Layered mantle structure beneath the western Guyana Shield, Venezuela: Evidence from diamonds and xenocrysts in Guaniamo kimberlites

Mantle xenoliths and xenocrysts from Guaniamo, Venezuela kimberlites record equilibration conditions corresponding to a limited range of sampling in the lithosphere (100-150 km). Within this small range, however, compositions vary considerably, but regularly, defining a strongly layered mantle sequence. Major and trace element compositions suggest the following lithologic sequence: highly depleted lherzolite from 100 to 115 km, mixed ultra-depleted harzburgite and lherzolite from 115 to 120 km, relatively fertile lherzolite from 120 to 135 km, and mixed depleted harzburgite and relatively fertile lherzolite from 135 to 150 km. Based on comparison with well-documented mantle peridotites and xenocrysts from elsewhere, we conclude that the Meso-proterozoic Cuchivero Province (host to the Guaniamo kimberlites) is underlain by depleted and ultra-depleted shallow Archean mantle that was underplated, and uplifted, by Proterozoic subduction, perhaps more than once. These Proterozoic subduction events introduced less-depleted oceanic lithosphere beneath the Archean section, which remains there and is the source of the abundant Guaniamo eclogite-suite diamonds that have ocean-floor geochemical signatures. Although diamond-indicative low-Ca Cr-pyrope garnets are abundant, they are derived primarily from the shallow depleted layer within the field of graphite stability, and the rare peridotite-suite diamonds are either metastably preserved at these shallow depths, or were derived from the small amount of depleted lithosphere sampled by these kimberlites that remains within the diamond stability field (the mixture of Archean and Proterozoic mantle in the depth range 135-150 km).     

The impact of vegetation on REE fractionation in stream waters of a small forested catchment (the Strengbach case)

Previous studies on waters of a streamlet in the Vosges Mountains (Eastern France) have shown that strontium and rare earth elements (REE) mainly originate from preferential dissolution of apatite during weathering. However, stream water REE patterns normalized to apatite are still depleted in the light REE (LREE, La-Sm) pointing to the presence of an additional LREE depleting process. Vegetation samples are strongly enriched in LREE compared to stream water and their Sr and Nd isotopic compositions are comparable with those of apatite and stream water. Thus, the preferential LREE uptake by vegetation might lead to an additional LREE depletion of surface runoff in the forested catchment. Mass balance calculations indicate, that the yearly LREE uptake by vegetation is comparable with the LREE export by the streamlet and, therefore, might be an important factor controlling LREE depletion in river water. This is underlined by the observation that rivers from arctic and boreal regions with sparse vegetation appear to be less depleted in LREE than rivers from tropical environments or boreal environments with a dense vegetation cover.     

Assessment of the mycoflora of commercial poultry feeds sold in the humid tropical environment of Imo State, Nigeria

This study was carried out to identify the common moulds growing in selected commercial poultry feed sold in Owerri, Imo State, Nigeria. Forty-eight bulk samples derived froml92 bags of feeds were collected from broiler starter (BS), broiler finishers (BF), grower mash (GM) and layer mash (LM) across 4 different brands of ommercial poultry feeds, which included Livestock (LF), Top (TF) Guinea (GF) and Vital (VF) feeds. The feed samples were collected during the rainy season months of June, July and August. The common moulds isolated from these feeds were Aspergillus sp., Peicillium sp.,Mucor sp., Yeast sp., Rhizopus sp., Epicoecum sp., Gymnoaseus sp., Cladosporium sp., Mortierella sp. as well as Bacteria. Generally, more organisms were isolated in June than the other months with Mortierella sp. being the only one not isolated in that month. Vital feed with 8 different isolates had the highest diversity of fungal species while the others had between 4 and 5 species. Prevalence rates across the feed types, generally ranged from 18.76% in layer mash to 30.03% in broiler finisher. The four Aspergillus sp. isolated came from GM and BF. This study highlights the need for constant monitoring of moulds in commercial feedstuff produced in the humid tropical environments such as Imo state, Nigeria. There is also the need to routinely include fungal growth nhibitors in commercial feeds since moulds are capable of reducing the nutritional values of feedstuff as well as elaborating pathogenic toxins.     

Visualization experiments of iron precipitates: Application for in-situ arsenic remediation

Laboratory experiments were carried out to acquire more insight and understanding of the phenomena associated with the in-situ arsenic remediation. Visualization techniques are the most informative for the detection of Fe(II) while flowing in soils. Green Rust (GR) was considered as representative of in-situ iron precipitates. In a visualization flat cell, the change in color of GR to orange, due to oxidation, was monitored by a digital camera and the images were analyzed giving the spatial and temporal distribution of Fe(II). Moreover, both oxygen and pH changes in time were recorded in two sections along the flow direction in the cell. The measured and calculated concentration profiles were compared and the actual reaction rates were predicted. The reaction rate constants measured in this study, under flowing conditions, are in a good agreement with the values obtained from batch experiments reported in the literature.     

Environmental impact of the former Pb–Zn mining and smelting in East Belgium

In the mining district of Plombières-La Calamine (East Belgium), extensive Pb–Zn mining activities resulted in an important contamination of overbank sediments along the Geul river. Moreover, a huge amount of heavy metals is stored in a dredged mine pond tailing, which is located along the river. In the dredged mine pond tailing sediments, Pb–Zn minerals control the solubility of Zn, Pb and Cd. Although Pb, Zn and Cd display a lower solubility in overbank sediments compared to the mine tailing pond sediments, elevated concentrations of Pb, Zn and Cd are still found in the porewater of the overbank sediments. The considerable ‘actual’ and ‘potential’ mobility of Zn, Pb and Cd indicates that the mine pond tailing sediments and the overbank sediments downstream from the mine pond tailing represent a considerable threat for the environment. Besides the chemical remobilisation of metals from the sediments, the erosion of overbank sediments and the reworking of riverbed sediments act as a secondary source of pollution.     

Holocene record of productivity in the NW Iberian continental shelf

A multiproxy approach (textural characteristics, heavy metal concentrations and benthic foraminifera data) was adopted to study the OMEX core KSGX 40, collected at the Galicia Mud Deposit (NW Iberian outer continental shelf) and recording the last ca. 4.8 ka cal BP. Geochemical profiles of Fe, Mn, Zn, Co, Cr, Cu, Ni, Pb, Al and Ca show significant temporal variations. Benthic foraminifera productivity proxy suggests that the recorded muddy events were contemporaneous of a higher C_org flux to the marine benthic environment. The higher flux of organic matter, following a long-term intensification of the upwelling and coinciding with a finer sedimentation, led to depressed levels of oxygen beneath the water-sediments interface and to early diagenetic processes of several redox-sensitive elements.