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81.
Continental rift systems and anorogenic magmatism 总被引:1,自引:0,他引:1
Precambrian Laurentia and Mesozoic Gondwana both rifted along geometric patterns that closely approximate truncated-icosahedral tessellations of the lithosphere. These large-scale, quasi-hexagonal rift patterns manifest a least-work configuration. For both Laurentia and Gondwana, continental rifting coincided with drift stagnation, and may have been driven by lithospheric extension above an insulated and thermally expanded mantle. Anorogenic magmatism, including flood basalts, dike swarms, anorthosite massifs and granite-rhyolite provinces, originated along the Laurentian and Gondwanan rift tessellations. Long-lived volcanic regions of the Atlantic and Indian Oceans, sometimes called hotspots, originated near triple junctions of the Gondwanan tessellation as the supercontinent broke apart. We suggest that some anorogenic magmatism results from decompression melting of asthenosphere beneath opening fractures, rather than from random impingement of hypothetical deep-mantle plumes. 相似文献
82.
Gas hydrate systems at Hydrate Ridge offshore Oregon inferred from molecular and isotopic properties of hydrate-bound and void gases 总被引:3,自引:0,他引:3
Alexei V. Milkov George E. Claypool Roger Sassen 《Geochimica et cosmochimica acta》2005,69(4):1007-1026
We report and discuss molecular and isotopic properties of hydrate-bound gases from 55 samples and void gases from 494 samples collected during Ocean Drilling Program (ODP) Leg 204 at Hydrate Ridge offshore Oregon. Gas hydrates appear to crystallize in sediments from two end-member gas sources (deep allochthonous and in situ) as mixtures of different proportions. In an area of high gas flux at the Southern Summit of the ridge (Sites 1248-1250), shallow (0-40 m below the seafloor [mbsf]) gas hydrates are composed of mainly allochthonous mixed microbial and thermogenic methane and a small portion of thermogenic C2+ gases, which migrated vertically and laterally from as deep as 2- to 2.5-km depths. In contrast, deep (50-105 mbsf) gas hydrates at the Southern Summit (Sites 1248 and 1250) and on the flanks of the ridge (Sites 1244-1247) crystallize mainly from microbial methane and ethane generated dominantly in situ. A small contribution of allochthonous gas may also be present at sites where geologic and tectonic settings favor focused vertical gas migration from greater depth (e.g., Sites 1244 and 1245). Non-hydrocarbon gases such as CO2 and H2S are not abundant in sampled hydrates. The new gas geochemical data are inconsistent with earlier models suggesting that seafloor gas hydrates at Hydrate Ridge formed from gas derived from decomposition of deeper and older gas hydrates. Gas hydrate formation at the Southern Summit is explained by a model in which gas migrated from deep sediments, and perhaps was trapped by a gas hydrate seal at the base of the gas hydrate stability zone (GHSZ). Free gas migrated into the GHSZ when the overpressure in gas column exceeded sealing capacity of overlaying sediments, and precipitated as gas hydrate mainly within shallow sediments. The mushroom-like 3D shape of gas hydrate accumulation at the summit is possibly defined by the gas diffusion aureole surrounding the main migration conduit, the decrease of gas solubility in shallow sediment, and refocusing of gas by carbonate and gas hydrate seals near the seafloor to the crest of the local anticline structure. 相似文献
83.
Solubility mechanisms of water in depolymerized silicate melts quenched from high temperature (1000°-1300°C) at high pressure (0.8-2.0 GPa) have been examined in peralkaline melts in the system Na2O-SiO2-H2O with Raman and NMR spectroscopy. The Na/Si ratio of the melts ranged from 0.25 to 1. Water contents were varied from ∼3 mol% and ∼40 mol% (based on O = 1). Solution of water results in melt depolymerization where the rate of depolymerization with water content, ∂(NBO/Si)/∂XH2O, decreases with increasing total water content. At low water contents, the influence of H2O on the melt structure resembles that of adding alkali oxide. In water-rich melts, alkali oxides are more efficient melt depolymerizers than water. In highly polymerized melts, Si-OH bonds are formed by water reacting with bridging oxygen in Q4-species to form Q3 and Q2 species. In less polymerized melts, Si-OH bonds are formed when bridging oxygen in Q3-species react with water to form Q2-species. In addition, the presence of Na-OH complexes is inferred. Their importance appears to increase with Na/Si. This apparent increase in importance of Na-OH complexes with increasing Na/Si (which causes increasing degree of depolymerization of the anhydrous silicate melt) suggests that water is a less efficient depolymerizer of silicate melts, the more depolymerized the melt. This conclusion is consistent with recently published 1H and 29Si MAS NMR and 1H-29Si cross polarization NMR data. 相似文献
84.
George W. Housner 《地震工程与工程振动(英文版)》2008,7(2):I0001-I0001
Destructive earthquakes have caused great damage in China and the United States and collapsing buildings havecaused many deaths and injuries. The field of earthquake engineering studies earthquake hazards, the occurrence ofearthquakes of various magnitudes, the nature of the ground shaking during an earthquake, the vibration of structuresduring earthquakes, the strengthening of existing structures and the design of new structures to be earthquake resistant,and finally, how to cope with earthquake damage and restore a city to normal functioning. Such efforts are in progressin both countries, but unfortunately, the language barrier interferes with the free flow of information between China andthe Untied States. It would be mutually beneficial if some means could be developed to promote the exchangeof information across the Pacific Ocean. This new journal has been established for this purpose and its success willbe an important step in promoting earthquake engineering in China and the United States. 相似文献
85.
A total of 120 grab samples of the surficial sediments in the Cap-Breton submarine canyon and surrounding continental shelf were collected and analyzed by grain-size sieving. A Q-mode Factor Analysis was made on the grain-size data in order to define the most meaningful facies types. Four distinct lithological facies were found to exist: silt and clay, very fine sand, fine sand, and coarse sand. Comparison with previous work and a 14C date on the silt and clay facies showed that the facies are not contemporaneous. The sands and coarse sands on the shelf were emplaced during the pre-Würm and Würm regressions, and later probably reworked during the Holocene (Flandrian) transgression. The silty clays found in the canyon and on the shelf to the south are younger and represent sediments brought in as suspended load by the Adour and other nearby rivers during the Holocene (Flandrian) transgression. 相似文献
86.
Richard H. Pierce Charles E. Olney George T. Felbeck 《Geochimica et cosmochimica acta》1974,38(7):1061-1073
A procedure was devised to study pp′-DDT [2,2-bis()-1,1,1-trichloroethane] adsorption and desorption to suspended particulates in aqueous solutions. DDT adsorption to a marine sediment, sediment fractions, clay and humic acid suspended in sea water was investigated and the humic fraction was found to have a greater adsorbing capacity than the clay or sediment. Removal of the humic fraction from sediment reduced the adsorption capacity to less than 60 per cent of the original sediment sample. It was concluded that suspended humic particulates may be important agents for transporting chlorinated hydrocarbons through the water column and for concentrating them in sediments and in detritus-feeding organisms. 相似文献
87.
George Luiz Luvizotto Thomas Zack Silke Triebold Hilmar von Eynatten 《Mineralogy and Petrology》2009,97(3-4):233-249
Metamorphic textures in medium-grade (~500–550°C) metasedimentary rocks from the Erzgebirge give evidence of prograde rutile crystallization from ilmenite. Newly-crystallized grains occur as rutile-rich polycrystalline aggregates that pseudomorph the shape of the ilmenites. In-situ trace element data (EMP and SIMS) show that rutiles from the higher-grade samples record large scatter in Nb content and have Nb/Ti ratios higher than coexisting ilmenite. This behavior can be predicted using prograde rutile crystallization from ilmenite and indicates that rutiles are reequilibrating their chemistry with remaining ilmenites. On the contrary, rutiles from the lowest grade samples (~480°C) have Nb/Ti ratios that are similar to the ones in ilmenite. Hence, rutiles from these samples did not equilibrate their chemistry with remaining ilmenites. Our data suggest that temperature may be one of the main factors determining whether or not the elements are able to diffuse between the phases and, therefore, reequilibrate. Newly-crystallized rutiles yield temperatures (from ~500 to 630°C, Zr-in-rutile thermometry) that are in agreement with the metamorphic conditions previously determined for the studied rocks. In quartzites from the medium-grade domain (~530°C), inherited detrital rutile grains are detected. They are identified by their distinct chemical composition (high Zr and Nb contents) and textures (single grains surrounded by fine grained ilmenites). Preliminary calculation, based on grain size distribution of rutile in medium-grade metapelites and quartzites that occur in the studied area, show that rutiles derived from quartzites can be anticipated to dominate the detrital rutile population, even if quartzites are a minor component of the exposure. 相似文献
88.
The structures of sodium silicate and aluminosilicate glasses quenched from melts at high pressure (6-10 GPa) with varying degrees of polymerization (fractions of nonbridging oxygen) were explored using solid-state NMR [17O and 27Al triple-quantum magic-angle spinning (3QMAS) NMR]. The bond connectivity in melts among four and highly coordinated network polyhedra, such as [4]Al, [5,6]Al, [4]Si, and [5,6]Si, at high pressure is shown to be significantly different from that at ambient pressure. In particular, in the silicate and aluminosilicate melts, the proportion of nonbridging oxygen (NBO) generally decreases with increasing pressure, leading to the formation of new oxygen clusters that include 5- and 6-coordinated Si and Al in addition to 4-coordinated Al and Si, such as [4]Si-O-[5,6]Si, [4]Si-O-[5,6]Al and Na-O-[5,6]Si. While the fractions of [5,6]Al increase with pressure, the magnitude of this increase diminishes with increasing degrees of ambient-pressure polymerization under isobaric conditions. Incorporating the above structural information into models of melt properties reproduces the anomalous pressure-dependence of O2− diffusivity and viscosity often observed in silicate melts. 相似文献
89.
Iron and Sulfur Chemistry in a Stratified Lake: Evidence for Iron-Rich Sulfide Complexes 总被引:5,自引:1,他引:5
George W. Luther III Brian Glazer Shufen Ma Robert Trouwborst Bradley R. Shultz Gregory Druschel Charoenwan Kraiya 《Aquatic Geochemistry》2003,9(2):87-110
A four month study of a man-made lake used for hydroelectric power generation in northeastern Pennsylvania USA was conducted
to investigate seasonal anoxia and the effects of sulfide species being transported downstream of the power generation equipment.
Water column analyses show that the system is iron-rich compared to sulfide. Total Fe(II) concentrations in the hypolimnion
are typically at least twice the total sulfide levels. In situ voltammetric analyses show that free Fe(II) as [Fe(H2O)6]2+ or free H2S as H2S/HS- are either not present or at trace levels and that iron-rich sulfide complexes are present. From the in situ data and total Fe(II) and H2S measurements, we infer that these iron-rich sulfide complexes may have stoichiometries such as Fe2SH3+ (or polymeric forms of this and other stoichiometries). These iron-rich sulfide complexes appear related to dissolution of
the iron-rich FeS mineral, mackinawite, because IAP calculations on data from discrete bottle samples obtained from bottom
waters are similar to the pKsp of mackinawite. Soluble iron-sulfide species are stable in the absence of O2 (both in lake waters and the pipeline) and transported several miles during power generation. However, iron-sulfide complexes
can react with O2 to oxidize sulfide and can also dissociate releasing volatile H2S when the waters containing them are exposed to the atmosphere downstream of the powerplant. Sediment analyses show that
the lake is rich in oxidized iron solids (both crystalline and amorphous). Fe concentrations in FeS solids are low (<5 μmole/grdry wt) and the pyrite concentration ranges from about equal to the solid FeS to 30 times the solid FeS concentration. The degree
of pyritization is below 0.12 indicating that pyrite formation is limited by free sulfide, which can react with the iron-rich
sulfide complexes. 相似文献
90.
FT-IR spectra of sillimanite samples from high grade regionally metamorphosed rocks belonging to the granulite terrain (amphibolite to pyroxene granulite facies) deciphers prominent OH features. Heating experiments indicate growth of prominent band at 3161cm−1. Heating above 1000°C all OH features disappear in intensity into broad features with slight shift of bands towards lower energies. Complete dehydration requires temperatures above 1000°C. Coexistence of boron and OH features are also observed in all sillimanite samples. The high temperature behaviour of sillimanite from the granulite terrain discerns that the hydrous species in sillimanite were incorporated much below 700°C, however, secondary hydration due to pegmatite activity, retrograde metamorphism and migmatisation is not ruled out. Thus a near anhydrous condition were probably not achieved during the granulite facies metamorphism in Eastern ghat granulite terrain. 相似文献