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11.
Carbon, oxygen and sulphur isotope data for transects across two pyrite-bearmg carbonate concretions, and their host sediments, from the Upper Lias of N.E. England show symmetrical zonation. δ13CPDB values of the calcite cement (?12.9 to ?15.4%.) indicate that most of it originated from organic matter by bacterial reduction of sulphate, augmented with marine and, to a lesser extent, fermentation derived carbonate. Organic carbon (δ13CPDB = ?26.1 to ?37.0%.). reflects the admixture of allochtho-nous terrestrial organic matter with marine material and the selective preservation of isotopically light organic material through microbiological degradation.Two phases of pyrite are present in each concretion. The earlier framboidal pyrite formed throughout the sediment prior to concretionary growth and has δ34SCD values of ?22 to ?26%. indicating formation by open system sulphate reduction. The later euhedral phase is more abundant and reaches values of ? 2.5 to ? 5.5%. at concretion margins. This phase of sulphate reduction provided the carbonate source for concretionary growth and occurred in a partially closed system. The δ13C and δ34S data are consistent with mineralogical and chemical evidence which suggest that both concretions formed close to the sediment surface. The δ18O values of the calcite in one concretion (δ18OPDB = 2.3 to ?4.8%.) indicate precipitation in pore waters whose temperature and isotopic composition was close to that of overlying seawater. The other concretion is isotopically much lighter (δ18OPDB?8.9 to ?9.9%.) and large δ18O differences between concretions in closely-spaced horizons imply that local factors control the isotopic composition of pore waters. 相似文献
12.
Diffusive isotopic fractionation factors are important in order to understand natural processes and have practical application in radioactive waste storage and carbon dioxide sequestration. We determined the isotope fractionation factors and the effective diffusion coefficients of chloride and bromide ions during aqueous diffusion in polyacrylamide gel. Diffusion was determined as functions of temperature, time and concentration. The effect of temperature is relatively large on the diffusion coefficient (D) but only small on isotope fractionation. For chlorine, the ratio, D35Cl/D37Cl varied from 1.00128 ± 0.00017 (1σ) at 2 °C to 1.00192 ± 0.00015 at 80 °C. For bromine, D79Br/D81Br varied from 1.00098 ± 0.00009 at 2 °C to 1.0064 ± 0.00013 at 21 °C and 1.00078 ± 0.00018 (1σ) at 80 °C. There were no significant effects on the isotope fractionation due to concentration. The lack of sensitivity of the diffusive isotope fractionation to anything at the most common temperatures (0 to 30 °C) makes it particularly valuable for application to understanding processes in geological environments and an important natural tracer in order to understand fluid transport processes. 相似文献
13.
Measurement of 17 laboratories on the stable isotope reference materials, NBS 22 lubricating oil and NBS 21 graphite, resulted in a revised calibration on the PDB scale. NBS 22 13C/12C ratios of all laboratories did not demonstrate a gausssian distribution. Therefore, values which resulted from direct comparison with NBS 20 carbonate standards were selected the mean value of which is ; this value is recommended as new value for NBS 22. The reported variations in δ13C for NBS 21 were consistent within the accuracies for single meaurements; the mean value is ?28.16 ± 0.01. The variations of D/H ratios of NBS 22 also were consistent with the accuracy quoted, with a mean value of ?119?. 相似文献
14.
Temporal and chemical connections between plutons and ignimbrites from the Mount Princeton magmatic center 总被引:1,自引:2,他引:1
The Mount Princeton magmatic center, located in central Colorado, consists of the epizonal Mount Princeton batholith, the nested Mount Aetna caldera, and volumetrically minor leucogranites. New CA-TIMS U/Pb zircon ages indicate the majority of the Mount Princeton batholith was emplaced during a period of regional ignimbrite quiescence. The structurally highest unit of quartz monzonite yields a 206Pb/238U age of 35.80 ± 0.10 Ma, and the youngest dated unit of the quartz monzonite is a porphyritic unit that yields a 206Pb/238U age of 35.37 ± 0.10 Ma. Using the exposed, dated volume of the quartz monzonite and new geochronology yields an estimated pluton filling rate of ~0.002 km3/a. This rate is comparable to the accumulation rates published for other plutons, and at least an order of magnitude slower than fluxes necessary to support accumulation of large eruptible magma volumes. Geochronology for the two large ignimbrites spatially associated with the batholith indicates a temporal disconnect between the vast majority of pluton building and explosive eruption of magma. The Wall Mountain Tuff erupted from a source in the same geographic area as the Mount Princeton batholith at 37.3 Ma (Ar/Ar sanidine), but no structural evidence of a caldera or temporally associated plutonic rocks is known. The Badger Creek Tuff erupted at 34.3 Ma (Ar/Ar sanidine) during the formation of the Mount Aetna caldera in the southern portion of the batholith. Our 206Pb/238U age for the Badger Creek Tuff is 34.47 ± 0.05. The only analyzed plutonic rocks of similar age to the Badger Creek Tuff are an extra-caldera dike with a 206Pb/238U age of 34.57 ± 0.08 Ma, a ring dike with a 206Pb/238U age of 34.48 ± 0.09 Ma, and a portion of the Mount Aetna pluton with a 206Pb/238U age of 34.60 ± 0.13 Ma. The small volume intrusions related to the eruption of the Badger Creek Tuff are chemically similar to the ignimbrite and show no signature of crystal–liquid separation in the shallow crust. 相似文献
15.
Research into fluvial dunes spans disciplines, studies at grain to reach scales, and methodological approaches that include theoretical, experimental, numerical and field investigations. Despite significant research efforts to date, it remains difficult to provide definitive answers to some fundamental questions regarding dunes. This paper reviews three notable challenges that remain regarding fluvial dunes, namely scale‐consistent linking of bed morphologies with turbulent flow fields, the intriguing question of what causes trains of highly‐ordered sediment waves to form in beds of river sediments, and how to define the important characteristics of a dune‐covered bed, including lengths, shapes, and their statistical nature. In each case, the particular challenge is discussed and then recent research and ways forward are presented. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
16.
Arwyn Rhys Jones 《Earth, Moon, and Planets》1987,37(2):171-185
The potential of remotely sensed imagery for Earth science applications has been widely recognised for many years. However,
with the advent of space borne sensors, a totally new perspective from which to view the Earth's surface has been available
to such disciplines as geology and geography. Primary in this development was the deployment of the Landsat series of Earth
observation satellites with their multispectral scanning capability. Although standard single band imagery and false-colour
composites contain much geomorphological detail, a great deal more information can be extracted from the digital output of
such scanners if the data is subjected to computer-assisted image processing. 相似文献
17.
A. E. Sveinbjornsdottir M. L. Coleman B. W. D. Yardley 《Contributions to Mineralogy and Petrology》1986,94(1):99-109
The Reykjanes and Krafla geothermal fields are both examples of active high temperature systems and show similar assemblages of alteration minerals, but the fluid at Reykjanes is dominantly sea water whereas that at Krafla is meteoric. Oxygen isotope analyses of surface rock and of drill chip samples from different depths are presented, together with results for the Krafla fluid, which is close to local precipitation (δ 18 O = ?11.9‰, δD= ?86.8‰). Calcite in both systems is apparently in equilibrium with the present deep fluid at the present field temperature, except for the upper 250 m at Reykjanes where the fluid may be more meteoric than at depth. Feldspar gives similar results. Quartz separates at Reykjanes are anomalously lighter than coexisting feldspar and give exceptionally high quartz-fluid temperatures. It is suggested that quartz originally grew when the fluid was more nearly meteoric (? glacial period) and has not re-equilibrated. Bulk-rock 18 O depletion supports this interpretation of the history of the Reykjanes system. Quartz in the Krafla system is mostly in equilibrium at the present field conditions but anomalies occur near the boundary between the upper and lower parts of the system, suggesting that this is not entirely stable. A high fluid:rock ratio (10–100 minimum) is indicated for the Krafla field. 相似文献
18.
P.B. Robertson L.C. Coleman E.J. Olsen S.A. Kissin 《Meteoritics & planetary science》1982,17(3):163-170
The recovery of several thousand meteorite fragments from Antarctica has led to speculation that accumulations may occur on the Earth's other major ice caps. Meteorites falling over the past 80,000 years on the Devon Island ice cap in the Canadian Arctic may be exposed at the surface near the ice cap margin. From the terrestrial meteorite flux, ice movement rates, and fragmentation factors it is calculated that 12,560 samples of 150 g mass are potentially concentrated in a 65 km2 zone along the northwest margin. A search of this region on foot and by helicopter in July, 1981, failed to recover any specimens. Although metre-sized gneissic boulders, plucked from the underlying Precambrian basement, were concentrated in this zone it is postulated that the unseasonal 30 cm snow cover on the ice prohibited the recognition of possible meteorite specimens, which may average only 5 cm in diameter. 相似文献
19.
Peri S.J. Coleman 《International Journal of Salt Lake Research》1998,7(1):25-40
Trials conducted at Dry Creek, a solar saltfield north of Adelaide, Australia, indicate that manganese, as manganous sulphate,
can increase both the purity and strength of salt crystals grown at an ambient or neutral-slightly acid pH range resulting
in a salt suitable for high purity uses. The improved load bearing strength of the crystals indicates that manganous sulphate
could also be useful in salt floor maintenance. However, the impact of an apparently slight rise in pH to 8.5 was significant.
Crystals produced at the higher pH from the manganese enriched brine were opaque with many tiny, almost colloidal crystals.
The salt did not drain well, had low load bearing strength and was notably impure. 相似文献
20.
Age of amphibolites associated with alpine peridotites in the Dinaride ophiolite zone,Yugoslavia 总被引:1,自引:0,他引:1
Marvin A. Lanphere Robert G. Coleman Steven Karamata Jakob Pamić 《Earth and Planetary Science Letters》1975,26(3):271-276
Amphibolites associated with alpine peridotites in the Central Ophiolite zone in Yugoslavia have K-Ar ages of 160–170 m.y. These amphibolites and associated peridotites underwent deep-seated metamorphism prior to tectonic emplacement into the sedimentary-volcanic assemblage of the Dinarides. The alpine peridotites and associated local rocks of the ophiolite suite are interpreted as Jurassic oceanic crust and upper mantle. 相似文献