Magnetic studies of magma-supply and sea-floor metamorphism: Troodos ophiolite dikes

Dikes of the eastern Troodos ophiolite of Cyprus intruded at slow ocean-spreading axes with dips ranging up to 15° from vertical and with bimodal strikes (now NE–SW and N–S due to post-88 Ma sinistral microplate rotation). Varied dike orientations may represent local stress fields during dike-crack propagation but do not influence the spatial-distributions or orientation-distributions of dikes' magnetic fabrics, nor of their palaeomagnetic signals. Anisotropy of magnetic susceptibility (AMS) integrates mineral orientation-distributions from each of 1289 specimens sampled from dikes at 356 sites over 400 km² in the eastern Troodos ophiolite of Cyprus. In 90% of dikes, AMS fabrics define a foliation (k_MAX–k_INT) parallel to dike walls and a lineation (k_MAX) that varies regionally and systematically. Magma-flow alignment of accessory magnetite controls the AMS with a subordinate contribution from the mafic silicate matrix that is reduced in anisotropy by sea-floor metamorphism. Titanomagnetite has less influence on anisotropy. Occasionally, intermediate and minimum susceptibility axes are switched so as to be incompatible with the kinematically reasonable flow plane but maximum susceptibility (k_MAX) still defines the magmatic flow axis. Such blended subfabrics of kinematically compatible mafic-silicate and misaligned multidomain magnetite subfabrics; are rare. Areas of steep magma flow (k_MAX plunge ≥ 70°) and of shallow magma-flow alternate in a systematic and gradual spatial pattern. Foci of steep flow were spaced 4 km parallel to the spreading axes and 6 km perpendicular to the spreading axes. Ridge-parallel separation of steep flow suggest the spacing of magma-feeders to the dikes whereas ridge-perpendicular spacing of 6 km at a spreading rate of 50 mm/a implies the magma sources may have been active for 240 Ka. The magma feeders feeding dikes may have been ≤ 2 km in diameter. Stable paleomagnetic vectors, in some cases verified by reversal tests, are retained by magnetite and titanomagnetite. In all specimens, the stable components were isolated by three cycles of low-temperature demagnetization (LTD) followed by ≥ 10 steps of incremental thermal demagnetization (TD). 47% of primary A-components [338.2 /+ 57.2 n = 207, α₉₅ = 3.9; mean T_UB = 397 ± 8 °C] are overprinted by a B-component [341.4 /+ 63.5, n = 96, α₉₅ = 8.7; mean T_UB = 182 ± 11 °C]. A- and B-components are ubiquitous and shared equally by the N–S and NE–SW striking dikes. A-component unblocking temperatures (T_UB) are zoned subparallel to the fossil spreading axis. Their spatial pattern is consistent with chemical remagnetization at some certain off-axis distance determined by sea-floor spreading. A-components indicate less microplate rotation and more northerly palaeolatitudes that are consistent with metamorphic remagnetization after some spreading from the ridge-axis. Thus, their magnetizations are younger than those of the overlying volcanic sequence for which ChRMs are commonly reported as 274 /+ 33 (88 Ma).     

An experimental study of the dissolution mechanism and rates of muscovite

Steady-state muscovite dissolution rates have been measured at temperatures from 60 to 201 °C and 1 ? pH ? 10.3 as a function of reactive solution K, Si, and Al concentration. The pegmatitic muscovite used in these experiments has a composition consistent with (Na_0.09, K_0.86)Fe_0.05Al_2.92Si_3.05O₁₀(OH_1.95, F_0.06). All experiments were performed in titanium mixed-flow reactors. All experiments were performed at far-from-equilibrium conditions with respect to muscovite. All reactive solutions were undersaturated with respect to secondary product phases other than for some experiments which were supersaturated with respect to bohemite and diaspore; steady-state dissolution was stoichiometric for all experiments that were undersaturated with respect to these phases.The variation of rates with reactive solution composition depends on the solution pH. At pH ? 7 rates were found to decrease significantly with increasing reactive fluid Al activity but be independent of aqueous SiO₂ activity. pH < 7 rates measured in the present study from 60 to 175 °C are consistent with

     

Hydroelectric Infrastructure and Potential Groundwater Contamination in the Brazilian Amazon: Altamira and the Belo Monte Dam

Heavy investments in hydroelectric development are occurring throughout the Amazon Basin, which holds 42.2 percent of Brazil’s hydroelectric potential. The Belo Monte dam is the most recent and largest project in this region. The prevalence of septic systems in the Amazon, coupled with the widespread use of water wells and rising water table from filling the reservoir, create sanitation and health concerns for upstream communities. Using spatial analytical data and terrain analyses, we identify high-risk locations within the most densely populated neighborhoods in Altamira, Belo Monte’s host city. The purpose of this research is to develop a heuristic for identifying areas susceptible to groundwater and well contamination in relation to existing and proposed hydroelectric projects. Altamira’s city center persists as a high-risk location for contamination of wells because of its population density and relatively low elevation compared to other parts of the city. The methods, tools, and analyses presented in this article provide a framework that can be used to identify vulnerability to groundwater and drinking well contamination. The results presented here can guide implementation of public health and sanitation efforts in areas affected by large hydroelectric projects to avoid future water quality crises. Key Words: Belo Monte, groundwater contamination, hydroelectric infrastructure, risk assessment, spatial and terrain analyses.     

Early biodégradation of ligneous organic materials and its relation to ore deposition in the tréves zn-pb ore body (gard, france)

Previous petrographical and geochemical studies on the organic content of mineralized and non-mineralized carbonate series in the Trèves Zn-Pb stratiform deposit led to the conclusion that ore deposition occurred during the diagenetic history of the sediments, in a mixed fresh/hydrothermal system (Gauthier and Disnar, 1984; Gauthier et al., 1985a). These circulations of fluids are probably responsible for the dolomitization of the original limestones and for the deposition of the ore through microbial activity (Gauthier, 1984; Gauthier el al., 1985b).The occurrence of this activity is supported by the formation of two kinds of organic components which are attributable to this uncommon bio-alteration process: alphatic-rich bitumen-like products and aromatic polyethers. The chemical composition of these two types of compounds suggests that they were produced from ligneous debris by (sulfate-reducing?) bacteria.Other indications of the occurrence of a biological activity have been found in the nature and relative abundance of microbial amino compounds. However, these constituents are thought to have been introduced only recently into the sediments whereas other evidence supports the idea that ore deposition occurred before diagenesis.     

Evolution du Complexe Alcalin de St.-Honoré (Québec) apres sa Mise en Place

Since its formation in earliest Cambrian time, the St-Honoré alkaline complex has fractured into large subhorizontal slabs under the effect of magmatic pulsations and thermal contraction. These slabs were rapidly segmented and cracked by the same stresses. The secondary permeability resulting from these fractures first aided the circulation of the rare earth mineralizing hydrothermal fluids in the center of the complex, and then, after the unroofing of the complex, instigated the development of a karstic drainage system within the carbonatites of the complex. The accumulated sediments in the karstic caverns derive from the periodic inwashing of the insoluble residues built up on the surface of erosion, were indurated after the sealing of the karstic openings with calcareous muds during the Ordovician transgression, and were faulted with their cover when the Saguenay graben formed. The last event, marked by low temperature regional mineralizating circulation (barite and calcite crystals coating geodes at NIOBEC), permitted the establishment of a new subterrean drainage system.     

Photoelastic and numerical investigation of stress distributions around fault models under biaxial compressive loading conditions

This paper presents the results of a photoelastic and numerical study of stress distributions (contours and trajectories) around fault models of various geometries, submitted to a biaxial compressive load. It aims to describe typical biaxial stress behaviours and emphasize the existing differences with the well-known uniaxial compressive load case. Stress trajectories are sometimes shown by joint sets acting as markers of a paleostress field, and they can be interpreted by particular shallow tension–compression situations. At depth fractures can be reactivated, or can dilate under conditions of triaxial compression, and behaviour is essentially controlled by a high stress ratio (high σ₃/σ₁ ratio). In spite of the potential importance of such stress states on fracture permeability and fluid flow, and although they are frequently found at depth in a reservoir context, such stress conditions are poorly investigated, particularly in terms of stress perturbations.The presented analogue experiments consisted of compression tests done on polymethylmethacrylate (PMMA) plates; this material has mechanical properties comparable to those of brittle rocks in the upper crust, and presents birefringence. The samples contained open defects acting as faults, and the stress trajectories around these faults were investigated using a photoelastic device. Comparable numerical experiments were realised with a finite-element code (Franc 2D), using frictionless fault models.First, the effect of an increasing biaxial compressive load ratio σ₃/σ₁ on stress trajectories around an isolated open defect was explored. It was shown that the stress trajectories were drastically modified when σ₃/σ₁ increased from 0.2 to 0.4, this result being consistent with previous studies. In particular, when σ₃/σ₁ was superior or equal to 0.4, external isotropic points around where trajectories diverged (called repulsive isotropic points) appeared near the tips, but away from the defects. They tended to move away from the defects towards the main load direction when σ₃/σ₁ increased. The described isotropic points were points of stress trajectory divergence, i.e. points where stress decreased, implying that zones around them were strongly unfavourable to shear reactivation.Second, stress trajectories around fault models of various geometries (oblique isolated defect, dilational jog, compressive jog, and complex patterns) were studied, the applied biaxial compressive load ratio being 0.7. These biaxial stress trajectories were compared with similar uniaxial stress trajectories in order to explore the existing differences between the two regimes. It enabled new stress trajectory geometries to be described and interpreted. In particular, numerous external repulsive isotropic points were observed, and defect tips were shown to be zones of high convergence of stress trajectories. Furthermore, in contrast to the uniaxial compressive load case, stress trajectories and stress contours were geometrically similar within dilational and compressive jogs under biaxial compressive load. For both jog types, the centre of the overlapping zone, and the areas along the wall of a jog-defining fault and facing the overlapping tip of the other fault, proved to be zones of low mean stress, implying that fluids may migrate towards them from the tips of the faults, in response to mean stress gradients. Furthermore, the centre of the jogs exhibiting a high differential stress was a favourable area in terms of fracture reactivation, which may facilitate fluids transfer and storage.It was also observed that for both the uniaxial and biaxial compressive loads, isotropic zones were localised at the acute angle between branching defects.     

Late Glacial-Holocene sequence of Lake Saint-Point (Jura Mountains, France): Detrital inputs as records of climate change and anthropic impact

A sediment sequence (SP05, 12.5 m long) was taken from the deep zone of Lake Saint-Point (850 m a.s.l.). Sedimentological analyses highlight two main contrasted periods of sedimentation: the Last Glacial Maximum (LGM)/Late Glacial characterized by high silicates and quartz contents the Holocene dominated by the carbonated fraction. At the beginning of the Holocene (11 400 years cal. BP), silicates fraction flux abruptly decreased. The shift between the Late Glacial and the Holocene periods may be explained by forest development in the catchment. From 10 200 to 6800 years cal. BP, silicates and detrital carbonate fractions remained stable before they progressively increased steady till 5000 years cal. BP. Both increases cannot be totally attributed to an anthropic impact since pollen data indicate continuous anthropic activities only dated back from 3000 years cal. BP. They thus resulted from a dominant climatic control. From 5000 years cal. BP, silicates content still increased while detrital carbonates input became steady due to a change in pedogenetic processes affecting the catchment. During the last millennium, silicates and detrital carbonate decreased, probably due to pastureland development.     

A Sr,Nd, Hf,and Pb isotope perspective on the genesis and long-term evolution of alkaline magmas from Erebus volcano,Antarctica

We report new Nd, Hf, Sr, and high-precision Pb isotopic data for 44 lava and tephra samples from Erebus volcano. The samples cover the entire compositional range from basanite to phonolite and trachyte, and represent all three phases of the volcanic evolution from 1.3 Ma to the present. Isotopic analyses of 7 samples from Mt. Morning and the Dry Valley Drilling Project (DVDP) are given for comparison. The Erebus volcano samples have radiogenic ²⁰⁶Pb/²⁰⁴Pb, unradiogenic ⁸⁷Sr/⁸⁶Sr, and intermediate ¹⁴³Nd/¹⁴⁴Nd and ¹⁷⁶Hf/¹⁷⁷Hf, and lie along a mixing trajectory between the two end-member mantle components DMM and HIMU. The Erebus time series data show a marked distinction between the early-phase basanites and phonotephrites, whose Nd, Hf, Sr, and Pb isotope compositions are variable (particularly Pb), and the current ‘phase-three’ evolved phonolitic lavas and bombs, whose Nd, Hf, Sr, and Pb isotope compositions are essentially invariant. Magma mixing is inferred to play a fundamental role in establishing the isotopic and compositional uniformity in the evolved phase-three phonolites. In-situ analyses of Pb isotopes in melt inclusions hosted in an anorthoclase crystal from a 1984 Erebus phonolite bomb and in an olivine from a DVDP basanite are uniform and identical to the host lavas within analytical uncertainties. We suggest that, in both cases, the magma was well mixed at the time melt inclusions were incorporated into the different mineral phases.