The Central Atlantic Magmatic Province (CAMP) is one of the largest igneous provinces on Earth, extending more than 5000 km north to south, on both sides of the Atlantic Ocean. Its emplacement occurred about 200 Ma ago, at the Triassic–Jurassic boundary, and is linked to the initial breakup of Pangaea. Two areas of the province are studied here: French Guyana/Surinam (South America) and Guinea (West Africa), in order to document the petrogenesis and geodynamical significance of high-Ti and low-Ti basaltic magmas from the CAMP.
In Guyana, doleritic and gabbroic dykes are located on the edge of the Guiana Shield, and represent limited volumes of magma. They display low SiO2 (47–50%), high TiO2 (2.5–3.5%) and high FeO tholeiitic trends and show variably enriched trace element patterns ((La/Yb)n=1.5–5.1). Their isotopic signature and ratios of very incompatible elements (εNdi=+5.8 to +4.2, (87Sr/86Sr)i=0.703–0.705, (207Pb/204Pb)i=15.46–15.64) match a depleted PREMA (prevalent mantle)-like source. Their genesis can be modeled by ca. 15% partial melting of a lherzolite source, and a subsequent limited fractional crystallization (5–10%) or a slight upper crustal assimilation–fractional crystallization (AFC, r=0.1, Proterozoic contaminant). In Guinea, in contrast, huge volumes of CAMP magmas were intruded along the Rockelides suture and the West African craton, forming the Fouta Djalon sills and the Kakoulima laccolith. The laccolith is more than 1000 m thick. These features consist of gabbros, dolerites, diorites and mafic (gabbro) and ultramafic (dunite, wherlite) cumulates. Guinean tholeiites show high SiO2 (51–58%), low TiO2 (0.7–1.2%) and FeO trends, with high LILE/HFSE ratios and slight negative Nb–Ta anomalies. Isotopic signatures (εNdi=+0.4 to −5.3, (87Sr/86Sr)i=0.705–0.710, (207Pb/204Pb)i=15.57–15.66) indicate a more enriched source than for Guyana as well as a higher rate of magma–upper crust interaction through an AFC process (r=0.3, Birimian crust contaminant) and, probably, an additional upper crustal contamination for the most differentiated sample.
This geochemical study supports the prevalence in Guinea, as for other low-Ti CAMP tholeiites, of a lithospheric mantle source, previously enriched during ancient subduction events, and preferentially reactivated in late Triassic times by edge-driven convection between cratonic and mobile belt domains. A larger contribution from a depleted asthenospheric source is required to generate high-Ti tholeiites in Guyana, which may reflect the development of CAMP rifting towards the initiation of the Central Atlantic oceanic crust. 相似文献
Semi-batch seawater experiments were conducted to follow the uptake and release of selected PAHs (anthracene, fluorathene, pyrene and B[a]P) and organochlorine pesticides (-HCH, aldrin, dieldrin, p,p′-DDT) in semi-permeable membrane devices (SPMDs) and green-lipped mussels (Perna viridis). Mathematical models were applied to describe the uptake and elimination curves of the contaminants for SPMDs, and kinetic parameters, such as uptake rate constants, and equilibrium triolein/water partitioning coefficients were calculated. SPMD data showed a good fit to estimate rate constant and partition coefficient equations, but only those contaminants which partitioned mainly in the dissolved phase (-HCH and dieldrin) were well explained for mussels. Poor conformity of the other contaminants indicated mussels uptake by routes other than diffusion, such as ingestion of algae. An apparent equilibrium state was only noted for -HCH in mussels. Aldrin was not detected in mussels in the first few days of exposure, indicating potential metabolism of this compound. B[a]P was not detected in the triolein of SPMDs, which suggests that the membrane may act as a reservoir. Loss of spiked B[a]P from the triolein was evident in a depuration experiment, which may indicate transfer to the membrane. Rate constants for mussels were higher than those for SPMDs, but the reverse was true for partition coefficients. Overall, mussels and SPMDs had similar uptake rates for all compounds in this study, excluding p,p′-DDT and dieldrin. Contaminant elimination took place more rapidly in mussels, implying that SPMDs are better candidates for detecting episodic discharge of organic contaminants. 相似文献
We consider groundwater steady flow in a heterogeneous porous formation of random and stationary log-conductivity Y = ln K, characterized by the mean 〈Y〉, and the two point correlation function CY which in turn has finite, and different horizontal and vertical integral scales I and Iv, respectively. The fluid velocity V, driven by a given head drop applied at the boundary, has constant mean value U ≡ (U, 0, 0). Approximate explicit analytical expressions for transverse velocity covariances are derived. The adopted methodology
follows the approach developed by Dagan and Cvetkovic (Spatial moments of kinetically sorbing plume in a heterogeneous aquifers,
Water Resour. Res. 29 (1993) 4053) to obtain a similar result for the longitudinal velocity covariance. Indeed, the approximate
covariances of transverse velocities are determined by requiring that they have the exact first order variances as well as
zero integral scale (G. Dagan, Flow and Transport in Porous Formations (Springer, 1989)) , and provide the exact asymptotic limits of the displacement covariance of the fluid particles obtained
by Russo (On the velocity covariance and transport modeling in heterogeneous anisotropic porous formations 1. Saturated flow,
Water Resour. Res., 31 (1995) 129). Comparisons with numerical results show that the proposed expressions compare quite well in the early time
regime, and for Ut/I >100. Since most of the applications, like assessing the effective mobility of contaminants or quantifying the potential
hazards of nuclear repositories, require predictions over higher times the proposed approximate expressions provide acceptable
results. The main advantage related to such expressions is that they allow obtaining closed analytical forms of spatial moments
pertaining to kinetically sorbing contaminant plumes avoiding the very heavy computational effort which is generally demanded.
For illustration purposes, we consider the movement of one contaminant species, and show how our approximate spatial moments
compare with the numerical simulations. 相似文献
The potential of shredded tyre chips, either on their own or mixed with gravel, to act as a permeable reactive barrier has been examined by carrying out tests for assessing sorption characteristics and establishing their mechanical properties. Sorption–desorption tests have been carried out using free product petrol and paraffin and aqueous phase phenol and cresols.
Compaction and compressibility tests have been carried out to establish optimum tyre–chip mixes in order to optimise the mix in terms of compressibility behaviour. The results demonstrate that tyre chips show significant promise for incorporation in a permeable reactive barrier. The evidence indicates that they would be particularly effective for the retardation of aqueous phase hydrophobic organic contaminants and residual free product. Optimum mixes of 2–6 mm tyre chips and 20 mm gravel, in terms of mechanical stability, will contain just under 10% of tyre chips. 相似文献
1 INTRODUCTION Sediments are the ultimate sink for many hydrophobic contaminants and represent biologically important environmental habitats. Exposure of the sediment-associated contaminants is not only influenced by the fate and transport of sediment but also is influenced by a variety of physical, chemical, and biological processes that involve no net movement of sediments. These processes include pore water transport processes such as advection and diffusion, and sediment mixing process… 相似文献