Timing and rate of isothermal decompression in Pan-African granulites from Rundvågshetta, East Antarctica     

Fraser  McDougall  Ellis  & Williams 《Journal of Metamorphic Geology》2000,18(4):441-454

Geochronological data, combined with field and petrological evidence, constrain the timing and rate of near‐isothermal decompression at granulite facies temperatures in rocks from the Lützow‐Holm Complex of East Antarctica. Granulite facies gneisses from Rundvågshetta in Lützow‐Holm Bay experienced a peak metamorphic temperature of over 900 °C at c. 11 kbar, as evidenced by primary orthopyroxene–sillimanite‐bearing assemblages, and secondary cordierite–sapphirine‐bearing assemblages in metapelites. Peak metamorphic assemblages show strong preferred mineral orientation, interpreted to have developed synchronously with pervasive ductile deformation. Zircon from a syndeformational leucosome has a U–Pb age of 517±9 Ma, which is interpreted as a melt crystallization age. This age provides the best estimate of the time of peak metamorphic conditions. The post‐peak metamorphic history is characterized by near‐isothermal decompression, recorded by mineral textures in a variety of rock compositions. Field and textural relations indicate that decompression post‐dated pervasive ductile deformation. K/Ar and ⁴⁰Ar/³⁹Ar ages from hornblende and biotite represent closure ages during cooling subsequent to decompression, and indicate cooling to temperatures between c. 350 and 300 °C by c. 500 Ma, thus placing a lower time limit on the duration of the high‐temperature isothermal decompression episode. The combination of the zircon age from a syndeformational melt with K/Ar and ⁴⁰Ar/³⁹Ar closure ages indicates that near‐isothermal decompression from c. 11 to c. 4 kbar at granulite facies temperatures, followed by cooling to c. 300 °C, took place within a time interval of 20±10 Myr. Simple one‐dimensional models for exhumation‐controlled cooling indicate that these data require exhumation rates of the order of c. 3 km Myr^?1 for several million years, then cessation of exhumation followed by relatively isobaric cooling during thermal re‐equilibration.  相似文献   

Synthesis of zinc oxide–montmorillonite composite and its effect on the removal of aqueous lead ions     

Egirani  Davidson E.  Poyi  Nanfe R.  Wessey  Napoleon 《中国地球化学学报》2019,38(1):120-130

Acta Geochimica - Lead adsorption of zinc oxide-coated ACOR montmorillonite was investigated in batches and under reducing conditions at ambient temperature. The presence of zinc oxide coating...  相似文献   

The age and formation mechanisms of Late Triassic fissure deposits,Gloucestershire, England: Comments on Mussini,G. et al. (2020). Anatomy of a Late Triassic Bristol fissure: Tytherington fissure 2     

Gordon M. Walkden  Nicholas C. Fraser  Michael J. Simms 《Proceedings of the Geologists' Association. Geologists' Association》2021,132(1):127-137

In the Late Triassic the landscape NE of present-day Bristol, SW England was dominated by Carboniferous Limestone ridges and cuestas that became progressively buried by continental Mercia Mudstones and finally inundated during the Rhaetian marine transgression. Mussini et al. (2020) adopt the assertions of earlier collaborators back to Whiteside and Marshall (2008) that terrestrial vertebrate assemblages from sediments contained within karstic fissure systems in the former limestone ridges at Cromhall, Tytherington and elsewhere are restricted to the Rhaetian. We review and reject the sedimentological, stratigraphic, geomorphological and topographic arguments for this and reassert a long pre-Rhaetian (Norian) history for the vertebrate-bearing fissure systems at both Tytherington and Cromhall. We also reject the contemporaneous Rhaetian freshwater-seawater mixing zone dissolution model for the fissure systems adopted by Mussini et al. (2020) and reaffirm that the Tytherington and Cromhall fissures developed as conduit caves with a long Triassic history. Applying a new regional study of the Rhaetian transgressive surface, we also show that whilst the fissures at Cromhall remained sealed after the Norian, those at nearby Tytherington were re-exposed in the Late Rhaetian. Already partially filled with Norian sediments, the Tytherington fissures were subject to reworking on the seabed. Internal collapses, probably triggered by well documented repeated regional seismicity, led to the chaotic state of the Tytherington fills when downward moving Rhaetian marine components came to lie amongst and mix with earlier Norian terrestrial sediments. The vertebrate associations in the Tytherington fissures therefore contain a substantial Rhaetian input whilst those at Cromhall do not.  相似文献   

SWIFT: An adaptive optics assisted I/z band integral field spectrograph     

Matthias Tecza  Niranjan Thatte  Fraser Clarke  Timothy Goodsall  Myrto Symeonidis 《New Astronomy Reviews》2006,49(10-12):647

SWIFT is an adaptive optics assisted integral field spectrograph covering the I and z astronomical bands (0.7–1.0 μm) at a spectral resolving power R  5000. At its heart is an all-glass image slicer with high throughput based on a novel de-magnifying design allowing a compact instrument. SWIFT profits from two recent developments: (i) the improved ability of second generation adaptive optics systems to correct for atmospheric turbulence in SWIFTS’s bandpass, and (ii) the availability of CCD array detectors with high quantum efficiency at very red wavelengths. It is a dedicated integral field spectrograph, specifically built to address a range of interesting astrophysical questions.  相似文献   

The luminosity function of the hot and cold Kuiper belt populations     

Wesley C. Fraser  Michael E. Brown  Megan E. Schwamb 《Icarus》2010,210(2):944-955

We have performed an ecliptic survey of the Kuiper belt, with an areal coverage of 8.9 square degrees to a 50% limiting magnitude of , and have detected 88 Kuiper belt objects, roughly half of which received follow-up 1–2 months after detection. Using this survey data alone, we have measured the luminosity function of the Kuiper belt, thus avoiding any biases that might come from the inclusion of other observations. We have found that the Cold population defined as having inclinations less than 5° has a luminosity function slope α_Cold = 0.82 ± 0.23, and is different from the Hot population, which has inclinations greater than 5° and a luminosity function slope α_Hot = 0.35 ± 0.21. As well, we have found that those objects closer than 38 AU have virtually the same luminosity function slope as the Hot population. This result, along with similar findings of past surveys demonstrates that the dynamically Cold Kuiper belt objects likely have a steep size distribution, and are unique from all of the excited populations which have much shallower distributions. This suggests that the dynamically excited population underwent a different accretion history and achieved a more evolved state of accretion than the Cold population. As well, we discuss the similarities of the Cold and Hot populations with the size distributions of other planetesimal populations. We find that while the Jupiter family comets and the scattered disk exhibit similar size distributions, a power-law extrapolation to small sizes for the scattered disk cannot account for the observed influx of comets. As well, we have found that the Jupiter Trojan and Hot populations cannot have originated from the same parent population, a result that is difficult to reconcile with scattering models similar to the NICE model. We conclude that the similarity between the size distributions of the Cold population and the Jupiter Trojan population is a striking coincidence.  相似文献   

Fluid inclusion evidence for rapid formation of the vapor-dominated zone at Sulphur Springs, Valles caldera, New Mexico, USA     

Masakatsu Sasada  Fraser Goff 《Journal of Volcanology and Geothermal Research》1995,67(1-3)

Microthermometric measurements were obtained for 618 fluid inclusions in hydrothermal quartz, fluorite and calcite and magmatic quartz phenocrysts in intracaldera tuffs from the VC-2A core hole in order to study evolutionary processes of the Sulphur Springs hydrothermal system in the Valles caldera. Relatively high T_h values in samples from shallow depths indicate erosion of about 200 m of caldera fill since deposition of hydrothermal minerals at shallow depths in the Sulphur Springs hydrothermal system, accompanied by a descent in the water table of the liquid-dominated reservoir. For samples collected below the current water level of the well, the minimum values of homogenization temperature (T_h) fit the present thermal profile, whereas minimum T_h values of samples from above the water level are several tens of degrees higher than the present thermal profile and fit a paleo-thermal profile following the boiling point curve for pure water, as adjusted to 92 °C at 20 m below the present land surface. This is attributed to development of an evolving vapor zone that formed subsequent to a sudden drop in the water table of the liquid-dominated reservoir. We suggest that these events were caused by the drainage of an intracaldera lake when the southwestern wall of the caldera was breached about 0.5 Ma. This model indicates that vapor zones above major liquid-dominated geothermal reservoirs can be formed due to dramatic changes in geohydrology and not just from simple boiling.  相似文献   

Observational Weighting Considerations In GPS Aerial Triangulation     

C. S. Fraser 《The Photogrammetric Record》1995,15(86):263-275

This paper addresses aspects of relative observational weighting for GPS camera station positions, ground control and image co-ordinate data in GPS aerial triangulation. Weighting implications for different control point and drift parameter scenarios are discussed, a two stage adjustment/transformation procedure is proposed and the implications of observational weighting on triangulation precision are reviewed. Bundle block adjustment results from a 626 photograph, 1650 point GPS aerial triangulation project are presented to illustrate the outcomes of different weighting and ground control strategies.  相似文献   

Role of melt during deformation in the deep crust   总被引：1，自引：0，他引：1  

Cameron Davidson  Stefan M. Schmid  Lincoln S. Hollister 《地学学报》1994,6(2):133-142

Deformation in the deep crust is strongly influenced by the presence of melt. Injected melt (or magma) weakens the crust because strain will tend to localize where melt is present. The amount of strain a pluton may accommodate is dependent on the length of time it takes for a pluton to crystallize and the strain rate. For plutons that intrude into rocks which are near the solidus temperature of the melt, crystallization times can be quite long (> 1Myr).
Partial melting of deep crustal rocks can lead to melt-enhanced embrittlement. This occurs because the volume change for most melting reactions is positive. Therefore, when the rate of melt production outpaces the rate at which melt can leave the system, the melt pressure increases. Eventually, the melt pressure may become sufficiently high that the melting rocks behave in a brittle fashion and fracture.
Conjugate sets of dilatant shear fractures filled with melt occur in migmatite from the Central Gneiss belt (Canada); this suggests that melt-enhanced embrittlement occurred in these rocks. An expression which relates the magnitude of differential stress to the angle between conjugate dilatant shear fractures is derived. Assuming that migmatite has a small tensile strength, differential stresses are ≤ 20 MPa in migmatitic rocks at the time melt-enhanced embrittlement occurs. The occurrence of melt-enhanced embrittlement shows that a switch in deformation mechanism from plastic flow to cataclasis is possible in the deep crust during melting. Furthermore, repeated episodes of melt-enhanced embrittlement in migmatitic rocks may be an efficient mechanism for extracting melt from partially melted terrains.  相似文献   

Variationally adjusted surface winds     

J. N. McEwen  M. B. Danard  G. A. Davidson 《Boundary-Layer Meteorology》1981,20(4):473-483

This paper describes a one-level variational adjustment process for producing mass-consistent surface winds in the Barrow Strait area, N.W.T. To achieve this result, the continuity equation is employed as a physical constraint. The variational technique adjusts mean winds (vertically averaged through the planetary boundary layer); therefore, a relationship is required between surface and mean winds. Use is made of existing velocity profiles, but interpolation may be used between surface and geostrophic winds. The model was found to be strongly dependent upon specification of boundary-layer height. Channeling effects are not readily seen until topography begins to protrude through the boundary layer. The model might thus be better suited to areas where shallow inversion layers are well defined. By application of the variational adjustment, errors in continuity are reduced by six orders of magnitude. Upon comparison of the variational technique with a diagnostic, one-level, primitive equation model, median errors between computed and observed surface winds were found to be comparable.  相似文献   

The use of oxygen isotope geothermometry on the granulites and related intrusives,Musgrave Ranges,central Australia     

Allan F. Wilson  David C. Green  Lawrence E. Davidson 《Contributions to Mineralogy and Petrology》1970,27(2):166-178

Oxygen isotope equilibration involving quartz, feldspar and magnetite has taken place within the granulites of the metamorphic aureole of the intrusive charnockitic Ernabella Adamellite, and a temperature of approximately 550° C is indicated by ¹⁸O/¹⁶O values from quartz-magnetite and plagioclase-magnetite pairs. Pyroxene-magnetite fractionations are larger than equilibrium fractionations at this temperature, and demonstrate that caution is necessary in assigning estimates of temperature to highly metamorphosed rocks.The intrusive adamellite and related fine-grained and pegmatitic phases appear to have attained oxygen isotope equilibrium with the granulites at a similar temperature (approx. 550° C) which is here regarded as a late metamorphic equilibration temperature rather than the maximum temperature (estimated to be about 780° C) reached within the aureole.Oxygen was extracted from the minerals by means of bromine pentafluoride, and the isotopes were successfully measured without the normal prior conversion of oxygen to carbon dioxide.  相似文献