Behavior of trace elements in relation to Lu–Hf and Sm–Nd geochronometers during metamorphic dehydration–hydration in the HP domain of Vårdalsneset, Western Gneiss Region, Norway

The geochemical and isotopic characterization of an eclogite and the associated retrogressive amphibolite at Vårdalsneset, WGR, Norwegian Caledonide was undertaken to investigate the mobility of REE and Hf and the behavior of Lu–Hf and Sm–Nd geochronometers during metamorphic dehydration/rehydration. Eclogitic garnets display a distinct core–rim chemical zoning. Thermodynamic modeling indicates that both cores (13–22 kbar, 500–580°C) and rims (>16 kbar, 610–660°C) crystallized under eclogite-facies conditions. The core–rim zoning corresponds to the dehydration of the system. This petrographic disequilibrium is associated with Lu–Hf and Sm–Nd disequilibrium, which prevents dating of the eclogitic stages. At the rock scale, the incoming fluid responsible for eclogite–amphibolite retrogression brought in Sm and Nd, leached Lu, and had no influence on Hf. At the grain scale, mass balance shows that Sm and Nd were stored in clinozoisite since the first eclogitic stage, whereas Lu and Hf, which were more thoroughly redistributed among minerals during retrogression, enable the dating of the amphibolitic facies at 378 ± 17 Ma.     

Phase equilibrium investigations of the Adirondack class basalts from the Gusev plains,Gusev crater,Mars

Abstract— Phase equilibrium experiments have been performed on a synthetic analog of the Gusev plains basalt composition from the Spirit landing site on Mars. Near‐liquidus phase relations were determined over the pressure range of 0.1 to 1.5 GPa and at temperatures from 1125 to 1390 °C in a piston cylinder apparatus and 1 atm gas mixing furnace. The composition is multiply saturated with olivine, orthopyroxene, and spinel near its liquidus at 1320 °C and 1.0 GPa, or 85 km depth on Mars, placing an upper limit constraint on the thickness of the Martian lithosphere at the time of eruption. Our experimental work suggests that the Gusev basalts are anhydrous batch melts of a primitive Martian mantle similar to the composition estimated by Dreibus and Wänke (1984). The temperature of multiple saturation indicates the persistence of high mantle potential temperatures on Mars, similar to those on the modern Earth, until at least the very latest Noachian (3.7 Ga). These high mantle temperatures would be responsible for persistent basaltic volcanism throughout the southern highlands during the first billion years of Mars's history. The source for Gusev basalts differs strongly from the source for shergottite meteorites, reinforcing the idea of the absence of global mantle convection and mixing on Mars. The existence of a relatively primitive mantle reservoir requires that at least part of the mantle underwent little modification during early planetary differentiation.     

Analytical Developments in Secondary Ion Mass Spectrometry 2003

This annual review of secondary ion mass spectrometry (SIMS) highlights significant progress in the application of the technology for the following areas: U-Pb geochronology (notably in the fields of reference material zircons), sources of uncertainty during analysis and secondary ion yields. Major publications introduced a new zircon reference sample and dealt with an intercomparison study of a suite of established calibrators, some of which have been shown to have certain limitations. Another publication claimed that the principal uncertainty in U-Pb dating is related to variations in the Pb and U relative emission yields over a complete analytical session. 2003 saw the introduction of an automated particle identification procedure applied to the analysis of a chondritic meteorite, as well as new geometries of SIMS hardware (NanoSIMS) and techniques (time-of-flight SIMS). NanoSIMS allows a two to three order of magnitude reduction in sampling volume as a result of a reduced beam diameter, and time-of-flight SIMS allows the study of sample surfaces, and can provide data for elements concurrently.     

Development of a high resolution runoff routing model, calibration and application to assess runoff from the LMD GCM

Large-scale runoff routing models (RRMs) are important as a validation tool for GCMs, and to close the hydrological cycle in fully-coupled climate models. The model RiTHM was developed to simulate the discharge of large rivers from the total runoff simulated by the LMD GCM. It uses a 1024×800 grid, nested in the 64×50 grid of the LMD GCM. The runoff simulated in a GCM grid cell is uniformly distributed over the underlying cells, where a series of two reservoirs accounts for the delay related to infiltration through the unsaturated zone and aquifers. The resulting riverflow is routed assuming pure translation along the drainage network, extracted with a GIS from a 5 min DEM. The transfer time from a cell to the outlet depends on topography, and on a basin-wide parameter, the time of concentration. RiTHM was calibrated in 11 river basins, using a realistic runoff forcing (computed by the land surface model SECHIBA from reanalyzed meteorological forcing). This led to a very satisfactory reproduction of observed hydrographs. The main problems were related to hydraulic processes neglected in RiTHM (reservoirs, diversion of riverflow because of flooding or irrigation). These results helped to validate SECHIBA, except for its snow processes, shown to be too simple. With the same parameters, RiTHM was also forced with runoff from the LMD GCM. This induced an important degradation of the simulated hydrographs, regarding both volume and timing. It was largely explained by errors in precipitation, and more generally climate, in the GCM. The direct calibration of RiTHM under the GCM-runoff forcing markedly improved the timing of simulated discharge, which could be interesting for land–atmosphere–ocean coupling. This work demonstrated that the usefulness of RRMs for GCMs strongly depends on their adequate calibration.     

Geodynamic significance of the Raspas Metamorphic Complex (SW Ecuador): geochemical and isotopic constraints

The Raspas Metamorphic Complex of southwestern Ecuador is regarded as the southernmost remnant of oceanic and continental terranes accreted in the latest Jurassic–Early Cretaceous. It consists of variably metamorphosed rock types. (1) Mafic and ultramafic rocks metamorphosed under high-pressure (HP) conditions (eclogite facies) show oceanic plateau affinities with flat REE chondrite-normalized patterns, Nd_{150 Ma} ranging from +4.6 to 9.8 and initial Pb isotopic ratios intermediate between MORB and OIB. (2) Sedimentary rocks metamorphosed under eclogitic conditions exhibit LREE enriched patterns, strong negative Eu anomalies, Rb, Nb, U, Th, Pb enrichments, low Nd_{150 Ma} values (from −6.4 to −9.5), and high initial ⁸⁷Sr/⁸⁶Sr and ^206,207,208Pb/²⁰⁴Pb isotopic ratios suggesting they were originally sediments derived from the erosion of an old continental crust. (3) Epidote-bearing amphibolites show N-MORB affinities with LREE depleted patterns, LILE, Zr, Hf and Th depletion, high Nd_{150 Ma} (>+10) and low initial Pb isotopic ratios.The present-day well defined internal structure of the Raspas Metamorphic Complex seems to be inconsistent with the formerly proposed interpretation of a “tectonic mélange”. The association of oceanic plateau rocks and continent-derived sediments both metamorphosed in HP conditions suggests that the thin edge of the oceanic plateau first entered the subduction zone and dragged sediments downward of the accretionary wedge along the Wadatti–Benioff zone. Subsequently, when its thickest part arrived into the subduction zone, the oceanic plateau jammed the subduction processes, due to its high buoyancy.In Ecuador and Colombia, the latest Jurassic–Early Cretaceous suture involves HP oceanic plateau rocks and N-MORB rocks metamorphosed under lower grades, suggesting a composite or polyphase nature for the latest Jurassic–Early Cretaceous accretionary event.     

Late Holocene local vegetation dynamics in the marsh of Gravgaz(southwest Turkey)

Late Holocene local vegetation succession is reconstructed in twodifferent sites in a small-scale open marsh ecosystem in southwest Turkey.This is done by comparison of the fossil local pollen assemblage zones in twocores with the local pollen data of 40 surface samples from the marsh. Thepollen data are supplemented with sedimentological and archaeological data. Theinsertion of the mean pollen data of the local pollen zones as passive samplesinto the canonical correspondence analysis triplot of the modern samples allowsus to detect modern analogues for the fossil pollen zones. From this numericalcomparative approach it is concluded that the marsh area was relatively dryuntil ca 2500 BP. After 2500 BP the area shifts towards a wet area dominated bySparganium and/or Typha angustifolia. A diversification of the marsh vegetationstarts at ca 2400/2300 BP. The area around one core site seems to have beensituated in an area with slowly flowing source water, whereas the other coresite is likely to have been characterised by damp conditions. The steadilyincreasing dryness of the marsh area starts after ca 680 BP. The drying upappears to be associated with recent agricultural and grazing pressure.     

Towards a microzonation of the Greater Beirut area: an instrumental approach combining earthquake and ambient vibration recordings

Lebanon is situated on the 1000 km long Levant transform fault that separates the Arabic from the African tectonic plates. In Lebanon, the Levant fault splits up into a set of ramifications that had, in the past, generated major destructive earthquakes causing a lot of destruction and thousands of casualties. The most devastating one was the 551 A.D. offshore earthquake that destroyed Beirut, the capital of Lebanon. This paper presents a site effect study in Beirut, aimed at proposing a framework for future microzonation works in the city. It includes two complementary parts. A 6-month, temporary seismological experiment was first conducted to estimate the site response at 10 sites sampling the main geological units of Beirut on the basis of local and regional earthquake recordings. This spatially sparse information was then complemented by a large number (615) of microtremor measurements covering the Beirut municipality and part of its suburbs with a 400 m dense grid. The recordings were analysed with the standard site-to-reference and horizontal-to-vertical spectral ratio methods for earthquake recordings, and the horizontal-to-vertical ratio for ambient noise recordings. Significant ground motion amplification effects (up to a factor of 8) are found in a few areas corresponding to recent deposits. The consistency between results from earthquake and microtremor recordings allows proposing a map of the resonance frequencies within the city and its suburbs, with frequencies ranging from 0.5 to 5 Hz for the deepest deposits, and 5–10 Hz for shallow areas. Finally, the results are discussed and a way to combine the results obtained from the temporary stations to the great number of recordings coming from the permanent Lebanese network is proposed.     

Sedimentary fabric characterized by X-ray tomography: A case-study from tsunami deposits on the Marquesas Islands,French Polynesia

X-ray tomography is used to analyse the grain size and sedimentary fabric of two tsunami deposits in the Marquesas Islands (French Polynesia, Pacific Ocean) which are particularly exposed to trans-Pacific tsunamis. One site is located on the southern coast of Nuku Hiva Island (Hooumi) and the other one is on the southern coast of Hiva Oa Island (Tahauku). Results are compared with other techniques such as two-dimensional image analysis on bulk samples (particle analyser) and anisotropy of magnetic susceptibility. The sedimentary fabric is characterized through three-dimensional stacks of horizontal slices (following a vertical step of 2·5 mm along the cores), while grain-size distribution is estimated from two-dimensional vertical slices (following a step of 2 mm). Four types of fabric are distinguished: (a) moderate to high angle (15 to 75°); (b) bimodal low-angle (<15°); (c) low to high angle with at least two different orientations; and (d) dispersed fabric. The fabric geometry in a tsunami deposit is not only controlled by the characteristics of the flow itself (current strength, flow regime, etc.) but also sediment concentration, deposition rate and grain-size distribution. There is a notable correlation between unimodal high-angle fabric – type (a) – and finely-skewed grain-size distribution. The two tsunami deposits studied represent two different scenarios of inundation. As demonstrated here, X-ray tomography is an essential method for characterizing past tsunamis from their deposits. The method can be applied to many other types of sediments and sedimentary rocks.     

Experimental study on the flow around two tandem cylinders with unequal diameters

In this paper, flow around two circular cylinders in tandem arrangement with unequal diameters has been investigated using the particle image velocimetry technique (PIV) in a water channel. The upstream to downstream diameter ratio was kept constant at d/D = 2/3, the centre-to-centre distance was varied from 1.2D to 5D and the Reynolds number was varied from 1200 to 4800. The flow characteristics were analyzed through ensemble-averaged patterns of velocity, vorticity, normalized Reynolds stress contours and streamlines. Based on ensemble-averaged and instantaneous flow fields, different flow patterns, including single-wake-shedding at small spacing ratio, bi-stable flow behavior (alternating behavior of reattachment and vortex shedding) at intermediate spacing ratio and co-shedding pattern at large spacing ratio were observed. The effects of Reynolds number and the centre-to-centre spacing ratio on flow patterns and turbulent characteristics were also investigated. It was found that the diameter ratio appears to have a certain effect on the flow patterns at intermediate spacing ratios, where the reattachment of shear layer depends on the lateral width of the wake flow in the lee of the upstream cylinder. Extensive discussion on the distributions of Reynolds stress and turbulent kinetic energy was presented.