Far-from-equilibrium batch dissolution experiments were carried out on the 2000–500, 500–250, 250–53 and 53–2 μm size fractions of the mineral component of the B horizon of a granitic iron humus podzol after removal of organic matter and secondary precipitates. The different size fractions were mineralogically and chemically similar, the main minerals present being quartz, alkali and plagioclase feldspar, biotite and chlorite. Specific surface area increased with decreasing grain size. The measured element release rates decreased in the order 53–2>>>2000–500>500–250>250–53 μm. Surface area normalised element release rates from the 2000–500, 500–250 and 250–53 μm size fractions (0.6–77×10−14 mol/m2/s) were intermediate between literature reported surface area normalised dissolution rates for monomineralic powders of feldspar (0.1–0.01×10−14 mol/m2/s) and sheet silicates (100×10−14 mol/m2/s) dissolving under similar conditions. Element release rates from the 53–2 μm fraction (400–3000×10−14 mol/m2/s) were a factor of 4–30 larger than literature reported values for sheet silicates. The large element release rate of the 53–2 μm fraction means that, despite the small mass fraction of 53–2 μm sized particles present in the soil, dissolution of this fraction is the most important for element release into the soil. A theoretical model predicted similar (within a factor of <2) bulk element release rates for all the mineral powders if observed thicknesses of sheet silicate grains were used as input parameters. Decreasing element release rates with decreasing grain size were only predicted if the thickness of sheet silicates in the powders was held constant. A significantly larger release rate for the 53–2 μm fraction relative to the other size fractions was only predicted if either surface roughness was set several orders of magnitude higher for sheet silicates and several orders of magnitude lower for quartz and feldspars in the 53–2 μm fraction compared to the other size fractions or if the sheet silicate thickness input in the 53–2 μm fraction was set unrealistically low. It is therefore hypothesised that the reason for the unpredicted large release rate from the 52–3 μm size fraction is due to one or more of the following reasons: (1) the greater reactivity of the smaller particles due to surface free energy effects, (2) the lack of proportionality between the BET surface area used to normalise the release rates and the actual reactive surface area of the grains and, (3) the presence of traces quantities of reactive minerals which were undetected in the 53–2 μm fraction but were entirely absent in the coarser fractions. 相似文献
Three major, interdependent processes control the genesis and distribution of mineral and thermal waters in the Rhenish Massif,
Central Europe: (a) Magmatic processes in the upper mantle provide most of the CO2 to produce bicarbonate waters in shallow aquifers. (b) Extension of the brittle upper crust enables the ascent of sodium
chloride waters. (c) Uplift and erosion shape the massif's relief, which determines the extent of flow systems and the distribution
of thermal springs. The chemistry of mineral waters further depends on the aquifers' mineral composition. A comprehensive
set of hydrological, chemical, tectonic and geophysical data on the Rhenish Massif has been compiled. It was used to classify
the mineral waters and to map the spatial distribution of water properties. The composition of cuttings from several representative
wells producing different water types shows that the hydrothermal alteration of the aquifer rocks consists mainly of kaolinization
of chlorite and dissolution of feldspar. Numerical transport simulations favour two modes of groundwater flow: topography-driven
flow and the pressure-driven ascent of basement brines along active faults. Thermal convection is less important. 相似文献
The 117.38 m of gabbroic core drilled during the Ocean Drilling Program (ODP) Leg 153 at Sites 921 to 924 in the Mid-Atlantic Ridge (MAR) between 23 °N and the Kane Fracture Zone, exhibits a remarkable primary compositional heterogeneity, such as magmatic layering, intrusive contacts and late magmatic veining, which express a succession of magmatic events. Textural indicators suggest that the cooling of the crystal mush occurred in a dynamic environment, with infiltration of progressively evolved liquids. Magmatic features include random shape fabric and magmatic lamination; the subsequent deformational overprint occurred in subsolidus conditions. The ductile deformation, generally concentrated in discrete domains of the gabbro, is associated with continuous re-equilibration of the metamorphic assemblages of (1) olivine + clinopyroxene + orthopyroxene + plagioclase + ilmenite + Ti-magnetite, (2) olivine + clinopyroxene + plagioclase + ilmenite + Ti-magnetite + red hornblende. At lower temperatures brittle deformation prevails and subsequent fractures control the development of metamorphic assemblages: (3) clinopyroxene + plagioclase + red brown hornblende + Ti-magnetite + magnetite (?) + ilmenite, (4) plagioclase + brown hornblende + Ti-magnetite + magnetite + hematite + titanite ± Ti-oxide, (5) plagioclase + green hornblende + magnetite + titanite, (6) plagioclase + actinolite + chlorite + titanite + magnetite, (7) albite + actinolite + chlorite + prehnite ± epidote ± titanite and (8) albite + prehnite + chlorite ± smectite. Assemblages 1 to 8 express increasing water/rock ratios and decreasing degrees of recrystallization.
During the ductile phase, red hornblende is stable and its abundance increases with deformation intensity, possibly as an effect of the introduction of hydrous fluids. During the brittle phase, water diffusion controls the development of the fracture-filling mineral assemblages and re-equilibration of the adjacent rock; temperatures decrease further, as demonstrated by mineral zoning and incompletely re-equilibrated assemblages. The lowest temperatures correspond to the development of hydrothermal assemblages.
Compared with oceanic gabbros from fast-spreading transform environments, high-temperature ductile phases (granulite and amphibolite) are well developed, whereas brittle phases are widespread, as microcracks, prevalent on fracturing associated with discrete veins. 相似文献
Orthogneisses are the major country rocks hosting eclogites in the Sulu UHP terrane, eastern China. All of the analyzed orthogneiss cores from the main drilling hole of the Chinese Continental Scientific Drilling Project (CCSD-MH) have similar major and trace element compositions and a granite protolith. These rocks have relatively high LREE/HREE ratios, strong negative Eu anomalies (Eu/Eu*=0.20–0.39), and negative Ba anomalies (Ba/Ba*=0.25–0.64). Coesite and coesite-bearing UHP mineral assemblages are common inclusions in zircons separated from orthogneiss, paragneiss, amphibolite, and (retrograded) eclogite of the CCSD-MH. This suggests that the eclogite, together with its country rocks, experienced in situ ultrahigh-pressure (UHP) metamorphism. Laser Raman spectroscopy and cathodoluminescence (CL) images show that zircons from the orthogneisses are zoned and that they have distinct mineral inclusions in the different zones. Most zircons retain early magmatic cores with abundant low-pressure mineral inclusions, which are mantled with metamorphic zircon-containing inclusions of coesite and other UHP minerals. The outermost rims on these grains contain low-pressure mineral inclusions, such as quartz and albite. SHRIMP U–Pb dating of the zoned zircons gives three discrete and meaningful groups of ages: Proterozoic ages for the protolith, 227±2 Ma for the coesite-bearing mantles, and 209±3 Ma for the amphibolite facies retrograde rims. The widespread occurrence of UHP mineral inclusions in zircons from the Sulu metamorphic belt dated at about 227 Ma suggests that voluminous continental crust experienced late Triassic subduction to depths of at least 120 km and perhaps more than 200 km. Eighteen million years later, the terrane was rapidly exhumed to midcrustal levels, and the UHP rocks were overprinted by amphibolite facies metamorphism. The exhumation rate deduced from the zircon age data and previously obtained metamorphic P–T data is estimated to be 5.6–11.0 km/Ma. Such rapid exhumation of the Sulu UHP terrane may be due to the buoyancy forces produced by subduction of low-density continental material into the deep mantle. 相似文献
The exsolution of volatile phases from silicate magmas controls physical and chemical magma properties and influences large-scale geologic phenomena and processes having major societal and economic implications including the release of climate-altering gases to the atmosphere, the explosivity of volcanic eruptions, hydrothermal alteration, and the generation of magmatic–hydrothermal mineralization. These volatile phases exsolve from a wide variety of magmas and cover a very broad spectrum of compositions.
The transition from the orthomagmatic to the hydrothermal stages has important bearing on these fundamentally important geologic phenomena, and this report summarizes the published results of a dozen scientific investigations on the magmatic–hydrothermal transition as applied to volcanic eruption and magmatic–hydrothermal mineralization. These studies involve a variety of analytical and experimental methodologies, and many focus on fluid and melt inclusions from mineralized magmatic systems. A primary goal of each study is to better understand the role of magmatic volatiles and the importance of the magmatic–hydrothermal transition on these geologic processes. 相似文献