The evolution of the polymetamorphic basement in the Central Alps unravelled by precise U−Pb zircon dating

The U–Pb age determinations of zircon and rutile from the Aar massif reveal a complex evolution of the Central Alpine basement. The oldest components are found in zircons of metasediments, which bear cores of Archean age; the U–Pb age of discordant prismatic zircons of the same rocks ranges between 580 and 680 Ma, an age that is typical for Pan-African metamorphism. The zircons are interpreted as Pan-African detritus with Archean inheritance. The provenance region of the Pan-African zircons is assumed to be a terrane of Gondwana-affinity, i.e. the W. African craton or the Pentevrian microplate. The Caledonian metamorphism left a pervasive structural imprint in amphibolite facies on the rocks of the Aar massif; it is dated at 456±2 and 445 Ma by zircons of a layered migmatitic gneiss and a migmatitic leucosome, respectively, both occurring in the northernmost zones of the massif. Hercynian metamorphism never exceeded greenschist-facies conditions and is recorded by zircon in a garnet-amphibolite and by rutile in a meta-psammite that yield an age of 330 Ma. Both zircon and rutile are considered to be products of retrograde mineral reactions and therefore do not date the peak conditions of Hercynian metamorphism. The Gastern granite at the western end of the Aar massif is a contaminated granite that intruded at 303±4 Ma, contemporaneously with the wide-spread late Hercynian post-collisional I-type magmatism. The study demonstrates the potential of isotope dilution U–Pb dating of single grains and microfractions in deciphering complex evolutionary histories of polymetamorphic terrains.     

A kinetic approach to describe trace-element distribution between particles and solution in natural aquatic systems

The partitioning of radioactive trace elements between seawater and particulate matter from surface sediments and sediment traps was investigated in laboratory experiments. For the elements Na, Zn, Se, Sr, Cd, Sn, Sb, Cs, Ba, Hg, Th and Pa (group I) constant distribution coefficients (K_d) were found after a few days of equilibration, whereas the elements Be, Mn, Co and Fe (group II) showed an increasing K_d over the whole time of observation of 108 days. The time dependence of K_d is described by an adsorption-desorption equilibrium (group I elements), followed by a lattice transport reaction step (group II elements). The reaction rate constants are compared to Mn oxidation rates and to adsorption rate constants derived from in situ measurements of the $\text{U}\text{Th}$ disequilibrium as available from literature.     

Quality control algorithms for rainfall measurements

One of the basic requirements for a scientific use of rain data from raingauges, ground and space radars is data quality control. Rain data could be used more intensively in many fields of activity (meteorology, hydrology, etc.), if the achievable data quality could be improved. This depends on the available data quality delivered by the measuring devices and the data quality enhancement procedures. To get an overview of the existing algorithms a literature review and literature pool have been produced. The diverse algorithms have been evaluated to meet VOLTAIRE objectives and sorted in different groups. To test the chosen algorithms an algorithm pool has been established, where the software is collected. A large part of this work presented here is implemented in the scope of the EU-project VOLTAIRE (Validation of multisensors precipitation fields and numerical modeling in Mediterranean test sites).     

Earthquakes in Switzerland and surrounding regions during 2006

This report of the Swiss Seismological Service summarizes the seismic activity in Switzerland and surrounding regions during 2006. During this period, 572 earthquakes and 91 quarry blasts were detected and located in the region under consideration. Of these earthquakes, two occurred in conjunction with the construction of the new Gotthard railway tunnel and 165 were induced artificially by the stimulation of a proposed geothermal reservoir beneath the city of Basel. With 20 events with M _L ≥ 2.5, five of which were artificially induced, the seismic activity in the year 2006 was far below the average over the previous 31 years. Nevertheless, six events were felt by the public, most prominently the strongest of the induced Basel events (M _L 3.4), which caused some non-structural building damage. Noteworthy are also the two earthquakes near Cortaillod (M _L 3.2), on the shore of Lake Neuchatel, and in Val Mora (M _L 3.5), between the Engadin and Val Müstair, as well as the 42 aftershocks of the M _L 4.9 Vallorcine earthquake, between Martigny and Chamonix, of September 2005. Editorial handling: Stefan Bucher     

Age and origin of magmatism along the Cenozoic Red River shear belt, China

To decipher the geodynamic significance of Cenozoic magmatism along the Red River shear belt, geochemical analyses, U-Pb and Rb-Sr dating, and Pb-Sr-Nd isotope tracing were undertaken. Zircon, monazite, titanite, and a Ti-U-oxide from foliated granitoid intrusions in the shear belt gneisses yield U-Pb emplacement ages of 33.1?±?0.2 (2σ), 31.9?±?0.3, 25.8?±?0.2 and 24.7?±?0.2?Ma, and an age of 35.0?±?0.3?Ma was obtained for the roughly 100?km long, adjacent Jinping (Phan Si Pang) alkali granite. Together with our previous data the new ages suggest that magmatism and left-lateral strike-slip movements occurred coevally during latest Eocene–Oligocene times from 33 to 22?Ma. The Rb-Sr dating of muscovite and biotite from the northernmost gneisses indicates that cooling to 500?°C occurred at 52.6?±?1.1?Ma, pre-dating the onset of magmatism, whereas further cooling to 300?°C took place at 28.9?±?0.6. This shows that unroofing in the north took place almost 9?million years earlier than in the central gneiss segments of the shear zone. Geochemical data substantiate two types of magmas: (1) amphibole-bearing intrusions of alkaline trend which are derived from sources with I_sr: 0.7065–0.7089 and _i ^Nd: ?3.7 to ?6.6; (2) leucogranitic layers and bodies having I_sr: 0.7084–0.7354 and _i ^Nd: ?3.3 to ?13.4. The former type of intrusion is found in both the gneisses and the adjacent unmetamorphosed cover rocks, whereas leucogranites are restricted to the shear belt gneisses. Source signatures of the alkaline intrusions lie adjacent to the those of OIB, plotting at the lower end of the Mantle Array. Contamination of these melts by continental material seems to be very limited. On the other hand, the leucogranitic layers are essentially crustal derived but none of the them has country rock isotope signatures, requiring melting of crust different from the actually exposed gneisses. Magma sources similar to those of ocean island basalt indicate magmatism to involve melting of light rare earth element and large ion lithophile element enriched mantle domains, most likely present in the lithosphere underneath the region. Since lithospheric thickening or subduction can be ruled out to produce both types of magmas, the presence of an important thermal anomaly is required, which is coevally active with left-lateral strike-slip shear. Adiabatic decompression and melting within the rising anomaly is the most plausible mechanism to produce the mantle magmas, which successively migrate through the crust to induce anatectic melting at 20–15?km crustal depth. Alkaline magmas largely dominate the volume of magmatism along the belt, being continuously present in the shear zone for millions of years. Such lubrication potentially explains how very large amounts of displacement can be absorbed in surprisingly narrow shear zones such as the Red River belt, possibly also playing a rôle for where and when zones of plate-scale lateral extrusion develop.     

Mineralogical alteration and associated permeability changes induced by a high-pH plume: Modeling of a granite core infiltration experiment

Within the framework of the HPF project (Hyperalkaline Plume in Fractured Rock) at the Grimsel Test Site (Switzerland), a small scale core infiltration experiment was performed at the University of Bern. A high-pH solution was continuously injected, under a constant pressure gradient, into a cylindrical core of granite containing a fracture. This high-pH solution was a synthetic version of solutions characteristic of early stages in the degradation of cement. The interaction between the rock and the solutions was reflected by significant changes in the composition of the injected solution and a decrease in the permeability of the rock. Changes in the mineralogy and porosity of the fault gouge filling the fracture were only minor. One-dimensional reactive transport modeling, using a modified version of the GIMRT code, was used to interpret the results of the experiment. Dispersive and advective solute transport, mineral reaction kinetics and a coupling between porosity and permeability changes were taken into account. In order to obtain a reasonable agreement between models and experimental results, reactive surface areas of the order of 10⁵ m²/m³ rock had to be used. These values are much smaller than the values measured for the fault gouge filling the fracture, which are in the order of 10⁶–10⁷ m²/m³ rock. However, the results could be improved by adding a small fraction of fine grained mineral, which could explain the high initial peaks in Al and Si concentration. With the inclusion of this fine grained fraction, the initial surface areas in the model were within the range of the measured specific surface areas of the fault gouge. The fact that the decrease in permeability was significant despite the minor changes in mineralogy, suggests that permeability may be controlled by changes in the structure of the rock (pore geometries) rather than by only the bulk volumetric (porosity) changes.     

U-Pb and Rb-Sr dating of a polymetamorphic nappe terrain: The Caledonian Jotun nappe,southern Norway

The Caledonian Jotun nappe in the Tyin area of southern Norway has been investigated using U-Pb analysis of zircon and sphene and Rb-Sr measurements of minerals and whole rocks, with special reference to the reaction of the isotope systems to various kinds of metamorphic event. The nappe consists of Precambrian basement rocks and their cover, inversely thrust over the Baltic shield and its parautochthonous, presumably lower Paleozoic sediments during the Caledonian orogeny. While the nappe basement retained its pre-Caledonian structures, the nappe cover was penetratively deformed and metamorphosed to lower greenschist facies conditions.U-Pb analyses of zircon and sphene ofnappe basement rocks point to the crystallization of a syenitic to monzonitic magma at 1694± 20m.y., the intrusion of gabbros into the syenites and monzonites at 1252_?25⁺²⁸ m.y., and the metamorphism (upper greenschist-lower amphibolite facies) and deformation (gneissification and mylonitization) of the whole complex at 909± 16m.y. Although this latest event caused strong lead loss in all zircon populations, it had no influence on the Rb-Sr system on a whole rock scale. The Caledonian movements did not disturb the U-Pb system of zircon and sphene but strongly influenced the Rb-Sr system in certain minerals and zircon and/or its inclusions (K-feldspar and brown biotite partially, green biotite completely reset).In contrast to the nappe basement, zircons from thecover rocks show pronounced lead loss during the Caledonian metamorphism/deformation — U-Pb analysis give discordia lower intercept ages of 415± 21 m.y. and 395± 6m.y. Again, however, the Rb-Sr whole-rock system has not been reset in Caledonian time. Minerals from the same rocks which provided the zircons give Rb-Sr isochron ages of 390± 11m.y. and 386 ± 20 m.y.Comparison of U-Pb and Rb-Sr results from the nappe basement demonstrates that only the analysis of U-Pb systematics in zircon and especially sphene resolved the 900-m.y. event. Rb-Sr mineral data alone yield ambiguous results.The results from the nappe cover confirm that U-Pb analyses from penetratively deformed sediments are very useful in dating a metamorphic/tectonic event in lower greenschist facies conditions.From the measured cell dimensions of the zircon populations it is concluded that lead can be completely retained in zircon during recrystallization.     

Microanalysis of nitrogen isotope abundances: association of nitrogen with noble gas carriers in Allende

Current research efforts to explore and account for the distribution of nitrogen isotope abundances in the ancient and present-day solar wind and in meteorites often require measurement of nitrogen abundances and isotopic compositions in very small samples of rare extraterrestrial materials. Isotopic analysis of ～ 1 μg of N₂ is possible with modern techniques of dynamic mass spectrometry, but even this high sensitivity is a limiting factor for certain critical samples. We have utilized a statistically operated mass spectrometer coupled to an ultrahigh vacuum gas extraction and processing system to lower this limit by approximately four orders of magnitude. Quantities of N₂ ranging from ～ 100 ng to < 100 pg are measurable with permil to percent precision in isotope ratios. Nitrogen and all noble gases evolved during stepwise combustion of fine-grained matrix material separated from the Allende meteorite have been meausred simultaneously in a pilot experiment using this technique. Isotopically heavy H-Xe (CCF-Xe) and isotopically light N are co-sited in a carbonaceous carrier phase, supporting a nucleosynthetic origin for¹⁵N-depleted nitrogen in Allende. The great isotopic uniformity of trapped Ar in all carrier phases indicates that simple, physical mass fractionation in gravitational escape of volatiles from the primitive nebula cannot have played a significant role in generating the nitrogen compositions observed in solar system matter.