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排序方式: 共有44条查询结果,搜索用时 78 毫秒
1.
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
2.
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 H2OCO2 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
3.
Laminated cyanobacterial mats in sediments of solar salt works: some sedimentological implications 总被引:1,自引:0,他引:1
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
4.
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
5.
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
6.
7.
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
8.
MIKE WALKER Martin J. Head John Lowe Max Berkelhammer Svante Bj
rck Hai Cheng Les C. Cwynar David Fisher Vasileios Gkinis Antony Long Rewi Newnham Sune Olander Rasmussen Harvey Weiss 《第四纪科学杂志》2019,34(3):173-186
The Holocene, which currently spans ~11 700 years, is the shortest series/epoch within the geological time scale (GTS), yet it contains a rich archive of evidence in stratigraphical contexts that are frequently continuous and often preserved at high levels of resolution. On 14 June 2018, the Executive Committee of the International Union of Geological Sciences formally ratified a proposal to subdivide the Holocene into three stages/ages, along with their equivalent subseries/subepochs, each anchored by a Global boundary Stratotype Section and Point (GSSP). The new stages are the Greenlandian (Lower/Early Holocene Subseries/Subepoch) with its GSSP in the Greenland NGRIP2 ice core and dated at 11 700 a b2k (before 2000 CE); the Northgrippian (Middle Holocene Subseries/Subepoch) with its GSSP in the Greenland NGRIP1 ice core and dated at 8236 a b2k; and the Meghalayan (Upper/Late Holocene Subseries/Subepoch) with its GSSP in a speleothem from Mawmluh Cave, north‐eastern India, with a date of 4250 a b2k. We explain the nomenclature of the new divisions, describe the procedures involved in the ratification process, designate auxiliary stratotypes to support the GSSPs and consider the implications of the subdivision for defining the Anthropocene as a new unit within the GTS. 相似文献
9.
McLAREN SANDRA; SANDIFORD MIKE; POWELL ROGER; NEUMANN NARELLE; WOODHEAD JON 《Journal of Petrology》2006,47(12):2281-2302
The effect of radiogenic heat production within the crust onthermal processes such as crustal anatexis is generally disregardedas bulk geochemical models suggest that crustal heat generationrates are too low to effect significant heating. However, theMount Painter Province in northern South Australia is characterizedby a total crustal contribution to surface heat flow of morethan twice the global average. The province is composed dominantlyof Proterozoic granites and granite gneisses with an area averageheat production of 16·1 µW/m3; individual lithologieshave heat production >60 µW/m3. These Proterozoic rocksare intruded by the British Empire Granite, a younger intrusivewhose origin has remained enigmatic. Isotope geochemistry suggestscrustal sources for the melt and it has a crystallization ageof 440450 Ma, which places the setting >750 km inboardof the nearest active plate boundary zone at this time. Phaseequilibria calculations suggest that temperatures of at least720750°C are required to produce the granite butthe intensity of crustal thickening during Palaeozoic deformation(12%) cannot account for these conditions. Here we describea model for the generation of the British Empire Granite inwhich the primary thermal perturbation for mid-crustal anatexiswas provided by the burial of the high heat-producing MountPainter basement rocks beneath the known thickness of Neoproterozoiccover sediments. The high heat-producing rocks at Mount Painterimply that the natural range and variability of crustal heatproduction is much greater than previously believed, with importantconsequences for our understanding of temperature-dependentcrustal processes including the exploitation of geothermal energyresources. KEY WORDS: geothermal energy; low-pressure anatexis; thermal conductivity; thermal regime 相似文献
10.
A computer code using sequential fragmentation/transport theory was used to deconvolute and characterize a large grain-size data set taken from the AD 79 Vesuvio deposits. The results allow us to interpret transport and deposition processes. Four principal morphological classes of grain-size spectra were recognized in the AD 79 deposits: 1 unimodal distributions with coarse modes and very good sorting; 2 polymodal distributions in which relative fractions of each subpopulation are considerably variable; 3 polymodal distributions, but with one mode greatly prevailing over the other ones; 4 flat spectra in which a large number of size classes show the same loading. Because different eruptive, transport and deposition conditions may have operated on pyroclasts which occur in the same bed, we have assigned grain-size subpopulations, with different modes to specific mechanisms of particle movement and sedimentation depending on the size range of the particles and the textures of the beds. The fragmentation/transport processes considered here occur either within dilute flows (as fall, traction, saltation and suspension loads) or in high-concentration flows (as a fluidized system or one with an extremely high sedimentation rate). Variation in strength and position of modes throughout the entire vertical section of AD 79 products illustrates changes in transport and deposition processes with time. Size spectra from Vesuvio quantitatively demonstrate contemporaneous deposition from fall and surge mechanisms as well as contributions from different levels of hydrovolcanic products. In contrast, vertical variations in size spectra within individual pyroclastic flow deposits suggest variation from high particle concentration near the base of the bed to more dilute depositional conditions towards the top. Lateral variations in size spectra for one marker horizon show how a local pyroclastic flow in a channel grades into a surge on the margins. This study supports the model of continuous modification in loadings of several discrete subpopulations during deposition from a single explosive cloud. 相似文献