Carbonate alteration of ophiolitic rocks in the Arabian–Nubian Shield of Egypt: sources and compositions of the carbonating fluid and implications for the formation of Au deposits

Ultramafic portions of ophiolitic fragments in the Arabian–Nubian Shield (ANS) show pervasive carbonate alteration forming various degrees of carbonated serpentinites and listvenitic rocks. Notwithstanding the extent of the alteration, little is known about the processes that caused it, the source of the CO₂ or the conditions of alteration. This study investigates the mineralogy, stable (O, C) and radiogenic (Sr) isotope composition, and geochemistry of suites of variably carbonate altered ultramafics from the Meatiq area of the Central Eastern Desert (CED) of Egypt. The samples investigated include least-altered lizardite (Lz) serpentinites, antigorite (Atg) serpentinites and listvenitic rocks with associated carbonate and quartz veins. The C, O and Sr isotopes of the vein samples cluster between ?8.1‰ and ?6.8‰ for δ¹³C, +6.4‰ and +10.5‰ for δ¹⁸O, and ⁸⁷Sr/⁸⁶Sr of 0.7028–0.70344, and plot within the depleted mantle compositional field. The serpentinites isotopic compositions plot on a mixing trend between the depleted-mantle and sedimentary carbonate fields. The carbonate veins contain abundant carbonic (CO₂±CH₄±N₂) and aqueous-carbonic (H₂O-NaCl-CO₂±CH₄±N₂) low salinity fluid, with trapping conditions of 270–300°C and 0.7–1.1 kbar. The serpentinites are enriched in Au, As, S and other fluid-mobile elements relative to primitive and depleted mantle. The extensively carbonated Atg-serpentinites contain significantly lower concentrations of these elements than the Lz-serpentinites suggesting that they were depleted during carbonate alteration. Fluid inclusion and stable isotope compositions of Au deposits in the CED are similar to those from the carbonate veins investigated in the study and we suggest that carbonation of ANS ophiolitic rocks due to influx of mantle-derived CO₂-bearing fluids caused break down of Au-bearing minerals such as pentlandite, releasing Au and S to the hydrothermal fluids that later formed the Au-deposits. This is the first time that gold has been observed to be remobilized from rocks during the lizardite–antigorite transition.     

Beach fetch distance and aeolian sediment transport

An experiment was conducted to examine the influence of fetch distance on aeolian sediment transport on a natural sand beach at Benone Strand, County Londonderry, Northern Ireland. The site consisted of a wide dissipative beach, approximately 150 m wide at low tide and 80 m wide during high tide. Surface moisture levels (and hence dry fetch distance) were dictated by both local groundwater, from a stream outlet across the beach, as well as local tidal levels. An abundant dry sediment supply was available during the experiment. High-resolution (1 Hz) measurements were made of wind speed and direction along with sediment flux. Wind velocity ranged from 2·1 to 8·1 m s^–1 during the study. Second-order polynomial sand transport equations were derived from the wind speed and trap results with r ² values of better than 0·93 for all data. When the data were sorted into velocity bins of 1 m s^–1, there was no discernible relationship between fetch distance and sand transport, with a measured fetch distance range of 10–58 m available during the experiment. Results show that fetch distance is unimportant when an adequate sand supply is available. However, it is suggested that fetch may restrict the development of steady-state transport under sediment-limited conditions. Sediment availability is thus identified as a key variable in aeolian transport studies on natural beaches.     

A computer modelling study of the interaction of organic adsorbates with fluorapatite surfaces

Computer modelling techniques were employed to investigate the adsorption of a selection of organic surfactant molecules to a range of fluorapatite surfaces, and new interatomic potential models for the apatite/adsorbate interactions are presented. The adsorbates coordinate mainly to the surfaces through interaction between their oxygen (or nitrogen) atoms to surface calcium ions, followed by hydrogen-bonded interactions to surface oxygen ions and, to a much lesser extent, surface fluorides. Bridging between two surface calcium ions is the preferred mode of adsorption, when the geometry of the adsorbates allows it, and multiple interactions between surfaces and adsorbate molecules lead to the largest adsorption energies. All adsorbates containing carbonyl and hydroxy groups interact strongly with the surfaces, releasing energies between approximately 100 and 215 kJ mol⁻¹, but methylamine containing only the –NH₂ functional group adsorbs to the surfaces to a much lesser extent (25–95 kJ mol⁻¹). Both hydroxy methanamide and hydroxy ethanal prefer to adsorb to some surfaces in an eclipsed conformation, which is a requisite for these functional groups. Sorption of the organic material by replacement of pre-adsorbed water at different surface features is calculated to be mainly exothermic for methanoic acid, hydroxy methanamide and hydroxy ethanal molecules, whereas methyl amine would not replace pre-adsorbed water at the fluorapatite surfaces. The efficacy of the surfactant molecules is calculated to be hydroxy aldehydes > alkyl hydroxamates > carboxylic acids ≫ alkyl amines. The results from this study suggest that computer simulations may provide a route to the identification or even design of particular organic surfactants for use in mineral separation by flotation.     

Euler deconvolution in a radial coordinate system

This paper introduces the conversion of Euler's equation from a Cartesian coordinate system to a radial coordinate system, and then demonstrates that for sources of the type 1/r^N (where r is the distance to the source, and N is the structural index) it can be solved at each point in space without the need for inversion, for a known structural index. It is shown that although the distance to the source that is obtained from Euler's equation depends on the structural index used, the direction to the source does not. For some models, such as the gravity and magnetic response of a contact, calculation of the analytic signal amplitude of the data is necessary prior to the application of the method. Effective noise attenuation strategies, such as the use of moving windows of data points, are also discussed. The method is applied to gravity and magnetic data from South Africa, and yields plausible results.     

Sublethal impact of short term exposure to the organophosphate pesticide azamethiphos in the marine mollusc Mytilus edulis

Concern has been raised that the increased use of pesticides in intensive aquaculture practices may cause adverse sublethal effects to non-target aquatic species. Azamethiphos is an organophosphate (OP) pesticide used to combat sea lice infestations in farmed salmonids. Here, the sublethal impact on the blue mussel, Mytilus edulis, of short term exposure to azamethiphos was determined. The testing regime included biomarkers of exposure (acetylcholinesterase activity), cytotoxicity (neutral red retention), immune function (phagocytic index) and physiological condition (feeding rate). The distribution and sensitivity of M. edulis acetylcholinesterase to inhibition by azamethiphos was first determined, yielding IC(50) values of 0.736 and 1.30 mg l(-1) for gill and haemolymph, respectively. Exposure of mussels to 0.1 mg l(-1) azamethiphos for periods of up to 24h caused a significant reduction in acetylcholinesterase activity in both the haemolymph (P<0.0002) and the gill (P<0.002), alteration in cell viability (P<0.02) and decrease in phagocytic index (P<0.03). The feeding rate remained unaffected. The results support the hypothesis that, in addition to its neurotoxic effects, azamethiphos can modulate haemocyte function and immune defence in M. edulis at environmentally relevant concentrations after only a few hours.     

Methods for reactive oxygen species (ROS) detection in aqueous environments

This review summarizes direct and indirect analytical methods for the detection and quantification of the reactive oxygen species (ROS): ¹O₂, O ₂ ^·? /HOO·, H₂O₂, HO·, and CO ₃ ^·? in aqueous solution. Each section briefly describes the chemical properties of a specific ROS followed by a table (organized alphabetically by detection method, i.e., absorbance, chemiluminescence, etc.) summarizing the nature of the observable (associated analytical signal) for each method, limit of detection, application notes, and reaction of the probe molecule with the particular ROS.     

Implications of ozone depletion for surface-water photochemistry: Sensitivity of clear lakes

Increased ultraviolet-B radiation (UV-B) in aquatic ecosystems brought about by stratospheric ozone depletion may result in increased formation rates of photochemical reaction products in the surface waters of aquatic ecosystems. In this study, the potential impact of increases in lower wave-length radiation on the formation of hydrogen peroxide (H₂O₂) and singlet oxygen (¹O₂) was modelled for lakes over a range of dissolved organic carbon (DOC) concentrations. The impact of increased UV-B radiation on the production of carbon monoxide (CO) was also calculated for two humic stained systems. The relative increases of H₂O₂ and¹O₂ production were greater in the clear lakes (70% increase in photochemical reaction rates near the surface) than the coloured systems (25%). Production of CO in the study sites also increased under depleted ozone conditions implying increased DOC losses to the overlying atmosphere.     

A tree-ring reconstruction of East Anglian (UK) hydroclimate variability over the last millennium

We present an annually resolved reconstruction of spring-summer precipitation variability in East Anglia, UK (52–53°N, 0–2°E) for the period AD 900–2009. A continuous regional network of 723 living (AD 1590–2009) and historical (AD 781–1790) oak (Quercus sp.) ring-width series has been constructed and shown to display significant sensitivity to precipitation variability during the March-July season. The existence of a coherent common growth signal is demonstrated in oaks growing across East Anglia, containing evidence of near-decadal aperiodic variability in precipitation throughout the last millennium. Positive correlations are established between oak growth and precipitation variability across a large region of northwest Europe, although climate-growth relationships appear time transgressive with correlations significantly weakening during the early twentieth century. Examination of the relationship between oak growth, precipitation, and the North Atlantic Oscillation (NAO), reveals no evidence that the NAO plays any significant role in the control of precipitation or tree growth in this region. Using Regional Curve Standardisation to preserve evidence of low-frequency growth variability in the East Anglian oak chronology, we produce a millennial length reconstruction that is capable of explaining 32–35% of annual-to-decadal regional-scale precipitation variance during 1901–2009. The full length reconstruction indicates statistically significant anomalous dry conditions during AD 900–1100 and circa-1800. An apparent prolonged wetter phase is estimated for the twelfth and thirteen centuries, whilst precipitation fluctuates between wetter and drier phases at near centennial timescales throughout the fourteenth to seventeenth centuries. Above average precipitation reconstructed for the twenty-first century is comparable with that reproduced for the 1600s. The main estimated wet and dry phases reconstructed here appear largely coherent with an independent tree-ring reconstruction for southern-central England.