Since the first workshop on ultrahigh-pressure (UHP) metamorphism at Stanford in 1994 and the special issue of The Island Arc `Ultrahigh-Pressure Metamorphism and Tectonics' published in December 1995, many symposia and special sessions specifically with regard to the UHP metamorphic terrane have been held. While we are still wondering how exhumation of UHP rocks from mantle depths to the surface takes place, the finding of possible records from the mantle transition zone at 300–400 km depths is astonishing. The study of the UHP regime has expanded to include input from mineral physics, experimental geochemistry and kinetics in addition to the petrochemical and tectonic study of a variety of HP–UHP rocks. It was with this theme that the second workshop for the task group III-6 `Ultrahigh-Pressure Metamorphism and Geodynamics in Collision-type Orogenic Belts' of the International Lithosphere Program was held during the International Geological Congress in Beijing, 1996. The Symposium 8–9 `Dynamic Metamorphic Rocks and High- and Ultrahigh-Pressure Metamorphism' (Cong Bolin & J. G. Liou conveners) had more than 25 presentations in two oral sessions and 70 papers in one poster session. This second special issue of The Island Arc includes nine papers from this symposium and a few related contributions to the geodynamics of HP–UHP metamorphism and tectonics. It is our hope that The Island Arc will continue to publish a special issue on this increasingly recognized subject that is essential to our understanding of continental collision, mantle dynamics and geochemical + fluid cycles. 相似文献
The behaviour of Fe-oxides was investigated during precipitation and co-precipitation, phase transformation and dissolution, while their ability to adsorb and incorporate trace components was examined. Some samples were synthesised and studied under controlled laboratory conditions and other samples were taken from experiments designed to test the effectiveness of waste treatment strategies using iron. Surface-sensitive and high-resolution techniques were used to complement information gathered from classical, macroscopic methods.
Adsorption isotherms for Ni2+ uptake on synthetic ferrihydrite (Fe5HO8·4H2O, often written simply Fe(OH)3), goethite (-FeOOH), hematite (-Fe2O3) and magnetite (Fe3O4) were all similar, increasing as expected at higher pH. Desorption behaviour was also similar, but one third or more of the Ni2+ failed to return to solution. In the past, “irreversible sorption” has been blamed on uptake into micro-fractures or pores, but during examination (using Atomic force microscopy, AFM) of hundreds of Fe-oxide particles, no evidence for such features could be found, leading to the conclusion that Ni2+ must become incorporated onto or into the solids. When solutions of Fe(II) are oxidised in controlled laboratory conditions or during treatment of ash from municipal waste incinerators, two-line ferrihydrite forms rapidly and on never-dried samples, AFM shows abundant individual particles with diameter ranging from 0.5 to several tens of nanometers. Aging in solution at 70°C promotes growth of the particles into hematite and goethite and their identification (by X-ray powder diffraction, XRPD, with Rietveld refinement) becomes possible at the same aging stage as mineral morphology becomes recognisable by AFM. In other experiments that were designed to mimic natural attack by organic acids, colloidal lepidocrocite (γ-FeOOH) was observed in situ by AFM, while reductive dissolution removed material on specific crystal faces. Lath ends are eroded fastest while basal planes are more stable.
In order to help elucidate mechanisms of contaminant immobilisation by Fe-oxides, we examined samples from a reactive barrier made with 90% quartz sand, 5% bentonite and 5% zero-valent iron filings that had reacted with a solution typical of leachate from coal-burning fly ash using time-of-flight secondary ion mass spectroscopy (TOF-SIMS). Fe(0) oxidised to Fe(III), while soluble and toxic Cr(VI) was reduced to insoluble Cr(III). Chemical maps show Fe-oxide coatings on bentonite; Cr is associated with Fe-oxides to some extent but its association with Ca in a previously undescribed phase is much stronger. Other samples taken from municipal waste incinerator ash that had been treated by aeration in Fe(II) solutions were examined with transmission electron microscopy (TEM), selected area electron diffraction (SAED) and energy dispersive X-ray spectroscopy (EDS). Pb and some Zn are seen to be dispersed throughout two-line ferrihydrite aggregates, whereas Sn and some Zn are incorporated simply as a result of entrainment of individual ZnSn-oxide crystallites.
Geochemical speciation models that fail to account for contaminant uptake in solid solutions within major phases or as thin coatings or entrained crystals of uncommon phases such as those described here risk to underestimate contaminant retardation or immobilisation. 相似文献
A passive sampling system for use with rhodamine WT (RWT) in groundwater tracing experiments was developed to assist in the
characterisation of groundwater flow paths. Amberlite XAD-7 resin was found to be suitable for adsorption of RWT, which can
then be extracted using an ethanol/water mix and analysed fluorometrically. Batch and column experiments showed that XAD-7
resin has a high RWT capacity. The adsorption was slightly dependent on pH, but was always above 75% under batch conditions.
The resin had a high percentage mass recovery at flow velocities around 1.5 m/day, but this decreased with increasing flow
velocities. Desorption of RWT off the resin in water is dependent on the flow velocity of water and the time after the peak
RWT has passed. The mass of RWT extracted from the resin bags correlated very well with both the RWT mass flux in the water
and the peak concentrations observed in the monitoring wells in a field experiment. The results of resin bags were reproducible
in the field with a mean coefficient of variation equal to 16%. This method has been successfully applied to two field situations
with different flow velocities to indicate groundwater flow paths.
Electronic Publication 相似文献
The kinetic parameters of generation have been obtained for different hydrocarbon classes, including methane, C2-C5 gas hydrocarbons, C6-C13 light hydrocarbons and C13+ heavy hydrocarbons, and vitrinite reflectance (R°) by the kinetic simulating experiment of kerogen cracking. Then, combined with the detailed geology of Sichuan Basin, the
effective gas-generating intensity of the Lower Cambrian source rock is approximately estimated by applying these parameters. 相似文献
The I-atom sensitised decomposition of ozone in air at 1 atm pressure and ambient temperature has been investigated. Iodine atoms were produced by photolysis of I2 using visible light or of CH3I using ultraviolet light. In both cases, the quantum yield for O3 decomposition was 1.25 (±0.11) per I atom. An important role is proposed for the self-reaction of IO radicals leading to higher oxides of iodine, IO+IO(+M)I2O2(+M)higher oxides, which predominated over the bimolecular reaction leading to regeneration of I atoms, IO+IO2I+O2, with k2a/k2b4. Simple computer modelling calculations indicate that reaction (2a) may be important in determining the fate of photolabile iodine species in the atmosphere. The consequences for the behaviour of radioiodine releases are also discussed. 相似文献
Abstract: Transportation of various kinds of elements occurred in wall rocks (Quaternary andesites) during the hydrothermal alteration accompanied by the Hishikari epithermal gold mineralization. For example, K2O and MgO contents of wall rocks decrease away from the gold-quartz veins, while (CaO+Na2O) content increases, and SiO2 content is variable near the veins. Hydrothermal alteration zoning and bulk compositional variations in wall rocks suggest that the mixing of hydrothermal solution and acidic groundwater took place an important role as the cause for the hydrothermal alteration and bulk compositional variations. The relationship between dissolved silica concentration and temperature of hydrothermal solution mixed with groundwater is obtained based on precipitation kinetics-fluid flow–mixing model, and the computed results are compared with the distribution of SiO2 minerals (quartz and cristobalite) in the hydrothermal alteration zones. This comparison suggests that the most reasonable flow rate of fluids migrating through hydrothermal alteration zones, and A/M (A: surface area of rocks interacting with fluid, M: mass of fluid) are estimated to be ca. 10-4.2 m/sec, and ca. 0.10 m2/kg, respectively. The mixing of two fluids (hydrothermal solution and acidic groundwater) can also explain δ18O zoning in the altered country rocks, hydrothermal alteration zoning from K-feldspar through K-mica to kaolinite from the center (veins) to margin, and deposition of gold. 相似文献
