Peridotitic mantle xenoliths from kimberlites on the Ekati Diamond Mine property, N.W.T., Canada: major element compositions and implications for the lithosphere beneath the central Slave craton

The composition, structure and thermal state of the lithosphere beneath the Slave craton have been studied by analysing over 300 peridotitic mantle xenoliths or multiphase xenocrysts entrained within kimberlites in the Lac de Gras area. These xenoliths are derived from seven kimberlites located on the Ekati Diamond Mine™ property and define a detailed stratigraphic profile through the central Slave lithosphere from less than 120 km down to 200 km. Two dominant peridotite types are present, namely garnet-bearing harzburgite and lherzolite with rare occurrences of chromite-facies peridotite, websterite and wehrlite. The pressures and temperatures (P–T's) defined by the entire data-set range from 28 to 62 kbar and 650 to 1250 °C, respectively, and approximately intersect the diamond stability field at 900 °C and 42 kbar. There is no apparent change in the geotherm with depth that is discernable beyond the resolution of the various thermobarometers. The peridotites can be divided into two compositional zones—a shallow layer dominated by garnet harzburgite that straddles the diamond–graphite boundary and a deeper layer that is strongly dominated by garnet lherzolite. Compositionally, the harzburgites (and to a lesser extent, the shallow lherzolites) are ultra-depleted relative to the more fertile deeper layer, irrespective of whether they reside within the graphite or diamond stability field. This ultra-depleted layer beneath Ekati continues to 150 km.     

Late Cenozoic magmatism of the Bolivian Altiplano

Small basalt to dacite volcanic centers are distributed sparsely over the Bolivian Altiplano, behind the Andean volcanic front. Most are Pliocene to Recent in age, and are characterized by textural and mineralogical disequilibrium with abundant xenoliths and xenocrysts. True phenocrysts are rare in the more mafic samples. Compared with Recent volcanic rocks from Andean stratovolcanoes, the Bolivian centers overlap in major element trends. Incompatible element contents tend to be higher, particularly in the eastern Altiplano. The ranges of isotopic compositions reflect ubiquitous crustal contamination. Pb isotope compositions are dominated by Pb from isotopically heterogeneous basement, resulting in a wide scatter of data lying between inferred crustal compositions and showing little overlap with possible mantle sources in the region. Rocks sampled from the Bolivian Altiplano were probably derived from asthenospheric mantle and subjected to extensive open system differentiation during ascent through the 70 km thick crust of the region. Major element trends are largely controlled by the fractionating phase assemblage (olivine, clinopyroxene and amphibole). Trace element and isotope systematics, however, defy realistic attempts at modeling due to the geographic scatter of samples, the uniformity of compositions at a given center, and the heterogeneity of the contaminant. Nevertheless, there are first order systematic trace element variations that appear to relate to the geometry of the subduction zone. In particular, LIFE/HFSE (exemplified by Ba/Nb), and Zr/Nb ratios decrease from the arc front eastward into the Altiplano. These variations are not easily reconciled with control by crustal contamination alone. A model is proposed in which the asthenospheric source is fluxed by high Ba/Nb slab-derived fluid to induce melting. Beneath the arc, high fluid flux increases the Ba/Nb ratio of the asthenosphre and leads to high degrees of partial melting (high Zr/Nb). To the east, lower or no fluid flux leads to low Ba/Nb and low degrees of partial melting (low Zr/Nb). Melting in the back arc region of the Altiplano may be facilitated by lithospheric delamination that leads to decompression melting of counter-flowing asthenosphere. There is no unequivocal evidence that requires a significant role for the lithospheric mantle.     

A Systematic Approach to Designing a Multiphase Unsaturated Zone Monitoring Network

A systematic approach is presented for the design of a multiphase vadose zone monitoring system recognizing that, as in ground water monitoring system design, complete subsurface coverage is not practical. The approach includes identification and prioritization of vulnerable areas: select ion of cost-effective indirect monitoring methods that will provide early warning of contaminant migration: selection of direct monitoring methods for diagnostic confirmation; identification of background monitoring locations; and identification of an appropriate temporal monitoring plan. An example of a monitoring system designed for a solid waste landfill is presented and utilized to illustrate the approach and provide details of system implementation. The example design described incorporates the use of neutron moisture probes deployed in both vertical and horizontal access tubes beneath the lcachate recovery collection system of the landfill. Early warning of gaseous phase contaminant migration is monitored utilizing whole-air active soil gas sampling points deployed in gravel- filled trenches beneath the subgrade. Diagnostic confirmation of contaminant migration is provided utilizing pore- liquid samplers. Conservative tracers can be used to distinguish between chemical species released by a landfill from those attributable to other (e.g. off-site) sources or present naturally in the subsurface. A discussion of background monitoring point location is also presented.     

Aerodynamic entrainment thresholds and dislodgement rates on impervious and permeable beds

In the literature it has been suggested that on permeable, granular beds, both the threshold and rate of aerodynamic entrainment may be affected significantly by seepage flows into and out of the bed induced by fluctuating pressures in the overlying turbulent boundary layer. Using a range of grain sizes and flow conditions, the series of laboratory experiments reported here compares directly the aerodynamic entrainment of loose grains overlying fixed permeable sediment beds with that occuring over fixed impervious beds. For a given granular material, no significant differences in entrainment dynamics on the two types of bed were observed and in the range of flow conditions examined both the threshold shear velocity (U _*T) and the aerodynamic entrainment coefficient (k) were found to be independent of bed permeability.     

Enforcement economics in Exclusive Economic Zones

The establishment of Exclusive Economic Zones places enormous burdens on the resources of coastal states' enforcement agencies. An economic problem looms in that decisions must be made as to how much to allocate to enforcement relative to the expected benefits that can be derived. In this paper, the objective is to review the economic methods available for analysis of enforcement programs. A second objective is to report on the enforcement problems and prospects in three areas with important fishery resources.     

The origin and evolution of magmas from the San Pedro-Pellado volcanic complex,S. Chile: multicomponent sources and open system evolution

The San Pedro-Pellado volcanic complex is located at 36° S in the Chilean Andes. The eruptive rocks of the complex record the development and collapse of a caldera, followed by voluminous, largely basaltic andesite, volcanism. At each stage of evolution, crystal fractionation was accompanied by variable degrees of contamination and mixing. Large variations in incompatible element ratios cannot be produced by closed system evolution. Correlations between indices of differentiation and incompatible element ratios, together with high ¹⁸O values, indicate that basaltic andesites have assimilated crust to generate the evolved volcanic rocks at San Pedro-Pellado. Even in the most mafic rocks, however, incompatible element characteristics are variable as a result of source heterogeneity and deep level processes. The restricted ranges in isotope ratios of Sr, Nd and Pb among San Pedro-Pellado rocks are due to the small contrast in isotopic compositions between magma and wallrock. Three source components are recognized as contributing to parental magmas at San Pedro-Pellado. Although the relative contributions of each cannot be quantified, the volumetrically dominant source component is the sub-arc asthenospheric mantle (MORB source). The major source of LILE is thought to be slab-derived fluids which modified the sub-arc mantle. Other incompatible elements may also have been enriched by interaction with the continental lithosphere (mantle and/or lower crust) during ascent.     

Polycyclic aromatic hydrocarbons in sediments of the Great Barrier Reef region,Australia

At present the concentrations of polycyclic aromatic hydrocarbons in sediments from the Great Barrier Reef are remarkably low. The baseline levels range from <0.01 μg kg^?1 dry wt. for benzo(k)fluoranthene and benzo(a)pyrene to <0.82 μg kg^?1 for chrysene. Measurable levels of polycyclic aromatic hydrocarbons have only been found in small areas close to sites frequently visited by power boats. Sediments from Townsville and Gladstone Harbours on the coast adjacent to the reef are polluted with polycyclic aromatic hydrocarbons to a similar degree to that reported for coastal sediments in similar locations in other parts of the world.     

Snow and ice perturbation during historical volcanic eruptions and the formation of lahars and floods

Historical eruptions have produced lahars and floods by perturbing snow and ice at more than 40 volcanoes worldwide. Most of these volcanoes are located at latitudes higher than 35°; those at lower latitudes reach altitudes generally above 4000 m. Volcanic events can perturb mantles of snow and ice in at least five ways: (1) scouring and melting by flowing pyroclastic debris or blasts of hot gases and pyroclastic debris, (2) surficial melting by lava flows, (3) basal melting of glacial ice or snow by subglacial eruptions or geothermal activity, (4) ejection of water by eruptions through a crater lake, and (5) deposition of tephra fall. Historical records of volcanic eruptions at snow-clad volcanoes show the following: (1) Flowing pyroclastic debris (pyroclastic flows and surges) and blasts of hot gases and pyroclastic debris are the most common volcanic events that generate lahars and floods; (2) Surficial lava flows generally cannot melt snow and ice rapidly enough to form large lahars or floods; (3) Heating the base of a glacier or snowpack by subglacial eruptions or by geothermal activity can induce basal melting that may result in ponding of water and lead to sudden outpourings of water or sediment-rich debris flows; (4) Tephra falls usually alter ablation rates of snow and ice but generally produce little meltwater that results in the formation of lahars and floods; (5) Lahars and floods generated by flowing pyroclastic debris, blasts of hot gases and pyroclastic debris, or basal melting of snow and ice commonly have volumes that exceed 10⁵ m³.The glowing lava (pyroclastic flow) which flowed with force over ravines and ridges...gathered in the basin quickly and then forced downwards. As a result, tremendously wide and deep pathways in the ice and snow were made and produced great streams of water (Wolf 1878).     

Isotope Ratio Determination in the Earth and Environmental Sciences: Developments and Applications in 2004/2005

This review documents developments and applications in the field of isotope ratio determination, as reflected in the literature for the Earth and environmental sciences for the years 2004 and 2005. The emphasis is largely one on applications, reflecting the enormous diversity of problems to which isotopic analysis is now applied, but seen against a background of rapid change in the uptake of new analytical technologies.     

Some properties of decorrelation techniques in the ambiguity space

The most recent contributions to ambiguity resolution techniques have mainly focused on resolution in the ambiguity domain. Two techniques utilizing a decorrelation approach are compared. These techniques are the least-squares ambiguity decorrelation adjustment method and the lattice basis reduction. The latter is also known as the LLL method. The main focus in this article is on the decorrelation performance of these state-of-the-art techniques, which are aiming at ambiguity space decorrelation through unimodular transformations. The performances of the two-decorrelation techniques are compared through their ability in making the ambiguity space as orthogonal as possible.