Mineral equilibrium constraints on the feasibility of diffusive H2O-fluxed melting in the continental crust     

Lorena A. Tafur  Johann F. A. Diener 《Journal of Metamorphic Geology》2020,38(7):719-742

Generation of granitic melt is believed to occur predominantly by melting through the breakdown of hydrous minerals. However, melting due to the influx of H₂O has been recognized in anatectic amphibolite facies tonalitic grey gneisses, metagreywackes and low-P metapelites, and has consequently been proposed as an alternative mechanism for the generation of granitic melt. Melting induced by H₂O addition is recognized from voluminous melt production at relatively low temperature, where hydrous minerals are stable and anhydrous minerals are preferentially consumed during melting. Mineral equilibrium modelling to determine the P–T conditions, melt volumes, melting reactions and viable H₂O sources reveals that the process is not restricted to specific compositions or P–T conditions, although lower pressure and lithologies with a low hydrous mineral content are more favourable. Melting reactions in all lithologies primarily consume quartz and feldspars to yield 5–6 mol.% melt for each mol.% of H₂O added. remains constant at ~0.70 to 0.77 during progressive melting as long as alkali feldspar is present. Once alkali feldspar is exhausted, plagioclase becomes the main reactant, producing more tonalitic melt compositions with gradually higher . Our results demonstrate that, at the site of melting, melting is driven by diffusion of H₂O into the target rock along chemical potential gradients, rather than the advective flow of a mechanically distinct water-rich fluid phase. Melting will initiate and proceed as long as a gradient exists between the H₂O source and target lithology. Our calculations show that an ordinary magma, such as an I-type magma with typical H₂O content, has a high enough to be a viable H₂O source, allowing diffusive H₂O-fluxed melting to produce melt proportions and fertility comparable to that of dehydration melting. However, high degrees of partial melting require a considerable amount of H₂O, which necessitates a continuously advecting H₂O source such as a magma conduit or melt-bearing shear zone. A magmatic H₂O source at emplacement level will undergo a similar amount of crystallization as the melt fraction produced in the target rock such that there will be no net melt production. Considering that shear-zone hosted magma conduits are localized features, diffusive H₂O-fluxed melting is likely to only be viable in a small fraction of the anatectic orogenic crust. Although it may play an important role in locally raising melt volumes and modifying magma chemistry through mingling and hybridization, it does not appear to, of itself, be able to generate significant volumes of granitic melt.  相似文献   

Effects of inter- and intraspecific competition on growth and development of Bufo viridis and Bufo bufo tadpoles     

Sophie KatzmannAndrea Waringer-Löschenkohl  Johann A. Waringer 《Limnologica》2003,33(2):122-130

Competition between the common toad Bufo bufo (L.) and the green toad Bufo viridisLaurenti was studied in an experimental design. Mixed and single-species groups of 40 larvae were reared in plastic tanks (water volume 411). Growth parameters such as body length, total length, body mass and developmental stage according to Gosner (1960) were measured weekly until metamorphic climax and analyzed by regression analysis. Body condition index was calculated. Tadpoles of B. bufo reached metamorphosis at the same time in mixed and single-species treatments. Larvae and toadlets were insignificantly larger in single-species groups, weighed more and had a higher body condition index than in mixed species designs. In B. viridis the larval period was six days shorter in mixed groups; tadpoles and toadlets were smaller but had a higher body condition index than in the single-species treatment. These results indicate high intraspecific competition in B. viridis and competitive inferiority of B. bufo to B. viridis under the experimental conditions.  相似文献   

基于透射电镜的香肠构造流变计研究   总被引：1，自引：0，他引：1  

樊春  曾佐勋  Franz Neubauer  杨光明  Johann Genser 《地质科技情报》2002,21(4):41-44

采用透射电镜(TEM)对采自奥地利东阿尔卑斯和北京西山的香肠构造中的香肠体和相应基质的样品进行了超微构造分析。结果表明细粒基质中的位错密度较粗颗粒香肠体中的位错密度明显低很多。故认为香肠体的变形机制以位错蠕变为主,而基质的变形机制则以扩散蠕变为主。这从一个侧面为香肠构造流变计的这一假设提供了超微观研究的依据。  相似文献   

Ar/Ar mineral ages from basement rocks in the Eastern Kunlun Mountains, NW China, and their tectonic implications     

Yongjiang Liu  Johann Genser  Franz Neubauer  Wei Jin  Xiaohong Ge  Robert Handler  Akira Takasu 《Tectonophysics》2005,398(3-4):199-224

⁴⁰Ar/³⁹Ar dating and estimates of regional metamorphic P–T conditions were carried out on the basement rocks of the Eastern Kunlun Mountains, Western China. Samples from the Jinshuikou, Xiaomiao, Kuhai, Wanbaogou, and Nachitai groups revealed distinct metamorphic events and four age groups. The age group in the range from 363 to 439 Ma is interpreted to represent cooling after Middle Silurian–Late Devonian granulite(?) and amphibolite facies metamorphism, which is dominated by low–middle pressure/high temperature conditions. This tectono-thermal event is related to the closure of an oceanic basin or marginal sea. An age group of 212–242 Ma represents cooling after Triassic metamorphic overprint, which is probably associated with magmatic intrusions. This thermal event, together with the Permo-Triassic ophiolite zone along the South Kunlun Fault, relates to the closure of a major ocean (between India and Eurasia) and the eventual N-ward accretion of the Qiangtang block in Permo-Triassic times. The significance of the age group of 104–172 Ma may be related to the ductile deformation along the Xidatan fault due to the northward-directed accretion of the Lhasa block. Biotites from Nachitai record a partial isotopic resetting at ca. 32 Ma that is interpreted to represent a late-stage exhumation caused by further crustal shortening.  相似文献   

Metasomatism in mantle xenoliths from the Letlhakane kimberlites: estimation of element fluxes   总被引：1，自引：3，他引：1  

Esmé van Achterbergh  William L. Griffin  Johann Stiefenhofer 《Contributions to Mineralogy and Petrology》2001,141(4):397-414

A suite of metasomatised xenoliths from the Letlhakane kimberlite (Botswana) forms a metasomatic sequence from garnet peridotite to garnet phlogopite peridotite to phlogopite peridotite. Before the modal metasomatism, most of the Letlhakane xenoliths were depleted harzburgites that had been subjected to an earlier cryptic metasomatic event. Modal phlogopite and clinopyroxene - Cr-spinel increase at the expense of garnet and orthopyroxene with increasing degrees of metasomatism. The most metasomatised xenolith is a wehrlite. With progressive modal metasomatism, the clinopyroxene becomes enriched in Sr, Sc and the LREE, orthopyroxene becomes depleted in Ca and Ni, but enriched in Al and Mn, and olivine becomes depleted in Al and V. Garnet chemical composition largely remains unchanged. The garnet replacement reaction seen in most xenoliths allows the measurement of the flux of trace elements through detailed modal analysis of the pseudomorphs. Mass balance calculations show that the modally metasomatised rocks became enriched in incompatible elements such as Sr, Na, K, the LREE and the HFSE (Ti, Zr and Nb). Major elements (Al, Cr and Fe) and garnet-compatible trace elements (V, Y, Sc, and the HREE) were removed during this metasomatic process. The modal metasomatism caused a strong depletion in Al, and the results challenge previous suggestions that this metasomatic process merely occurred within an Al-poor environment. The data suggest that the xenoliths represent the mantle wallrock adjacent to a major conduit for an alkaline basic silicate melt (with high contents of volatile and incompatible elements). The volatile and incompatible element-enriched component of this melt percolated into the wallrock along a strong temperature gradient and caused the observed range of metasomatism.  相似文献   

TL stratigraphy of pre-Weichselian loess from NW Europe using feldspar coarse grains     

S. Balescu  Ch. Dupuis  Y. Quinif 《Quaternary Science Reviews》1988,7(3-4)

A new feldspar relative TL dating method is proposed that enables dating of pre-Weichselian loess older than 130 ka, by applying the additive γ dose technique and the Mejdahl's exponential extrapolation to alkali feldspar coarse grains. The method is applied to loess deposits from various reference loessic sections of NW Europe. Our relative TL age estimates are consistent with the information provided by the regional stratigraphy; furthermore, they lead to a clear separation of the pre-Weichselian periods of loess deposition in the time period 130 ka–300 ka.  相似文献   

Vertical mixing in Überlinger See,western part of Lake Constance     

Gerhard Heinz  Johann Ilmberger  Michael Schimmele 《Aquatic Sciences - Research Across Boundaries》1990,52(3):256-268

Depth variable vertical eddy diffusion coefficients for heat (K _z) were calculated from continuously measured temperature profiles in Überlinger See (western part of Lake Constance). The temperatures were averaged over vertical intervals of 10 m yielding 14 discrete values (maximum depth of Überlinger See: 147 m). A linear fit from 10 June to 29 September 1987 was used to smooth the significant temperature fluctuations caused by internal seiches of Lake Constance.Assuming horizontal homogeneity for the smoothed data the Gradient-Flux-Method was applied to compute vertical diffusion coefficientsK _z at different depths using the depth variable volumes and surfaces of the 14 layers. The resulting mean diffusion coefficients for the period from June to September are 0.04 cm²/s near the thermocline and up to 0.8 cm²/s in deeper strata (accuracy: ± 50%). It is shown that horizontal mixing between Überlinger See and Obersee (main lake) alters the computation ofK _z by less than 50%.A relationship betweenK _z and stability (Brunt-Väisälä) frequencyN is found which corresponds well to the theory of internal wave induced turbulence.Combining the diffusion coefficients with measured phosphorus profiles, a phosphorus flux from the hypolimnion to the epilimnion of (0.7 ± 0.4) mg P m^–2 d^–1 was calculated, corresponding to about 20% of the average external loading per area of Lake Constance in 1986.  相似文献   

Vertical mixing in Überlingersee (Lake Constance) traced by SF6 and heat     

Manfred Maiss  Johann Ilmberger  Karl Otto Münnich 《Aquatic Sciences - Research Across Boundaries》1994,56(4):329-347

Vertical mixing in Überlingersee is studied by releasing sulfur hexafluoride (SF₆) as a tracer at a central hypolimnic depth of 60 m and measuring its subsequent vertical dispersion over a period of three months. The experiment started with a streaky tracer injection of 1 liter gaseous SF₆ (STP) in August 1990. At that time the lake showed a typical strong summer stratification which in a weakened form lasts until November. From the SF₆ profiles of fifteen surveys at three sampling sites vertical diffusivitiesK _z are calculated compensating internal seiche displacement and horizontal tracer loss. Except of the bottom region no sampling site or time period is marked by significant differences in the hypolimnicK _z profile. So vertical mixing in the whole Überlingersee is described by mean diffusivities decreasing from 1.7 cm²/s at 120 m depth to 0.4 cm²/s in 30 m. The minimal value of 0.3 cm²/s in the thermocline region at 20 m depth is only based on observations in autumn. For a strong summer stratification it is certainly lower. The gradient-flux-method for heat was applied to compute a meanK _z (T) profile from continuously measured temperature profiles. Significant differences resulting from the two tracers showed, that theK _z (T) values are underestimated by up to a factor of 5 if cooling by lateral exchange is neglected. Particularly, internal seiche pumping of colder water from the adjacent Lake Obersee over the separating sill of Mainau into the deep Überlingersee basin is observed in 1990 from August onward, obviously controlling the heat budget below the sill level.  相似文献   

Field characteristics of laterally emplaced dikes: Anatomy of an exhumed Miocene dike swarm in Reydarfjördur, eastern Iceland     

Johann Helgason  Marcos Zentilli 《Tectonophysics》1985,115(3-4)

Many dikes of the Tertiary Breiddalur dike swarm in Reydarfjördur have a great lateral extent but are vertically (upward and/or downward) discontinuous. Field relationships indicate that they are not the feeders of the lava pile they intrude. Their characteristics can be best interpreted as them constituting the fossil expression of lateral magma emplacement analogous to that occurring in the actively spreading Krafla volcanic center of the neovolcanic zone. Average dike thickness increases clearly with depth and the thickness versus depth relationship is different from that described for other Tertiary dike swarms in eastern Iceland. It is suggested that dikes represent the extent of finite spreading which at higher levels in the crust is expressed by fissures and other extensional structures.  相似文献   

Coupling of groundwater and surface water at Lake Willersinnweiher: Groundwater modeling and tracer studies   总被引：1，自引：0，他引：1  

Ute Wollschläger  Johann Ilmberger  Margot Isenbeck-Schröter  Andreas M. Kreuzer  Christoph von Rohden  Kurt Roth  Wolfgang Schäfer 《Aquatic Sciences - Research Across Boundaries》2007,69(1):138-152

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