The lateritic profile of Balkouin, Burkina Faso: geochemistry, mineralogy and genesis

This study reports on the geochemical and mineralogical characterization of a lateritic profile cropping out in the Balkouin area, Central Burkina Faso, aimed at obtaining a better understanding of the processes responsible for the formation of the laterite itself and the constraints to its development. The lateritic profile rests on a Paleoproterozoic basement mostly composed of granodioritic rocks related to the Eburnean magmatic cycle passing upwards to saprolite and consists of four main composite horizons (bottom to top): kaolinite and clay-rich horizons, mottled laterite and iron-rich duricrust. In order to achieve such a goal, a multi-disciplinary analytical approach was adopted, which includes inductively coupled plasma (ICP) atomic emission and mass spectrometries (ICP-AES and ICP-MS respectively), X-ray powder diffraction (XRPD), scanning electron microscopy with energy dispersive spectrometry (SEM-EDS) and micro-Raman spectroscopy.

The geochemical data, and particularly the immobile elements distribution and REE patterns, show that the Balkouin laterite is the product of an in situ lateritization process that involved a strong depletion of the more soluble elements (K, Mg, Ca, Na, Rb, Sr and Ba) and an enrichment in Fe; Si was also removed, particularly in the uppermost horizons. All along the profile the change in composition is coupled with important changes in mineralogy. In particular, the saprolite is characterized by occurrence of abundant albitic plagioclase, quartz and nontronite; kaolinite is apparently absent. The transition to the overlying lateritic profile marks the breakdown of plagioclase and nontronite, thus allowing kaolinite to become one of the major components upwards, together with goethite and quartz. The upper part of the profile is strongly enriched in hematite (+ kaolinite). Ti oxides (at least in part as anatase) and apatite are typical accessory phases, while free aluminum hydroxides are notably absent. Mass change calculations emphasize the extent of the mass loss, which exceeds 50 wt% (and often 70 wt%) for almost all horizons; only Fe was significantly concentrated in the residual system.

The geochemical and mineralogical features suggest that the lateritic profile is the product of a continuous process that gradually developed from the bedrock upwards, in agreement with the Schellmann classic genetic model. The laterite formation must have occurred at low pH (? 4.5) and high Eh (? 0.4) values, i.e., under acidic and oxidizing environments, which allowed strongly selective leaching conditions. The lack of gibbsite and bohemite is in agreement with the compositional data: the occurrence of quartz (± amorphous silica) all along the profile was an inhibiting factor for the formation of free aluminum hydroxides.

Integrating petrography, mineralogy and hydrochemistry to constrain the influence and distribution of groundwater contributions to baseflow in poorly productive aquifers: Insights from Gortinlieve catchment, Co. Donegal, NW Ireland

Identifying groundwater contributions to baseflowforms an essential part of surfacewater body characterisation. The Gortinlieve catchment (5 km2) comprises a headwater stream network of the Carrigans River, itself a tributary of the River Foyle, NW Ireland. The bedrock comprises poorly productive metasediments that are characterised by fracture porosity. We present the findings of a multi-disciplinary study that integrates new hydrochemical and mineralogical investigations with existing hydraulic, geophysical and structural data to identify the scales of groundwater flow and the nature of groundwater/bedrock interaction (chemical denudation). At the catchment scale, the development of deep weathering profiles is controlled by NE-SW regional scale fracture zones associated with mountain building during the Grampian orogeny. In-situ chemical denudation of mineral phases is controlled by micro- to meso-scale fractures related to Alpine compression during Palaeocene to Oligocene times. The alteration of primary muscovite, chlorite (clinochlore) and albite along the surfaces of these small-scale fractures has resulted in the precipitation of illite, montmorillonite and illite/montmorillonite clay admixtures. The interconnected but discontinuous nature of these small-scale structures highlights the role of larger scale faults and fissures in the supply and transportation of weathering solutions to/from the sites of mineral weathering. The dissolution of primarily mineral phases releases the major ions Mg, Ca and HCO3 that are shown to subsequently formthe chemical makeup of groundwaters. Borehole groundwater and stream baseflow hydrochemical data are used to constrain the depths of groundwater flow pathways influencing the chemistry of surface waters throughout the stream profile. The results show that it is predominantly the lower part of the catchment, which receives inputs from catchment/regional scale groundwater flow, that is found to contribute to the maintenance of annual baseflow levels. This study identifies the importance
of deep groundwater in maintaining annual baseflow levels in poorly productive bedrock systems.

GPCR structure, function, drug discovery and crystallography: report from Academia-Industry International Conference (UK Royal Society) Chicheley Hall, 1-2 September 2014

G-protein coupled receptors (GPCRs) are the targets of over half of all prescribed drugs today. The UniProt database has records for about 800 proteins classified as GPCRs, but drugs have only been developed against 50 of these. Thus, there is huge potential in terms of the number of targets for new therapies to be designed. Several breakthroughs in GPCRs biased pharmacology, structural biology, modelling and scoring have resulted in a resurgence of interest in GPCRs as drug targets. Therefore, an international conference, sponsored by the Royal Society, with world-renowned researchers from industry and academia was recently held to discuss recent progress and highlight key areas of future research needed to accelerate GPCR drug discovery. Several key points emerged. Firstly, structures for all three major classes of GPCRs have now been solved and there is increasing coverage across the GPCR phylogenetic tree. This is likely to be substantially enhanced with data from x-ray free electron sources as they move beyond proof of concept. Secondly, the concept of biased signalling or functional selectivity is likely to be prevalent in many GPCRs, and this presents exciting new opportunities for selectivity and the control of side effects, especially when combined with increasing data regarding allosteric modulation. Thirdly, there will almost certainly be some GPCRs that will remain difficult targets because they exhibit complex ligand dependencies and have many metastable states rendering them difficult to resolve by crystallographic methods. Subtle effects within the packing of the transmembrane helices are likely to mask and contribute to this aspect, which may play a role in species dependent behaviour. This is particularly important because it has ramifications for how we interpret pre-clinical data. In summary, collaborative efforts between industry and academia have delivered significant progress in terms of structure and understanding of GPCRs and will be essential for resolving problems associated with the more difficult targets in the future.

Mineralogy, chemistry and biological contingents of an early-middle Miocene Antarctic paleosol and its relevance as a Martian analogue

Fossil mesofauna and bacteria recovered from a paleosol in a moraine situated adjacent to the inland ice, Antarctica, and dating to the earliest glacial event in the Antarctic Dry Valleys opens several questions. The most important relates to understanding of the mineralogy and chemistry of the weathered substrate habitat in which Coleoptera apparently thrived at some point in the Early/Middle Miocene and perhaps earlier. Here, Coleoptera remains are only located in one of six horizons in a paleosol formed in moraine deposited during the alpine glacial event (> 15 Ma). A tendency for quartz to decrease upward in the section may be a detrital effect or a product of dissolution in the early stage of profile morphogenesis when climate was presumably milder and the depositing glacier of temperate type. Discontinuous distributions of smectite, laumontite, and hexahydrite may have provided nutrients and water to mesofauna and bacteria during the early stage of biotic colonization of the profile. Because the mesofauna were members of burrowing Coleoptera species, future work should assess the degree to which the organisms occupied other sites in the Dry Valleys in the past. Whereas there is no reasonable expectations of finding Coleoptera/insect remains on Mars, the chemistry and mineralogy of the paleosol is within a life expectancy window for the presence of microorganisms, principally bacteria and fungi. Thus, parameters discussed here within this Antarctic paleosol could provide an analogue to identifying similar fossil or life-bearing weathered regolith on Mars.

How Porosity and Permeability Vary Spatially With Grain Size, Sorting, Cement Volume, and Mineral Dissolution In Fluvial Triassic Sandstones: The Value of Geostatistics and Local Regression

Although it is well known that sandstone porosity and permeability are controlled by a range of parameters such as grain size and sorting, amount, type, and location of diagenetic cements, extent and type of compaction, and the generation of intergranular and intragranular secondary porosity, it is less constrained how these controlling parameters link up in rock volumes (within and between beds) and how they spatially interact to determine porosity and permeability. To address these unknowns, this study examined Triassic fluvial sandstone outcrops from the UK using field logging, probe permeametry of 200 points, and sampling at 100 points on a gridded rock surface. These field observations were supplemented by laser particle-size analysis, thin-section point-count analysis of primary and diagenetic mineralogy, quantitiative XRD mineral analysis, and SEM/EDAX analysis of all 100 samples. These data were analyzed using global regression, variography, kriging, conditional simulation, and geographically weighted regression to examine the spatial relationships between porosity and permeability and their potential controls. The results of bivariate analysis (global regression) of the entire outcrop dataset indicate only a weak correlation between both permeability porosity and their diagenetic and depositional controls and provide very limited information on the role of primary textural structures such as grain size and sorting. Subdividing the dataset further by bedding unit revealed details of more local controls on porosity and permeability. An alternative geostatistical approach combined with a local modelling technique (geographically weighted regression; GWR) subsequently was used to examine the spatial variability of porosity and permeability and their controls. The use of GWR does not require prior knowledge of divisions between bedding units, but the results from GWR broadly concur with results of regression analysis by bedding unit and provide much greater clarity of how porosity and permeability and their controls vary laterally and vertically. The close relationship between depositional lithofacies in each bed, diagenesis, and permeability, porosity demonstrates that each influences the other, and in turn how understanding of reservoir properties is enhanced by integration of paleoenvironmental reconstruction, stratigraphy, mineralogy, and geostatistics.

The importance of solid-phase distribution on the oral bioaccessibility of Ni and Cr in soils overlying Palaeogene basalt lavas, Northern Ireland

Potentially toxic elements (PTEs) including nickel and chromium are often present in soils overlying basalt at concentrations above regulatory guidance values due to the presence of these elements in underlying geology. Oral bioaccessibility testing allows the risk posed by PTEs to human health to be assessed; however, bioaccessibility is controlled by factors including mineralogy, particle size, solid-phase speciation and encapsulation. X-ray diffraction was used to characterise the mineralogy of 12 soil samples overlying Palaeogene basalt lavas in Northern Ireland, and non-specific sequential extraction coupled with chemometric analysis was used to determine the distribution of elements amongst soil components in 3 of these samples. The data obtained were related to total concentration and oral bioaccessible concentration to determine whether a relationship exists between the overall concentrations of PTEs, their bioaccessibility and the soils mineralogy and geochemistry. Gastric phase bioaccessible fraction (BAF %) ranged from 0.4 to 5.4 % for chromium in soils overlying basalt and bioaccessible and total chromium concentrations are positively correlated. In contrast, the range of gastric phase BAF for nickel was greater (1.4–43.8 %), while no significant correlation was observed between bioaccessible and total nickel concentrations. However, nickel BAF was inversely correlated with total concentration. Solid-phase fractionation information showed that bioaccessible nickel was associated with calcium carbonate, aluminium oxide, iron oxide and clay-related components, while bioaccessible chromium was associated with clay-related components. This suggests that weathering significantly affects nickel bioaccessibility, but does not have the same effect on the bioaccessibility of chromium.

Laterite as a disinfectant.

A groundwater programme monitoring flow and quality of a potable water spring in a slum district in Kampala, Uganda revealed that although latrines acted as the principal means of organic waste disposal for the 1000 plus people living in the spring’s catchment, levels of faecal indicator bacteria (TVC 45 Deg C) in spring discharge remained at or below detection during the dry season, despite the presence of high levels of chloride (45mg/l-56mg/l) and nitrate (23mg/l – 30mg/l NO3-N), indicating sewage impacts. A programme of column and batch testing of laterite underlying the area provided a means of investigating the soil’s attenuation capacity under more controlled conditions.
X-ray diffraction analyses revealed the laterite to be dominated by quartz and kaolinite with minor (<5% by volume) quantities of haematite. Batch studies revealed that over 99% of bacteriophage adsorbed to haematite in less than 5 minutes. By contrast batch tests on haematite-free soil samples from the Blue Hills in Australia showed that although they had comparable dominant mineralogy and iron coverage on their surfaces (determined from Energy dispersive X-ray fluorescence) they had negligible ability to adsorb H40/1.
Based on the results of the batch studies using natural soils, a programme of batch studies, undertaken using pure haematite showed the mineral to have an extremely high capacity to adsorb bacteriophage, and suggested that it was responsible for the levels of attenuation observed.
The results of column studies were in keeping with the findings of batch experiments. Injection of 20 pore volumes of 300 pfu/mL of the bacteriophage H40/1 into a 20mm diameter glass column packed with sand sized (Ø>500µm) laterite revealed that the column could irreversibly remove over 2.5 log10 bacteriophage over its 10cm length.
Importance:
Mineralogical and batch test data provide convincing evidence to show that laterite can potentially act as an inexpensive means of removing micro organisms from water. The material, particularly in nodular form, displays considerable potential to act as an alternative filter material to conventional quartz filter sands.

Rapid magmatic processes accompany arc-continent collision: the Western Bismarck arc, Papua New Guinea

New U-Th-Ra, major and trace element, and Sr-Nd-Pb isotope data are presented for young lavas from the New Britain and Western Bismarck arcs in Papua New Guinea. New Britain is an oceanic arc, whereas the latter is the site of an arc-continent collision. Building on a recent study of the Manus Basin, contrasts between the two arcs are used to evaluate the processes and timescales of magma generation accompanying arc-continent collision and possible slab detachment. All three suites share many attributes characteristic of arc lavas that can be ascribed to the addition of a regionally uniform subduction component derived from the subducting altered oceanic crust and sediment followed by dynamic melting of the modified mantle. However, the Western Bismarck arc lavas diverge from the Pb isotope mixing array formed by the New Britain and the Manus Basin lavas toward elevated Pb-208/Pb-204. We interpret this to reflect a second and subsequent addition of sediment melt at crustal depth during collision. U-238 and Ra-226 excesses are preserved in all of the lavas and are greatest in the Western Bismarck arc. High-Mg andesites with high Sr/Y ratios in the westernmost arc are attributed to recent shallow mantle flux melting at the slab edge. Data for two historical rhyolites are also presented. Although these rhyolites formed in quite different tectonic settings and display different geochemical and isotopic compositions, both formed from mafic parents within millennia.