Melt Generation and Fluid Flow in the Thermal Aureole of the Bushveld Complex   总被引：2，自引：0，他引：2  

HARRIS  NIGEL; McMILLAN  ANDY; HOLNESS  MARIAN; UKEN  RON; WATKEYS  MIKE; ROGERS  NICK; FALLICK  ANTHONY 《Journal of Petrology》2003,44(6):1031-1054

Granite sheets emplaced into the migmatite zone of the easterncontact aureole of the Bushveld Complex resulted from fluid-enhanced,incongruent biotite melting of the underlying Silverton Formationshales during prograde metamorphism. Ba concentrations are extremein both the sheets (>1000 ppm) and the hornfels (>800ppm) into which they have been emplaced. We conclude that aBa-rich, hydrothermal fluid induced melting in the aureole,and that fluid transport of Ba²⁺, and to a lesser extent, Sr²⁺and Eu²⁺, persisted in the melt zones under subsolidus conditions.Sr-isotope systematics from high-Ba localities define an errorchronof 2161 ± 106 Ma with an initial (⁸⁷Sr/⁸⁶Sr) ratio of0·705 ± 0·001. Metasedimentary rocks unaffectedby fluid infiltration were homogenized at the same time butwith an increased initial ratio, suggesting that whereas isotopehomogenization was achieved between outcrops permeated by fluids,there is no evidence of regional homogenization. Oxygen-isotopecompositions of psammitic metasediments in the aureole are uncorrelatedwith distance from the contact, suggesting the infiltratingfluid equilibrated isotopically with the metasediments. Theirelevated  相似文献   

Lithological and Structural Controls on Regional 3-D Fluid Flow Patterns during Greenschist Facies Metamorphism of the Dalradian of the SW Scottish Highlands   总被引：3，自引：0，他引：3  

SKELTON  ALASDAIR D. L.; GRAHAM  COLIN M.; BICKLE  MIKE J. 《Journal of Petrology》1995,36(2):563-586

Greenschist facies metabasite sills enclosed within phyllitesand psammites of the Dalradian Supergroup of the SW ScottishHighlands have been used as quantitative sensors of fluid flow,from which ‘fossil’ fluid flow-paths have been mapped.Infiltration of H₂O–CO₂ fluid caused carbonation of themetabasite assemblage. Consequently, metabasite sills have developeda characteristic asymmetric zonation of reacted (carbonate-bearing)margins and unreacted (carbonate-absent) interiors. Reactionfront advection theory has been used to constrain the time-integratedfluid flux and flow direction in one dimension, for each sill,from the widths of the reacted margins. Three-dimensional fluxesand flow directions have been constrained from one-dimensionalfluxes by geometrical computation, and regional fluid flow patternshave been deduced. On a regional scale, fluid flow was channelled through phyllitesand focused towards the axial zone of major antiformal foldstructures. Fluid flow was channelled through the 6 km thickArdrishaig Phyllites, within which fluid fluxes were found toincrease exponentially with proximity to the axial surface ofthe Ardrishaig Anticline. Fluid flow through the psammites waslimited to narrow zones of intense deformation associated withmetabasic or phyllitic intercalations. Although time-integratedfluid fluxes were locally high (4 10² m³/m²), average fluxes(100 m¹/m²) are compatible with availability of fluid from underlyingdevolatilizing rocks. Complete isotopic homogenization of phyllites,psammites and metabasites within the axial region of the ArdrishaigAnticline is predicted by these high measured fluxes. KEY WORDS: 3-D flow; Dalradian; SW Scotland; greenschist facies; metabasites ^*Corresponding author. Telephone: 0131-650-5885 Fax: 0131-668-3184 E-mail: askelton{at}glg.ed.ac.uk  相似文献   

Drag reduction in turbulent muddy seawater flows and some sedimentary consequences   总被引：1，自引：0，他引：1  

J. L. BEST  M. R. LEEDER 《Sedimentology》1993,40(6):1129-1137

Turbulent saline flows which contain low concentrations of suspended clay (< 10 g 1^?1) have previously been reported to possess dramatically different boundary layer characteristics from clearwater flows and to exhibit the phenomenon of drag reduction. Drag reduction causes significantly lower friction factors and higher erosion thresholds in muddy saline flows than predicted by experimental data gained from fresh-water flows using the law of the wall to estimate bed shear stress. Confirmation is given by experimental results on drag reduction using non-intrusive laser Doppler measurements in seawater flows with <2·2 g 1^?1 maximum concentration of suspended clay. Increasing the concentration of clay caused progressively lower velocities near the wall due to a gradual thickening of the buffer region of the turbulent boundary layer. Several aspects of sediment transport, deposition and bedform development in natural marine environments are thus considered to be significantly affected by the drag reduction process.  相似文献   

Bedload stresses and sediment transport theory—a correction     

MICHAEL R. LEEDER 《Sedimentology》1984,31(2):277-278

A previous proposal (Leeder, 1977) to test the magnitude of the solid-transmitted stresses due to bedload transport failed to confirm Bagnold's theory because the applied fluid stress contribution was not corrected for sidewall drag. Once this correction is made there is a reasonable correspondence of theory and experiment.  相似文献   

Laminated cyanobacterial mats in sediments of solar salt works: some sedimentological implications   总被引：1，自引：0，他引：1  

ANNIE CORNÉE  MIKE DICKMAN†  GEORGES BUSSON 《Sedimentology》1992,39(4):599-612

Formation of microlaminated sediments in solar salt works along the Mediterranean coast in southern France only occurs within a restricted salinity range of 60–150 gl^?1. These salinities are associated with development of a laminated cyanobacterial mat composed primarily of the filamentous cyanobacteria Microcoleus chthonoplastes interbedded with detrital laminae. Transplants of the cyanobacterial mat to a less saline zone (36–60 gl^?1) indicated that the cyanobacterial mats failed to colonize the less saline waters due to herbivorous snails and competition for light from floating algal masses of Cladophora and Enteromorpha. Neither the snails nor the Cladophora and Enteromorpha masses are tolerant of salinities above 60 gl^?1, and therefore the Microcoleus mats are restricted to those areas of the solar salt works with these higher salinities. Analyses of salinity, conductivity, dissolved oxygen and pH in shallow salt pans (with salinities of 60–150 gl^?1) established a relationship between the daily development of oxygen supersaturation and cyanobacterial photosynthesis. Sediments are unlaminated in those portions of the solar salt works where there are no cyanobacterial mats. These mats are frequently drained of their overlying water, and thus desiccation cracks divide them into polygonal plates. The development and translocation of these plates is enhanced by gas bubbles which form under the surface of the mats. No correlation between the microlaminae in sections from two cores located approximately 1 m apart was observed. This was consistent with the hypothesis that the surface of the desiccation crack polygons can be removed by currents and redeposited on the top of other cyanobacterial mat polygons. This process results in a ‘patchwork quilt’of young and old cyanobacterial mat polygons with an irregular microlamination pattern. The presence of such an irregular pattern of laminae permits an important distinction to be made between sediments associated with stromatolite formation and those associated with the very fine and horizontal varved sediments of stratified (meromictic) water bodies. The sedimentological significance of these observations is reviewed in relation to the processes of stromatolite genesis.  相似文献   

Environmental impacts of the Norse settlement: palaeoenvironmental data from Myvatnssveit, northern Iceland   总被引：2，自引：0，他引：2  

IAN T. LAWSON  FREDERICK J. GATHORNE-HARDY  MIKE J. CHURCH  ANTHONY J. NEWTON  KEVIN J. EDWARDS  REW J. DUGMORE  AARNI EINARSSON 《Boreas: An International Journal of Quaternary Research》2007,36(1):1-19

Lawson, I. T., Gathorne‐Hardy, F. J., Church, M. J., Newton, A. J., Edwards, K. J., Dugmore, A. J. & Einarsson, Á. 2007 (January): Environmental impacts of the Norse settlement: palaeoenvironmental data from Mývatnssveit, northern Iceland. Boreas, Vol. 36, pp. 1–19. Oslo. ISSN 0300–9483. The first stratigraphically continuous pollen profile spanning the Norse and Medieval periods from the archaeologically‐rich Mvatnssveit region of northern Iceland is presented. Detailed analyses were made of the tephra, sediment characteristics, pollen and chironomids of a 3 kyr sediment sequence from Helluvastjörn, a small, shallow lake. The pollen data show a steady decline in the percentage abundance of tree birch (Betula pubescens) pollen between the Norse settlement (landnám, c. AD 870) and c. AD 1300, a pattern that contrasts with the abrupt fall in birch pollen percentages immediately following the Norse colonization at almost all previously studied sites in Iceland. Some lines of evidence suggest that the gradual birch decline could be a result of reworking of soil pollen, but independent evidence suggests that this may not necessarily be the case. The pollen record indicates that birch woodland was replaced by acidophilic taxa (notably Empetrum nigrum and Sphagnum), again contrasting with the more usual pattern of Poaceae expansion seen in post‐landnám pollen diagrams from mires close to farm sites. Chironomid and Pediastrum accumulation data show that the limnic environment became more productive immediately after landnám, probably because of anthropogenic disturbance. An increase in sedimentation rate after landnám appears initially to have been caused by increased lake productivity, while reworked inorganic soil materials became a significant contributor to the sediments after c. AD 1200. The data suggest that the impact of settlement on terrestrial vegetation may have been more variable than previously thought, while freshwater ecosystems experienced significant and rapid change.  相似文献   

A simulation model of alluvial stratigraphy   总被引：8，自引：0，他引：8  

JOHN S. BRIDGE  MICHAEL R. LEEDER 《Sedimentology》1979,26(5):617-644

The quantitative model presented simulates the development of a two-dimensional alluvial sedimentary succession beneath a floodplain traversed by a single major river. Several inter-related effects which influence the distribution of channel-belt sand and gravel bodies within overbank fines are accounted for. These are (a) laterally variable aggradation, (b) compaction of fine sediment, (c) tectonic movement at floodplain margins, and (d) channel avulsion. Selected experiments with the model show how the interconnectedness and areal density of channel-belt deposits decrease with increasing floodplain width/channel-belt size, mean avulsion period, and channel-belt aggradation rate. Separation of stream patterns based on interconnectedness and channel deposit density is difficult. Tectonic movements do not have a significant influence upon the successions unless a preferred direction of tilting is maintained (half-graben). Then channel-belt deposits showing offlap tendencies tend to cluster adjacent to the active floodplain margin, leaving dominantly fine-grained alluvium to accumulate on the inactive side. Individual channel-belt deposits thicken during aggradation, although a self-regulating limit to such thickening is likely to operate. ‘Multistorey’features resulting from aggradation may be difficult to tell apart from those arising through superposition of distinct channel-belt deposits of avulsive origin.  相似文献   

Criteria for the instability of upper-stage plane beds     

J. R. L. ALLEN  M. R. LEEDER 《Sedimentology》1980,27(2):209-217

Calculations of the critical dimensionless bed stresses that obtain when upper-stage plane beds should revert to ripple and dune bed forms are presented. Strong support is given to the Bagnold ‘universal’ plane-bed instability criterion and to a modified criterion suggested by Allen over a wide range of solids grain size. A reinterpretation of the mechanism of plane bed instability is based upon the extent to which significant grain concentrations in plane bed flows increase apparent fluid viscosity and decrease turbulence production over potential bed defects, thereby preventing ripple or dune propagation and growth.  相似文献   

Preparing For Hyperseed MAC: An Observing Campaign To Monitor The Entry Of The Genesis Sample Return Capsule     

PETER JENNISKENS  PAUL WERCINSKI  JOE OLEJNICZAK  GARY ALLEN  PRASUN N. DESAI  GEORGE RAICHE  DEAN KONTINOS  DOUG REVELLE  JASON HATTON  RICHARD L. BAKER  RAY W. RUSSELL  MIKE TAYLOR  FRANS RIETMEIJER 《Earth, Moon, and Planets》2004,95(1-4):339-360

The imminent return of the Genesis Sample Return Capsule (SRC) from the Earth’s L1 point on September 8, 2004, represents the first opportunity since the Apollo era to study the atmospheric entry of a meter-sized body at or above the Earth’s escape speed. Until now, reentry heating models are based on only one successful reentry with an instrumented vehicle at higher than escape speed, the 22 May 1965 NASA “FIRE 2” experiment. In preparation of an instrumented airborne and ground-based observing campaign, we examined the expected bolide radiation for the reentry of the Genesis SRC. We find that the expected emission spectrum consists mostly of blackbody emission from the SRC surface (T∼ ∼2630 K@peak heating), slightly skewed in shape because of a range of surface temperatures. At high enough spectral resolution, shock emission from nitrogen and oxygen atoms, as well as the first positive and first negative bands of N₂⁺, will stand out above this continuum. Carbon atom lines and the 389-nm CN band emission may also be detected, as well as the mid-IR 4.6-μm CO band. The ablation rate can be studied from the signature of trace sodium in the heat shield material, calibrated by the total amount of matter lost from the recovered shield. A pristine collection of the heat shield would also permit the sampling of products of ablation.  相似文献   

Implications of a Century of Anthropogenic Erosion for Future Land Use in the Gisborne-East Coast Region of New Zealand     

MIKE PAGE  NOEL TRUSTRUM  BASIL GOMEZ 《New Zealand geographer》2000,56(2):13-24

The East Coast of the North Island of New Zealand is world renowned for its severe erosion, flooding, and sedimentation. Extensive deforestation between 1880–1920 initiated this period of dramatic landscape transformation, and today reforestation is seen as the panacea. However, a century of pastoral farming has left a legacy of a highly degraded landscape, which is currently redistributing the products of this erosion. The rate and level of landscape recovery will influence the ability of communities to carry out future land use. This paper uses the results of a decade of geomorphic research into the controls and processes of landscape change to illustrate some of the likely future impacts on the landscape and its land use, and to identify some still unanswered questions. This increasing understanding, together with changing community attitudes, provides the opportunity to maximise the benefits of reforestation and other management interventions.  相似文献