four basaltic flows from bottom to top, which are tholeiitic in character and intercalated with Pliocene sedimentary rocks, were erupted along this fault zone. Chemical compositions of these flows reveal some differences between the first flow and others in terms of high-field strength elements (HFSEs) (e.g. Ti, Zr, Nb). Limited variations in compositions within the first flow and upper flows suggest a limited fractionation range. Trace-element patterns exhibit that all the flows have similar and OIB-like patterns without positive peak at Pb and a trough at Nb—Ta, indicating minimal or no crustal contribution. Rare-earth element (REE) patterns indicate that the first flow has flat patterns with negative Eu anomaly, whereas the upper flows have variable enrichments in LREE and depletions in HREE. La/Sm_N, Dy/Yb_N and Zr/Y ratios exhibit that the degree of partial melting decreases from the first flow to upper flows. Higher values of La/Yb_N ratio for the upper flows and depletions at Y and Yb on the trace-element patterns suggest the presence of garnet as a residual phase, which imply that the depth of partial melting took place solely in the garnet-stability field. OIB-like trace-element patterns and trace-element ratios (e.g. La/Nb, Ce/Y and Zr/Nb) emphasize that the melts forming the Arguvan basalt were originated from the asthenospheric mantle rather than the lithospheric mantle.     

Geochemical Evidence for Slab Melting in the Trans-Mexican Volcanic Belt

Geochemical studies of Plio-Quaternary volcanic rocks from theValle de Bravo–Zitácuaro volcanic field (VBZ) incentral Mexico indicate that slab melting plays a key role inthe petrogenesis of the Trans-Mexican Volcanic Belt. Rocks fromthe VBZ are typical arc-related high-Mg andesites, but two differentrock suites with distinct trace element patterns and isotopiccompositions erupted concurrently in the area, with a traceelement character that is also distinct from that of other Mexicanvolcanoes. The geochemical differences between the VBZ suitescannot be explained by simple crystal fractionation and/or crustalassimilation of a common primitive magma, but can be reconciledby the participation of different proportions of melts derivedfrom the subducted basalt and sediments interacting with themantle wedge. Sr/Y and Sr/Pb ratios of the VBZ rocks correlateinversely with Pb and Sr isotopic compositions, indicating thatthe Sr and Pb budgets are strongly controlled by melt additionsfrom the subducted slab. In contrast, an inverse correlationbetween Pb(Th)/Nd and ¹⁴³Nd/¹⁴⁴Nd ratios, which extend to lowerisotopic values than those for Pacific mid-ocean ridge basalts,indicates the participation of an enriched mantle wedge thatis similar to the source of Mexican intraplate basalts. In addition,a systematic decrease in middle and heavy rare earth concentrationsand Nb/Ta ratios with increasing SiO₂ contents in the VBZ rocksis best explained if these elements are mobilized to some extentin the subduction flux, and suggests that slab partial fusionoccurred under garnet amphibolite-facies conditions. KEY WORDS: arcs; mantle; Mexico; sediment melting; slab melting     

Magma Genesis and Mantle Dynamics at the Harrat Ash Shamah Volcanic Field (Southern Syria)

A geochemical and petrological study of Miocene to recent alkalibasalts, basanites, hawaiites, mugearites, trachytes, and phonoliteserupted within the Harrat Ash Shamah volcanic field was performedto reconstruct the magmatic evolution of southern Syria. Themajor element composition of the investigated lavas is mainlycontrolled by fractional crystallization of olivine, clinopyroxene,± Fe–Ti oxides and ± apatite; feldspar fractionationis restricted to the most evolved lavas. Na₂O and SiO₂ variationswithin uncontaminated, primitive lavas as well as variably fractionatedheavy rare earth element ratios suggest a formation by variabledegrees of partial melting of different garnet peridotite sourcestriggered, probably, by changes in mantle temperature. The isotopicrange as well as the variable trace element enrichment observedin the lavas imply derivation from both a volatile- and incompatibleelement-enriched asthenosphere and from a plume component. Inaddition, some lavas have been affected by crustal contamination.This effect is most prominent in evolved lavas older than 3·5Ma, which assimilated 30–40% of crustal material. In general,the periodicity of volcanism in conjunction with temporal changesin lava composition and melting regime suggest that the Syrianvolcanism was triggered by a pulsing mantle plume located underneathnorthwestern Arabia. KEY WORDS: ⁴⁰Ar/³⁹Ar ages; intraplate volcanism; mantle plume; partial melting; Syria     

Three-dimensional simulations of air flow and momentum transfer in partially harvested forests

In order to predict wind loading on trees (canopy height h) in partially harvested forests, it is necessary to characterize air flow and momentum transfer in progressively more complex patterns where groups of trees (or aggregates) are retained. In this study, we used large-eddy simulation to explore the effects of aggregate size, inter-aggregate spacing, and the ratio between the aggregate size and inter-aggregate spacing on air flow and momentum transfer. Forty-five grid points across an aggregate were needed to achieve an adequate level of turbulence. Using grid sizes of h/15 throughout was too viscous for the smaller aggregates. Vertical and horizontal flow deflection by the leading aggregates sheltered some of the downstream aggregates to varying degrees where turbulence increased for subsequent rows. The number of rows of protected aggregates decreased as aggregate dimensions and the space between aggregates increased. A theoretical treatment of time-dependent wind is presented for the lead aggregate and a simulation case is presented for the case of a gust of reduced wind passing through the aggregate pattern. The leading aggregate responded with decreasing moment for decreasing ambient wind speed as predicted by theory. However, downwind aggregates experienced substantial increases in bending moment. The overall results of the disruptive aspects of time dependence agrees with arguments regarding the role of irrotational (potential) flow to this problem. Our treatment of retention pattern design is only a first step and further research suggestions are presented.     

中国地表植被覆盖变化及其与气候因子关系

本文利用１９８５－１９９０年连续６９个月的ＮＯＡＡ时间序列数据,进行中国植被覆盖变化的空间,以及时间序列分析,并且结合同期的平均气温,降水数据,进行植被覆盖变化与气候因子相关性分析,该文证实了在中国植被覆盖随时间的推移规律,空间分布规律,以及植被覆盖变化与气温,降水的定量关系。     

元素地球化学场及其地学意义

有关岩浆花岗岩成因的原地重熔说,揭示了各种化学元素在内生过程中的迁移规律。据此并结合元素在周期表中的位置,得出了元素地球化学场的概念,元素地球化学场不但总结了元素在原地重熔过程中的聚散趋势,而且揭示了元素自身组构与其空间分布规律的关系,展示出三个不同层次的地质断面,即花岗岩体上部和顶部盖层断面,大陆地壳断面及地球球体断面。     

Petrogenesis of the Tanzawa plutonic complex, central Japan: Exposed felsic middle crust of the Izu–Bonin – Mariana arc

The Miocene Tanzawa plutonic complex, consisting mainly of tonalite intrusions, is exposed at the northern end of the Izu–Bonin – Mariana (IBM) arc system as a consequence of collision with the Honshu Arc. The Tanzawa plutonic rocks belong to the calc-alkaline series and exhibit a wide range of chemical variation, from 43 to 75 wt% SiO₂. They are characterized by relatively high Ba/Rb and Ce/Nb ratios, and low abundances of K₂O, LIL elements, and rare earth elements (REE). Their petrographic and geochemical features indicate derivation from an intermediate parental magma through crystal fractionation and accumulation processes, involving hornblende, plagioclase, and magnetite. The Tanzawa plutonic complex is interpreted to be the exposed middle crust of the IBM arc, which was uplifted during the collision. The mass balance calculations, combining data from melting experiments of hydrous basaltic compositions at lower-to-middle crustal levels, suggest that parental magma and ultramafic restite were generated by dehydration partial melting (∼ 45% melting) of amphibolite chemically similar to low-K tholeiitic basalt. Partial melting of hydrated mafic lower crust might play an important role in felsic middle-crust formation in the IBM arc.