Geochronology and geochemistry of eclogites from the Mariánské Lázně Complex,Czech Republic: Implications for Variscan orogenesis

The Mariánské Lázn complex (MLC) is located in the Bohemian Massif along the north-western margin of the Teplá-Barrandian microplate and consists of metagabbro, amphibolite and eclogite, with subordinate amounts of serpentinite, felsic gneiss and calcsilicate rocks. The MLC is interpreted as a metaophiolite complex that marks the suture zone between the Saxothuringian rocks to the north-west and the Teplá-Barrandian microplate to the south-east. Sm-Nd geochronology of garnet-omphacite pairs from two eclogite samples yields ages of 377±7, and 367±4 Ma. Samples of eclogite and amphibolite do not define a whole rock Sm-Nd isochron, even though there is a large range in Sm/Nd ratio, implying that the suite of samples may not be cogenetic. Eclogites do not have correlated _Nd values and initial ⁸⁷Sr/⁸⁶Sr ratios. Five of the eight eclogite samples have high _Nd values (+10.2 to +7.1) consistent with derivation from a MORB-like source, but variable ⁸⁷Sr/⁸⁶Sr ratios (0.7033 to 0.7059) which probably reflect hydrothermal seawater alteration. Three other eclogite samples have lower _Nd values (+ 5.4 to –0.8) and widely variable ⁸⁷Sr/⁸⁶Sr ratios (0.7033 to 0.7096). Such low _Nd values are inconsistent with derivation from a MORB, source and may reflect a subduction or oceanic island basalt component in their source. The MLC is an important petrotectonic element in the Bohemian Massif, providing evidence for Cambro-Ordovician formation of oceanic crust and interaction with seawater, Late Devonian (Frasnian-Famennian) high- and medium-pressure metamorphism related to closure of a Saxothuringian ocean basin, Early Carboniferous (Viséan) thrusting of the Teplá terrane over Saxothuringian rocks and Late Viséan extension.     

Io's sodium emission cloud and the voyager 1 encounter

Io's neutral sodium emission cloud was monitored during the period of Voyager 1 encounter from two independent ground-based sites. Observations from Table Mountain Observatory verified the continued existence of the “near-Io cloud” (d < 1.5 × 10⁵ km, for 4πI > 1 kR; R denotes Rayleigh) while those from Wise Observatory showed a deficiency in the weaker emission at greater distances from Io. The sodium cloud has been monitored from both observatories for several years. These and other observations demonstrate that the behavior of the cloud is complex since it undergoes a variety of changes, both systematic and secular, which can have both time and spatial dependencies. The cloud also displays some characteristics of stability. Table Mountain images and high-dispersion spectra (resolution $\text{～0.2}\text{A}\text{?}$) indicate that the basic shape and intensity of the “near cloud” have remained relatively constant at least since imaging observations began in 1976. Wise Observatory low-dispersion spectra (resolution $\text{～1}\text{A}\text{?}$) which have been obtained since 1974 demonstrate substantial variability of the size and intensity of the “far cloud” (d ? 1.5 × 10⁵ km) on a time scale of months or less. Corresponding changes in the state of the plasma associated with the Io torus are suggested, with the period of Voyager 1 encounter represented as a time of unusually high plasma temperature and/or density. Dynamic models of the sodium cloud employing Voyager 1 plasma data provide a reasonable fit to the Table Mountain encounter images. The modeling assumptions of anisotropic ejection of neutral sodium atoms from the leading, inner hemisphere of Io with a velocity distribution characteristic of sputtering adequately explain the overall intensity distribution of the “near cloud”. During the Voyager 1 encounter period there appeared a region of enhanced intensity projecting outward from Io's orbit and inclined to the orbital plane. This region is clearly distinguished from the sodium emission normally aligned with the plane of Io's orbit. The process responsible for this phenomenon is not yet understood. Similar but less pronounced features are also present in several Table Mountain images obtained over the past few years.     

A model of the origin of the Jovian ring

Starting with the assumption that the micron-sized particles which make up the bright Jovian ring are fragments of erosive collisions between micrometeoroid projectiles and large parent bodies, a physical model of the ring is calculated. The physics of high-velocity impacts leads to a well-defined size distribution for the ejecta, the optical properties of which can be compared with observation. This gives information on the ejecta material (very likely silicates) and on the maximum size of the projectiles, which turns out to be about 0.1 μm. The origin of these projectiles is discussed, and it is concluded that dust particles ejected in volcanic activity from Io are the most likely source. The impact model leads quite naturally to a distribution in ejecta sizes, which in turn determines the structure of the ring. The largest ejecta form the bright ring, medium-sized ejecta form a disk extending all the way to the Jovian atmosphere, and the small ejecta form a faint halo, the structure of which is dominated by electromagnetic forces. In addition to the Io particles, interaction with interplanetary micrometeoroids is also considered. It is concluded that μm-sized ejecta from this source have ejection velocities which are several orders of magnitude too large, and thus cannot contribute significantly to the observed bright ring. However, the total mass ejection rate is significant. Destruction of these ejecta by the Io particles may provide additional particles for the halo.     

Intercomparison of measurements of stratospheric hydrogen fluoride

Observations of the vertical profile of hydrogen fluoride (HF) vapor in the stratosphere and of the vertical column amounts of HF above certain altitudes were made using a variety of spectroscopic instruments in the 1982 and 1983 Balloon Intercomparison Campaigns. Both emission instruments working in the far infrared spectral region and absorption instruments using solar occultation in the 2.5m region were employed. No systematic differences were seen in results from the two spectral regions. A mean profile from 20–45 km is presented, with uncertainties ranging from 20% to 50%. Total columns measured from ground and from 12 km are consistent with the profile if the mixing ratio for HF is small in the tropophere and low stratosphere.     

Surface-layer flow in complex terrain: Comparison of models and full-scale observations

Four models of surface boundary-layer flow in complex terrain are compared with observations made at Blashaval Hill, North Uist, Scotland. The field experiment is described by Mason and King (1985). Three of the models are derived from the two-dimensional theory of Jackson and Hunt (1975) and are described in Mason and King (1985), Walmsley et al. (1986) and Troen and Petersen (1989). The fourth is a mass-consistent code based on Traci et al. (1979). The model results are in good agreement with each other and are generally within the observed range of variation ( ~ ± 16%) in normalized wind speed. For most wind direcions (7 of 11), model results of normalized wind speed at the summit were within 7% of the observed mean values. For some wind directions, calculations using the Guidelines of Walmsley et al. (1989) suggested that variations in surface roughness were important. This led us to apply one of our models incorporating nonuniform surface roughness. The lack of significant improvement for cases when water lay upstream of Blashaval Hill is attributed to compensating changes at summit and reference sites and to very local effects on the wind data. Sensitivity to topography lying to the west and northwest of Blashaval was also investigated. Results suggested an influence from those distant topographic features for some wind directions. When those features were incorporated, maximum errors in normalized wind speed at the summit were reduced from 18 to 13%.     

H,O, Sr,Nd, and Pb isotope geochemistry of the Latir volcanic field and cogenetic intrusions,New Mexico,and relations between evolution of a continental magmatic center and modifications of the lithosphere

Over 200 H, O, Sr, Nd, and Pb isotope analyses, in addition to geologic and petrologic constraints, document the magmatic evolution of the 28.5–19 Ma Latir volcanic field and associated intrusive rocks, which includes multiple stages of crustal assimilation, magma mixing, protracted crystallization, and open- and closed-system evolution in the upper crust. In contrast to data from younger volcanic centers in northern New Mexico, relatively low and restricted primary ¹⁸O values (+6.4 to +7.4) rule out assimilation of supracrustal rocks enriched in ¹⁸O. Initial ⁸⁷Sr/⁸⁶Sr ratios (0.705 to 0.708), ¹⁸O values (-2 to-7), and ²⁰⁶Pb/²⁰⁴Pb ratios (17.5 to 18.4) of metaluminous precaldera volcanic rocks and postcaldera plutonic rocks suggest that most Latir rocks were generated by fractional crystallization of substantial volumes of mantle-derived basaltic magma that had near-chondritic Nd isotope ratios, accompanied by assimilation of crustal material in two main stages: 1) assimilation of non-radiogenic lower crust, followed by 2) assimilation of middle and upper crust by inter-mediate-composition magmas that had been contaminated during the first stage. Magmatic evolution in the upper crust peaked with eruption of the peralkaline Amalia Tuff (26 Ma), which evolved from metaluminous parental magmas. A third stage of late, roofward assimilation of Proterozoic rocks in the Amalia Tuff magma is indicated by trends in initial ⁸⁷Sr/⁸⁶Sr and ²⁰⁶Pb/²⁰⁴Pb ratios from 0.7057 to 0.7098 and 19.5 to 18.8, respectively, toward the top of the pre-eruptive magma chamber. Highly evolved postcaldera plutons are generally fine grained and are zoned in initial ⁸⁷Sr/⁸⁶Sr and ²⁰⁶Pb/²⁰⁴Pb ratios, varying from 0.705 to 0.709 and 17.8 to 18.6, respectively. In contrast, the coarser-grained Cabresto Lake (25 Ma) and Rio Hondo (21 Ma) plutons have relatively homogeneous initial ⁸⁷Sr/⁸⁶Sr and ²⁰⁶Pb/²⁰⁴Pb ratios of approximately 0.7053 and 17.94 and 17.55, respectively. ¹⁸O values for all the postcaldera plutons overlap those of the precaldera rocks and Amalia Tuff, except for those for two late-stage rhyolite dikes associated with the Rio Hondo pluton that have ¹⁸O values of-8.6 and-9.5; these dikes are the only Latir rocks which may be largely crustal melts.Chemical and isotopic data from the Latir field suggest that large fluxes of mantle-derived basaltic magma are necessary for developing and sustaining large-volume volcanic centers. Development of a detailed model suggests that 6–15 km of new crust may have been added beneath the volcanic center; such an addition may result in significant changes in the chemical and Sr and Nd isotopic compositions of the crust, although Pb isotope ratios will remain relatively unchanged. If accompanied by assimilation, crystallization of pooled basaltic magma near the MOHO may produce substantial cumulates beneath the MOHO that generate large changes in the isotopic composition of the upper mantle. The Latir field may be similar to other large-volume, long-lived intracratonal volcanic fields that fundamentally owe their origins to extensive injection of basaltic magma into the lower parts of their magmatic systems. Such fields may overlie areas of significant crustal growth and hybridization.     

Molecular simulations of metal adsorption to bacterial surfaces

The atomic-scale interactions that occur between cations and the metal-binding cell wall components common to many gram-positive bacteria were investigated using molecular simulations techniques. We examined the adsorption of Cd and Pb onto peptidoglycan and teichoic acid components of the bacterial cell wall using classical energy force field methods. Within the framework of molecular mechanics and the Cerius² modeling software, we used energy minimization, conformational analysis, and molecular dynamics to examine the different components of the cell wall and to determine relative binding energies and structural configurations of the cell wall components, both with and without the metals present. Electronic structure calculations of representative metal-organic complexes validate the more practical classical methods required in simulating the large number of atoms associated with the cell wall components. The classical force field simulations were conducted in both gas phase and solvated periodic cells. Force field-based simulation techniques can adequately describe the interactions of Cd with the cell wall, defining both metal ion coordinations and binding distances. However, the classical force field approach is inconsistent in describing the observed Pb-cell wall interactions due to possible limitations in the force field parameters, the propensity for Pb to form hydroxides at circumneutral pH, or the dominance of other adsorption mechanisms.     

Diffusion-limited REE uptake by eclogite garnets and its consequences for Lu–Hf and Sm–Nd geochronology

Garnets from the Zermatt-Saas Fee eclogites contain narrow central peaks for Lu + Yb + Tm ± Er and at least one additional small peak towards the rim. The REE Sm + Eu + Gd + Tb ± Dy are depleted in the cores but show one prominent peak close to the rim. These patterns cannot be modeled using Rayleigh fractionation accompanied by mineral breakdown reactions. Instead, the patterns are well explained using a transient matrix diffusion model where REE uptake is limited by diffusion in the matrix surrounding the porphyroblast. Observed profiles are well matched if a roughly linear radius growth rate is used. The secondary peaks in the garnet profiles are interpreted to reflect thermally activated diffusion due to temperature increase during prograde metamorphism. The model predicts anomalously low ¹⁷⁶Lu/¹⁷⁷Hf and ¹⁴⁷Sm/¹⁴⁴Nd ratios in garnets where growth rates are fast compared to diffusion of the REE, and these results have important implications for Lu–Hf and Sm–Nd geochronology using garnet.     

A 500 hPa synoptic wildland fire climatology for large Canadian forest fires, 1959–1996

Summary In Canada, the average annual area of burned forest has increased from around 1 million ha in the 1970’s to over 2.5 million ha in the 1990’s. A previous study has identified the link between anomalous mid-tropospheric circulation at 500 hPa over northern North America and wildland fire severity activity in various large regions of Canada over the entire May to August fire season. In this study, a northern North American study region of the hemispheric gridded 5° latitude by 10° longitude 500 hPa dataset is identified and analysed from 1959 to 1996 for a sequence of six monthly periods through the fire season, beginning in April and ending in September. Synoptic types, or modes of upper air behavior, are determined objectively by the eigenvector method employing K-means cluster analysis. Monthly burned areas from the Canadian Large Fire Database (LFDB) for the same period, 1959 to 1996, are analysed in conjunction with the classified monthly 500 hPa synoptic types. Relationships between common monthly patterns of anomalous upper flow and spatial patterns of large fire occurrence are examined at the ecozone level. Average occurrence of a monthly synoptic type associated with very large area burned is approximately 18% of the years from 1959 to 1996. The largest areas burned during the main fire (May to August) season occur in the western Boreal and Taiga ecozones – the Taiga Plains, Taiga Shield, Boreal West Shield and Boreal Plains. Monthly burned areas are also analysed temporally in conjunction with a calculated monthly zonal index (Zi_m) for two separate areas defined to cover western and eastern Canada. In both western and eastern Canada, high area burned is associated with synoptic types with mid-tropospheric ridging in the proximity of the affected region and low Zi_m with weak westerlies and strong meridional flow over western Canada. Received April 3, 2001 Revised July 13, 2001