Ophicalcites from the northern Pyrenean belt: a field,petrographic and stable isotope study

Brecciated and fractured peridotites with a carbonate matrix, referred to as ophicalcites, are common features of mantle rocks exhumed in passive margins and mid-oceanic ridges. Ophicalcites have been found in close association with massive peridotites, which form the numerous ultramafic bodies scattered along the North Pyrenean Zone (NPZ), on the northern flank of the Pyrenean belt. We present the first field, textural and stable isotopic characterization of these rocks. Our observations show that Pyrenean ophicalcites belong to three main types: (1) a wide variety of breccias composed of sorted or unsorted millimeter- to meter-sized clasts of fresh or oxidized ultramafic material, in a fine-grained calcitic matrix; (2) calcitic veins penetrating into fractured serpentine and fresh peridotite; and (3) pervasive substitution of serpentine minerals by calcite. Stable isotopic analyses (O, C) have been conducted on the carbonate matrix, veins and clasts of samples from 12 Pyrenean ultramafic bodies. We show that the Pyrenean ophicalcites are the product of three distinct genetic processes: (1) pervasive ophicalcite resulting from relatively deep and hot hydrothermal activity; (2) ophicalcites in veins resulting from tectonic fracturing and cooler hydrothermal activity; and (3) polymictic breccias resulting from sedimentary processes occurring after the exposure of subcontinental mantle as portions of the floor of basins which opened during the mid-Cretaceous. We highlight a major difference between the eastern and western Pyrenean ophicalcites belonging, respectively, to the sedimentary and to the hydrothermal types. Our data set points to a possible origin of the sedimentary ophicalcites in continental endorheic basins, but a post-depositional evolution by circulation of metamorphic fluids or an origin from relatively warm marine waters cannot be ruled out. Finally, we discuss the significance of such discrepancy in the characteristics of the NPZ ophicalcites in the frame of the variable exhumation history of the peridotites all along the Pyrenean realm.     

Holocene environmental changes and human impact in the northern Swiss Jura as reflected by data from the Delémont valley

The construction of the A16-Transjurane motorway revealed evidence of Holocene sediment sequences in the Delémont valley (Canton of Jura, Switzerland). Certain processes begin during the Younger Dryas. Pine forests dominate this cold period, which was unfavourable for pedogenesis; they remain throughout the first half of the Holocene. The meandering river system then becomes stable for more than four millennia. The first signs of human impact on the vegetal cover begin to appear around 3,500 cal BC (Middle Neolithic). An increase in hydric activity occurs between 3,600 and 2,500 cal BC. However, the earliest evidence of the cultivation of cereals dates only to about 2,000 cal BC (Early Bronze Age). Extensive forest clearing and the emergence of cultivated plants occur after 1,400 cal BC. On the cleared slopes the soil erodes and at their foot colluvium deposits accumulate. The densification of the settlement from the Late Bronze Age (1,350–800 cal BC) until the beginning of the Iron Age (800–650 cal BC) contributes to alluvial destabilization. A palaeosol has been identified in all of the Holocene deposits in the valley. The deforestation intensifies between 400 and 100 cal BC while hydric activity decreases. The first centuries of our era record very limited pedo-sedimentary phenomena. However, human presence becomes less marked after 350 cal AD. The slopes are stabilized and the soil develops. From 550 cal AD, an important increase in hydric activity takes place, a probable consequence of a wet fluctuation in the climate. Contemporary forest clearing causes deep gullies. After 750 cal AD, drier conditions set in. This period of stability, marked by occasional rises in the water level, continues until 1,250–1,300 cal AD (Late Middle Ages). Then superficial flows resume and entrenchment of the main waterways occurs, combined consequences of the Little Ice Age and the upsurge in human activity.     

Evaluation of the regional climate model ALADIN to simulate the climate over North America in the CORDEX framework

In this study, an ensemble of four multi-year climate simulations is performed with the regional climate model ALADIN to evaluate its ability to simulate the climate over North America in the CORDEX framework. The simulations differ in their driving fields (ERA-40 or ERA-Interim) and the nudging technique (with or without large-scale nudging). The validation of the simulated 2-m temperature and precipitation with observationally-based gridded data sets shows that ALADIN performs similarly to other regional climate models that are commonly used over North America. Large-scale nudging improves the temporal correlation of the atmospheric circulation between ALADIN and its driving field, and also reduces the warm and dry summer biases in central North America. The differences between the simulations driven with different reanalyses are small and are likely related to the regional climate model’s induced internal variability. In general, the impact of different driving fields on ALADIN is smaller than that of large-scale nudging. The analysis of the multi-year simulations over the prairie and the east taiga indicates that the ALADIN 2-m temperature and precipitation interannual variability is similar or larger than that observed. Finally, a comparison of the simulations with observations for the summer 1993 shows that ALADIN underestimates the flood in central North America mainly due to its systematic dry bias in this region. Overall, the results indicate that ALADIN can produce a valuable contribution to CORDEX over North America.     

Superimposed tectonic and hydrothermal events during the late-orogenic extension in the Western French Massif Central: a structural and 40Ar/39Ar study

In the Western French Massif Central, the Argentat fault is a major structure through which As–Au fluids percolated in the Late Carboniferous along brittle fractures. New petrostructural investigations show that an early ductile normal-dextral faulting, coeval to leucogranite emplacement took place during the Late Visean syncollisional extension of the belt and was accompanied by a hydrothermal event marked by the growth of muscovites whose ⁴⁰Ar/³⁹Ar ages cluster around 335 Ma. This early fluid channelling is associated with brittle deformation only in the hangingwall of the Argentat fault, whereas ductile deformation is restricted to the footwall. These results provide new evidence for the upper crust implication during the syncollisional extension in the French Massif Central. This study stresses the interest of a detailed multimethod analysis to characterize hydrothermal processes, especially in basement areas where the tectonic, plutonic and metamorphic evolution is polyphased.     

A View to the Future: Ultraviolet Studies of the Solar System

We discuss the status of ultraviolet knowledge of Solar System objects. We begin with a short historical survey, followed by a review of knowledge gathered so far and of existing observational assets. The survey indicates that UV observations, along with data collected in other spectral bands, are necessary and in some cases essential to understand the nature of our neighbors in the Solar System. By extension, similar observations are needed to explore the nature of extrasolar planets, to support or reject astro-biology arguments, and to compose and test scenarios for the formation and evolution of planetary systems.We propose a set of observations, describing first the necessary instrumental capabilitites to collect these and outlining what would be the expected scientific return. We identify two immediate programmatic requirements: the establishment of a mineralogic database in the ultraviolet for the characterization of planetary, ring, satellite, and minor planet surfaces, and the development and deployment of small orbital solar radiation monitors. The first would extend the methods of characterizing surfaces of atmosphere-less bodies by adding the UV segment. The latter are needed to establish a baseline against which contemporaneous UV observations of Solar System objects must be compared.We identify two types of UV missions, one appropriate for a two-meter-class telescope using almost off-the-shelf technology that could be launched in the next few years, and another for a much larger (5–20 meter class) instrument that would provide the logical follow-up after a decade of utilizing the smaller facility.Michel Festou, our co-author and a very important contributor to this paper, passed away while this paper was being completed. We dedicate it to his memory.Deceased 11 May 2005     

The ground calibration setup of OMEGA and VIRTIS experiments: description and performances

Hyperspectral imagery is an essential technique for remote sensing of surfaces and atmospheres of planetary objects. However, given the instrumental complexity of coupling imaging and spectroscopy, in particular in the infrared, an in-depth ground calibration is mandatory to enable an unbiased and optimized data reduction.This paper presents the ground calibration setup designed and implemented for the visible and near infrared imaging spectrometers VIRTIS/ROSETTA, and OMEGA/MARS-EXPRESS, and summarizes the main results obtained in validating the required performances.     

Exposure history of the St‐Robert (H5) fall

Abstract— The compositionally typical H5 chondrite St‐Robert has an exposure age, 7.8 Ma, indistinguishable from that of the main cluster of H chondrites. Small values of the cosmogenic ²²Ne/²¹Ne ratio in interior samples imply a pre‐atmospheric radius on the order of 40 cm. Sample depths based on tracks and the production rates of Bhattacharya et al. (1973) range from 6 to ～40 cm and are generally larger than depths estimated from published ⁶⁰Co activities, perhaps because the track production rates adopted are too high. Depth profiles of the production rates of ¹⁴C, ³⁶Cl, ²⁶Al, ¹⁰Be, and ²¹Ne in stony material show increases with depth and reach levels 5% to 15% higher than expected from modeling calculations. The maximum concentrations in St‐Robert are, however, generally comparable to those measured for the L5 chondrite, Knyahinya, whose pre‐atmospheric radius of ～45 cm is thought to lead to the maximum possible production rates in chondrites. We infer that the pre‐atmospheric radius of St‐Robert was within 5 cm of the value that supports maximum production rates (i.e., 45 ± 5 cm). This radius corresponds to a pre‐atmospheric mass of (1.3 ± 0.4) × 10³ kg. The agreement of exposure ages for St‐Robert obtained in several different ways and the similarity of the depth profiles for ¹⁴C, ²⁶Al, ¹⁰Be, and ²¹Ne argue against a lengthy pre‐exposure of St‐Robert on the parent body and against a two‐stage exposure after launch from the parent body. Following Morbidelli and Gladman (1998), we suggest that St‐Robert was chipped from deep in its parent body, spent the next 7–8 Ma without undergoing a major collision, was nudged gradually into an orbital resonance with Jupiter, and then traveled quickly to Earth.     

Interferometry: The tool to study giant, supergiant and Mira stars

Late-type giant stars have been traditional targets for infrared interferometers. They are bright and big and are therefore easy targets to resolve and to detect. Considerable progress has been and is being made on the spatial structure of these objects thanks to existing interferometers. Beyond the classical measurement of their diameters, pulsations have been directly detected, spatial intensity distributions are more and more understood and more important, consistent scenarios for both spectroscopic and interferometric measurements are on the verge to be validated. All this has been possible with prototype instruments having a small number of baselines and very limited spectral capabilities. AMBER and MIDI will surely open a new era with high spectral resolution, high efficiency and imaging capabilities. This is not an exhaustive review of all the work done in the field but rather a presentation of the context. A recent review of Mira star observations with interferometers was recently written (Scholz, 2003).A brief science case is first introduced in this paper. Achievements with high angular resolution single-telescope techniques are presented. The contributions of optical and infrared interferometers are then explained. Eventually, some hints about the possible progress with VLTI are discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Geochemistry and timing of post-metamorphic dyke emplacement in the Mersin Ophiolite (southern Turkey): New age constraints from 40Ar/39Ar geochronology

The Mersin ophiolite, which is a relic of the late Cretaceous Neotethyan ocean domain in the eastern Mediterranean, is situated on the southern flank of the central Tauride belt. The ophiolite body is cross-cut at all structural levels by numerous mafic dyke intrusions. The dykes do not intrude the underlying melange of platform carbonates. Therefore, dyke emplacement post-dates the formation of the opholite and metamorphic sole but pre-dates the final obduction onto the Tauride platform. The post-metamorphic dyke swarms suggest the geochemical characteristics of Island Arc Tholeiites (IAT). ⁴⁰Ar/³⁹Ar geochronology of the post-metamorphic microgabbroic-diabasic dykes cutting both mantle tectonites and metamorphic sole revealed ages ranging from 89.6 ± 0.7–63.8 ± 0.9 Myr old, respectively, indicating widespread magmatic activity during the Late Cretaceous-early Palaeocene in the Neotethyan ocean. These data suggest that island arc development in the Neotethyan ocean in southern Turkey was as early as Late Cretaceous.