Trace-element study and age dating of zircon from chromitites of the Bushveld Complex (South Africa)

The layered Bushveld Complex hosts a number of chromitite layers, which were found to contain significant amounts of zircon grains compared with adjacent silicate rocks. Cathodoluminescent-dark, partially metamict cores and transparent rims of composite zircon grains were analyzed for trace elements with SIMS and LA-ICPMS techniques. The cores are enriched in REE, Y, Th and U and are characterized by distinctly flatter REE patterns in contrast to those of the rims and transparent homogenous crystals. Zircon from the different stratigraphic units has specific Th/U ratios, the highest of which (1.5–4) occurs in a Merensky Reef zircon core. The Ti content of Bushveld zircon ranges from 12 to 52 ppm correlating to a crystallization temperature range of 760–930 °C. The geochemical characteristics of the first zircon generation are consistent with its high-temperature crystallization as the first major U, Th and REE acceptor from a highly-evolved residue of the high-Mg basalt magma, whereas the rims and coreless crystals have crystallized from percolating intercumulus liquid of new influx of the same magma. U-Pb SHRIMP dating of zircon cores and rims does not reveal a distinguishable difference between their ages indicating the absence of inherited zircon. Concordia ages of 2,051?±?9 Ma (2σ, MSWD?=?0.1) and 2,056?±?5 Ma (2σ, MSWD?=?0.05) for zircons from the Merensky Reef and the Upper Platreef located equally near the top of the Critical Zone are in agreement with published ages for the Merensky Reef. Zircon from the deeper-seated Lower Group, Middle Group and Lower Platreef chromitites yields younger concordia ages that may reflect prolonged late-stage volatile activity.     

Modification of the subcontinental mantle beneath East Serbia: Evidence from orthopyroxene-rich xenoliths

Orthopyroxene-rich olivine websterite xenoliths (OWB₂) in Palaeogene basanites in East Serbia are mostly composed of tabular low-Al₂O₃ orthopyroxene (> 70 vol.%, Mg# 85–87) containing tiny Cr spinel inclusions. Orthopyroxene shows a slightly U-shaped primitive mantle-normalized trace element pattern with strong peaks at U and Pb, similar to that of orthopyroxene from normal regional peridotitic mantle. In between the orthopyroxenes are interstitial spaces composed of partially altered olivine (Mg# 85–87), clinopyroxene, Ti-rich spinel, Mg-bearing calcite, K-feldspar, apatite, ilmenite and relicts of a hydrous mineral. Clinopyroxene appears as selvages around orthopyroxene and as coarser euhedral crystals. Trace element patterns of the clinopyroxene selvages resemble those of adjacent orthopyroxene, whereas the coarser ones have flatter and more LREE- and LILE-enriched patterns, similar to that of metasomatic clinopyroxene. The OWB₂ xenoliths are interpreted as having formed in two stages. During Stage I orthopyroxene crystallized, along with some spinel, olivine and probably hydrous phase(s). This original OWB₂ lithology was a hydrous olivine-bearing orthopyroxenite that crystallised from subduction-related SiO₂-saturated, boninite-like magmas. During Stage II the interstitial minerals formed due to infiltration of a low-SiO₂, high-CaO and CO₂-rich external melt, accompanied by decomposition of original H₂O-bearing minerals. The calculated composition of the infiltrating liquid corresponds to a mafic alkaline melt similar to the basanitic host but more enriched in CO₂, LREE and LILE. Metasomatism is interpreted in terms of small degree melts related to the Palaeogene mafic alkaline magmatism.     

The Double Solid Reactant Method for modeling the release of trace elements from dissolving solid phases: I. Outline and limitations

A Double Solid Reactant Method was elaborated from a suggestion of Marini (Geological sequestration of carbon dioxide: Thermodynamics, kinetics, and reaction path modeling. Developments in Geochemistry, Elsevier, Amsterdam, 2007) to simulate the release of trace elements during the progressive dissolution of solid phases. The method is based on the definition, for each dissolving solid, of both an entity whose thermodynamic and kinetic properties are known (either a pure mineral or a solid mixture) and a special reactant, that is, a material of known stoichiometry and unknown thermodynamic and kinetic properties. The special reactant is utilised to take into account the concentrations of trace elements in the dissolving solid phase. In this communication, the influence of several trace elements on the ΔG _f ^o, ΔG _r ^o and log K of the minerals considered by Lelli et al. (Environ Geol, 2007) and Accornero and Marini (Geobasi, 2007a; Proceedings of IMWA symposium, Cagliari, 27–31 May 2007b) was evaluated assuming ideal mixing in the solid state. These effects were found to be negligible for albite and the leucite–latitic glass, limited for muscovites and chlorites, and slightly more important for apatites. These influences become progressively higher with increasing concentration of trace elements in these minerals. Based on these deviations in thermodynamic parameters, special reactants should not include oxide components with molar fractions higher than 0.003. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.

High‐resolution evolution of terrigenous sediment yields in the Provence Basin during the last 6 Ma: relation with climate and tectonics

Basin‐wide correlation of Messinian units and Plio‐Quaternary chronostratigraphic markers (5.3 Ma, 2.6 Ma, 0.9 Ma and 0.45 Ma), the mapping of total sediment thickness and the determination of overall sedimentary volumes enabled us to provide a high‐resolution quantitative history of sediment volumes for the last 6 Ma along the Gulf of Lions margin. The results point to (i) a dramatic increase in terrigenous sediment input during the Messinian Salinity Crisis. This increased sedimentation reflects enhanced regional fluvial erosion related to the dramatic fall of Mediterranean base‐level. Stronger weathering due to a regional wetter climate probably also increased erosional fluxes. (ii) A sediment input three times higher during the Plio‐Quaternary compared to the Miocene seems in agreement with published measurements from World's ocean. However, the timing of this increase being uncertain, it implies that the trigger(s) also remain(s) uncertain. (iii) A decrease in detrital volume around 2.6 Ma is attributed to a regional change in the drainage pattern of rivers in the northwestern Alps. (iv) This study also highlights the Mid‐Pleistocene Revolution around 0.9 Ma, which resulted in an almost doubling of sediment input in the Provencal Basin.     

Experiments with Higdon's absorbing boundary conditions for a number of wave equations

Simulation of wave propagation for seismic purposes is usually restricted to a small portion of the earth. Artificial boundary conditions are required where the subsurface model is truncated. Absorbing boundaries should ensure that waves hitting the artificial boundaries are not reflected. The vast amount of literature on the subject suggests that “good” conditions have not been found, and only “reasonable” solutions exist. A cursory overview of existing and a few new ideas is presented that may guide the construction of suitable boundary conditions. Because the intended application of the boundary conditions was a high-order finite-difference code that runs on a parallel computer, we have restricted our attention to local boundary conditions. A fundamental problem in the design of accurate local boundary conditions is pointed out: accuracy is required to keep the amount of reflected energy small, but at the same time allows for growing low-frequency modes. We have settled for Higdon’s boundary conditions. Higdon proposes to include some damping to suppress the growing low-frequency modes. We show that third-order conditions provide acceptable results for the simple scalar wave equation and the acoustic equation. In the elastic case, an additional low-frequency growing mode may occur. This mode can be suppressed by using a dissipative boundary scheme and by increasing the amount of damping. The increase in damping results in an increase in the amount of reflected energy, which is larger than in the scalar case. Numerical experiments exhibit a reasonable performance, although some improvement would be useful, particularly in the anisotropic elastic case. This revised version was published online in July 2006 with corrections to the Cover Date.     

Preparation and application of engineering and environmental geological maps in The Netherlands

The Netherlands has no nation-wide mapping programme for either engineering geology or environmental geology, but on request the Geological Survey of The Netherlands, together with other geoscientific institutes, prepares: (1) detailed (1:5,000) engineering geological maps of urban areas, specifically meant for urban planning and management as well as for feasibility studies for civil engineering works; and (2) small-scale (1:250,000 to 1:50,000) thematic environmental geological maps, meant for regional landuse planning. The preparation and application of these maps is discussed, and some examples are given.     

Sustainable urban drainage systems: examining the potential for green infrastructure-based stormwater management for Sub-Saharan cities

Green infrastructure (GI)-based approaches to urban drainage such as sustainable urban drainage systems (SUDS) could provide Sub-Saharan cities with an opportunity to address projected climate change impacts and existing deficits in their drainage infrastructure, even more so due to the synergies between an enhanced green infrastructure stock and sustainable urban development. The objective of this paper was to assess the theoretical value of using green infrastructure for stormwater management as an alternative and supplement to conventional pipe-based stormwater management systems. A SWOT analysis is performed to assess the potential that SUDS hold if adopted and implemented in Sub-Saharan cities. This analysis is based on a review of sustainable stormwater management as well as urban planning and governance literature. Results show that despite seemingly significant barriers to the adoption of SUDS in Sub-Saharan cities such as low prioritization on the urban agenda and lack of data among others, the concept may hold valuable potential for flood risk reduction, even more so due to its multi-functionality and synergies with urban agriculture, amenity and water supply. In the light of the existing threats and weaknesses, it is recommended that GI-based SUDS may be best approached initially as experiments at a local community scale.     

Mt. Etna volcano high-resolution topography: airborne LiDAR modelling validated by GPS data

High-resolution digital topography is essential for land management and planning in any type of territory as well as the reproduction of the Earth surface in a geocoded digital format that allows several Digital Earth applications. In a volcanic environment, Digital Elevation Models are a valid reference for multi-temporal analyses aimed to observe frequent changes of a volcano edifice and for the relative detailed morphological and structural analyses. For the first time, a DTM (Digital Terrain Model) and a DSM (Digital Surface Model) covering the entire Mt. Etna volcano (Italy) derived from the same airborne Light Detection and Ranging (LiDAR) are here presented. More than 250 million 3D LiDAR points have been processed to distinguish ground elements from natural and anthropic features. The end product is the highly accurate representation of Mt. Etna landscape (DSM) and ground topography (DTM) dated 2005. Both models have a high spatial resolution of 2?m and cover an area of 620?km². The DTM has been validated by GPS ground control points. The vertical accuracy has been evaluated, resulting in a root-mean-square-error of ±?0.24?m. The DTM is available as electronic supplement and represents a valid support for various scientific studies.     

The Var turbidite system (Ligurian Sea, northwestern Mediterranean)—morphology, sediment supply, construction of turbidite levee and sediment waves: implications for hydrocarbon reservoirs

The Var turbidite system is a small sandy system located in the Ligurian Basin. It was deposited during the Pliocene-Quaternary in a flat-floored basin formed during the Messinian salinity crisis. The system was fed through time by the Var and Paillon canyons that connect directly to the Var and Paillon rivers. It is still active during the present sea-level highstand. Two main mechanisms are responsible for gravity-flow triggering in the Var turbidite system: (1) mass-wasting events affect mainly the upper part of the continental slope, in areas where volumes of fresh sediment delivered by rivers are highest, and result from the under-consolidation state of slope sediments and earthquakes, and (2) high-magnitude river floods resulting from melting of snow and convective rainfall during fall and spring seasons, and generating hyperpycnal turbidity currents at river mouths when the density of freshwater transporting suspended particles exceeds that of ambient seawater. Failure- and flood-induced gravity flows are involved through time in the construction of the Var Sedimentary Ridge, the prominent right-hand levee of the Var system, and sediment waves. Processes of construction of both the Var Ridge and sediment waves are closely connected. Sandy deposits are thick and abundant in the eastern (downchannel) part of the ridge. Their distribution is highly constrained by the strong difference of depositional processes across the sediment waves, potentially resulting through time in the individualization of large and interconnected sand bodies.