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761.
Sarah K. Appleby Martin R. Gillespie Colin M. Graham Richard W. Hinton Grahame J. H. Oliver Nigel M. Kelly 《Contributions to Mineralogy and Petrology》2010,160(1):115-132
In contrast to I-type granites, which commonly comprise infracrustal and supracrustal sources, S-type granites typically incorporate
predominantly supracrustal sources. The initial aim of this study was to identify the sources of three Scottish Caledonian
(~460 Ma) S-type granites (Kemnay, Cove and Nigg Bay) by conducting oxygen, U–Pb and Hf isotope analyses in zircon in order
to characterise one potential end-member magma involved in the genesis of the voluminous late Caledonian (~430–400 Ma) I-type
granites. Field, whole-rock geochemical and isotopic data are consistent with the generation of the S-type granites by melting
their Dalradian Supergroup country rocks. While Hf isotope compositions of magmatic zircon, U–Pb data of inherited zircons,
and high mean zircon δ18O values of 9.0 ± 2.7‰ (2SD) and 9.8 ± 2.0‰ for the Kemnay and Cove granites support this model, the Nigg Bay Granite contains
zircons with much lower δ18O values (6.8 ± 2.1‰), similar to those found in Scottish I-type granites. This suggests that the Nigg Bay Granite contains
low-δ18O material representing either altered supracrustal material, or more likely, an infracrustal source component with mantle-like
δ18O. Mixing trends in plots of δ18O vs. εHf for S-type granite zircons indicate involvement of at least two sources in all three granites. This pilot study
of Scottish Caledonian S-type granites demonstrates that, while field and whole-rock geochemical data are consistent with
local melting of only supracrustal sources, the oxygen isotopic record stored in zircon reveals a much more complex petrogenetic
evolution involving two or more magma sources. 相似文献
762.
We studied clinopyroxenes from spinel-facies peridotite xenoliths sampled by the Quaternary intra-plate volcanism of the Middle Atlas (Morocco) and present new trace element and Sr-Nd-Hf isotope data. However, we focus in particular on Pb isotope data and 238U/204Pb and 232Th/204Pb ratios of these clinopyroxenes. This data allows us to investigate: (a) the timing of metasomatic events, (b) the prevalence and persistence of elevated 238U/204Pb, 232Th/238U and 232Th/204Pb in continental mantle roots and (c) the 238U/204Pb and 232Th/204Pb composition of putative basaltic melts generated from such metasomatised sub-continental lithospheric mantle (SCLM).Incompatible trace element concentrations in these clinopyroxenes are elevated, marked by high-field strength element depletion and fractionated elemental ratios (e.g., U/Nb, Zr/Hf) most consistent with enrichment due to carbonatitic liquids. Sr, Nd and Hf isotopes have an affinity to HIMU.U, Th and Pb abundances in the clinopyroxenes generally exceed estimates of primitive mantle clinopyroxene. Pb isotope compositions of these clinopyroxenes are radiogenic and vary between 206Pb/204Pb = 19.93-20.25, 207Pb/204Pb = 15.63-15.66 and 208Pb/204Pb = 39.72-40.23. These Pb isotope systematics result in generally negative Δ7/4 but positive Δ8/4; setting these samples distinctly apart from typical HIMU. These Pb isotope compositions are also distinct from the associated host volcanic rocks. 238U/204Pb and 232Th/204Pb of these clinopyroxenes, which range from 26 to 81 and 136 to 399, respectively, are elevated and more extreme than estimates of MORB- and HIMU-source mantle.The Pb isotope evolution of the clinopyroxenes suggests that the metasomatic enrichment is younger than 200 Ma, which discounts the volcanic activity due to the opening of the Atlantic and the onset of the collision of the African and Eurasian plates as processes generating the lithophile element and isotope composition of this continental mantle root. Instead, the enrichment is thought to be associated with the Quaternary intra-plate volcanism in the Middle Atlas. However, the erupted mafic melts have unradiogenic Pb isotopes and lower 238U/204Pb, 232Th/204Pb and 232Th/238U relative to the clinopyroxene and do not seem to have equilibrated with the clinopyroxenes. The high Th abundances and the high 232Th/238U also suggest that the metasomatism was due to carbonatitic liquids.When literature data for Pb isotopes in mantle minerals are considered, the Pb isotope range of Archean, Proterozoic and Phanerozoic continental mantle roots is remarkable in that they are similar to the convecting mantle. This observation does not support the existence of sub-continental lithospheric mantle with high 238U/204Pb and 232Th/204Pb for long periods of time. Consequently, the narrow range of Pb isotopes in SCLM worldwide suggests that only the youngest metasomatic events are recorded by incompatible elements such as U, Th and Pb. Numerical modelling of putative magmas generated from Middle Atlas SCLM by fractional, non-modal melting calculations yield extremely high 238U/204Pb and 232Th/204Pb ratios. For example, pure SCLM magmas generated from 0.5% to 10% melting are anticipated to have 232Th/204Pb ratios exceeding those known from terrestrial basalts. 相似文献
763.
Steven N. Bacon Eric V. McDonald Todd G. Caldwell Graham K. Dalldorf 《Quaternary Research》2010,73(3):425-438
The integration of geomorphic mapping, soil stratigraphy, and radiocarbon dating of alluvial deposits offers insight to the timing, magnitude, and paleoclimatic context of Holocene fan sedimentation near Yuma, Arizona. Mapping of 3400 km2 indicates about 10% of the area aggraded in the late Holocene and formed regionally extensive alluvial fan and alluvial plain cut-and-fill terraces. Fan deposits have weakly developed gravelly soils and yielded a date of 3200–2950 cal yr BP from carbonized wood. Alluvial plain deposits have weakly developed buried sandy soils and provided a date of 2460–2300 cal yr BP from a terrestrial snail shell. Precipitation records were analyzed to form historical analogues to the late Holocene aggradation and to consider the role of climatic variability and extreme hydrologic events as drivers of the sedimentation. The historical precipitation record indicates numerous above-average events correlated to the Southern Oscillation Index (SOI) in the region, but lacks any significant reactivation of alluvial fan surfaces. The timing of aggradation from 3200 to 2300 cal yr BP correlates well with other paleoclimatic proxy records in the southwestern U.S. and eastern Pacific region, which indicate an intensification of the El Niño-Southern Oscillation (ENSO) climatic pattern and rapid climate change during this period. 相似文献
764.
Christopher I. Massey Vernon Manville Graham H. Hancox Harry J. Keys Colin Lawrence Mauri McSaveney 《Landslides》2010,7(3):303-315
The summit crater of Mt Ruapehu volcano normally hosts a 15.4-ha warm lake, whose water has been repeatedly wholly or partly
ejected by explosive and extrusive eruptions. Some of the larger eruptions have modified the lake outlet by burying it under
unconsolidated tephra (volcanic ash and blocks), creating a dam-break flood hazard independently of the occurrence of an eruption.
Eruptions in 1995 and 1996 followed this sequence; a break-out flood was anticipated and a warning system was installed to
mitigate the risk from this event and subsequent lahars in the same catchment. The 11-year filling time allowed much planning
and rehearsal. The warning system involved manual inspections of dam integrity, and seepage and lake-level monitoring to constrain
the likely failure window, and telemetered instruments including a tripwire and geophones to detect breaching of the dam and
propagation of the outbreak flood. The dam-collapse sequence, captured by a time-lapse camera, involved a series of retrogressing
landslides initiated and accelerated by seepage forces and toe scour when the lake was 1.1 m below overtopping. The barrier
failed in two phases on 18th March, 2007, beginning at 09:55 (NZST), with rapid retreat of one of the erosion scarps on the
downstream slope of the eastern barrier, initiated by internal erosion. Headward retrogression of the scarp into the barrier
formed an initial breach in the dam, after which increasing outflow led to erosion and undercutting of the wider downstream
toe of the western barrier. A final, larger dam breach occurred between 11:21 and 11:22 as slope instability caused retrogressive
failure of the remaining barrier. Five-hundred meters downstream of the dam, a large landslide was reactivated by toe scour
during the flood, contributing about a million cubic meters of solid material to the volumetric bulking of the outflow, which
reached the coast, 215 km away, 17 h later. The success of the planning and warning system allowed the whole event to occur
with little damage to infrastructure and without causing injury. 相似文献
765.
Summary This paper argues that the study of crime and the fear of crime in rural areas reveals much about the geography of crime, policing and rural society. Drawing upon a crime and safety survey conducted with residents of a rural parish in Worcestershire it establishes a link between fear of crime and 'cultural threats' to residents' dominant constructions of rurality. It concludes by considering the 1998 Crime and Disorder Act and its implications for rural policing and society. 相似文献
766.
Elspeth Graham 《The Professional geographer》1999,51(1):76-89
Research traditions exert a powerful influence over the thinking of academic researchers. In population geography an entrenched empiricism and a reluctance to engage in wider debates on theory and method within human geography have resulted in a degree of separate development. Such methodological complacency has, until recently, threatened to undermine population geography's role within the discipline as a whole. Thus the discussion of multi-method research designs is of particular significance because it offers population geographers an opportunity to break out from the confines of a dominant research tradition and participate with other human geographers in an important methodological debate. This paper seeks not only to provide a critical overview of the current debate within population geography (as represented in the preceding papers in this Focus section) but also to extend that debate by raising issues of much more general concern. I argue that there are dangers in drawing the terms of the debate too narrowly. If we are to understand the nature and potential of multi-method research, we must first pose fundamental questions about the interrelationships among methods, data, and research problems. Pragmatic views on the choice of research method are inadequate because they fail to recognise the theory-driven nature of research. Only once we have achieved a better understanding of the philosophical grounding of research strategies will the opportunities afforded by multi-method research be fully realised. 相似文献
767.
J. Graham Cogley 《Geografiska Annaler: Series A, Physical Geography》1999,81(4):497-507
Correlograms from multiple time series of point mass balance, measured on White Glacier (Axel Heiberg Island, Canada) and Abramov Glacier (Alai Range, Kirgizia), show that the correlation decreases with the difference in elevation between the points. The correlogram is used to calculate an integral spatial scale or effective sample area which, when divided into the area of the glacier, yields an estimate of the number of degrees of freedom in a stake-based estimate of the whole-glacier balance. This number – the 'effective sample size'– is a small fraction of the number of point measurements; indeed, it is independent of the number of measurements. Estimates of average balance for elevation bands are at one remove from raw stake measurements, but they are amenable to a principal component analysis which confirms that the effective sample size is very small. The small effective sample size means that uncertainty in a typical measurement of whole-glacier mass balance cannot be much less than the large value implied by the conservative assumption that stakes are perfectly correlated. One way around this difficulty would be to increase the role of prior physical understanding by seeking to model the spatial variability of mass balance. A successful model would need only a few parameters, and would allow for the joint estimation of both magnitude and uncertainty; the uncertainty in mass balance could be derived objectively from the uncertainty in the parameters. This, however, would require good estimates of the variability of mass balance at the elevation-band scale, which might in turn require that many measurement networks be redesigned. 相似文献
768.
Landslides control the spatial and temporal variation of channel width in southern Taiwan: Implications for landscape evolution and cascading hazards in steep,tectonically active landscapes 下载免费PDF全文
Brian J. Yanites Nate A. Mitchell Joshua C. Bregy Grace A. Carlson Kirstyn Cataldo Margaret Holahan Graham H. Johnston Amelia Nelson Jeffery Valenza Matthew Wanker 《地球表面变化过程与地形》2018,43(9):1782-1797
Intense precipitation or seismic events can generate clustered mass movement processes across a landscape. These rare events have significant impacts on the landscape, however, the rarity of such events leads to uncertainty in how they impact the entire geomorphic system over a range of timescales. Taiwan is steep, tectonically active, and prone to landslide and debris flows, especially when exposed to heavy rainfall events. Typhoon Morakot made landfall in Taiwan in August of 2009, causing widespread landslides in southern Taiwan. The south to north trend in valley relief in southern Taiwan leads to spatial variability in landslide susceptibility providing an opportunity to infer the long‐term impact of such landslide events on channel morphology. We use pre‐ and post‐typhoon imagery to quantify the propagating impact of this event on channel width as the debris is routed through the landscape. The results show the importance of cascading hazards from landslides on landscape evolution based on patterns of channel width (both pre‐ and post‐typhoon) and hillslope gradients in 20 basins along strike in southern Taiwan. Prior to Typhoon Morakot, the river channels in the central part of the study area were about 3–10 times wider than the channels in the south. Following the typhoon, aggradation and widening was also a maximum in these central to northern basins where hillslope gradients and channel steepness is high, accentuating the pre‐typhoon pattern. The results further show that the narrowest channels are located where channel steepness is the lowest, an observation inconsistent with a detachment‐limited model for river evolution. We infer this pattern is indicative of a strong role of sediment supply, and associated landslide events, on long‐term channel evolution. These findings have implications across a range of spatial and temporal scales including understanding the cascade of hazards in steep landscapes and geomorphic interpretation of channel morphology. Copyright © 2018 John Wiley & Sons, Ltd. 相似文献
769.
Jacob Mulder Graham Hagen-Peter Teresa Ubide Rasmus Andreasen Ellen Kooijman Melanie Kielman-Schmitt Yue-Xing Feng Bence Paul Andreas Karlsson Christian Tegner Charles Lesher Fidel Costa 《Geostandards and Geoanalytical Research》2023,47(2):311-336
Laser ablation multi-collector mass spectrometry (LA-MC-ICP-MS) has emerged as the technique of choice for in situ measurements of Sr isotopes in geological minerals. However, the method poses analytical challenges and there is no widely adopted standardised approach to collecting these data or correcting the numerous potential isobaric inferences. Here, we outline practical analytical procedures and data reduction strategies to help establish a consistent framework for collecting and correcting Sr isotope measurements in geological materials by LA-MC-ICP-MS. We characterise a new set of plagioclase reference materials, which are available for distribution to the community, and present a new data reduction scheme for the Iolite software package to correct isobaric interferences for different materials and analytical conditions. Our tests show that a combination of Kr-baseline subtraction, Rb-peak-stripping using βRb derived from a bracketing glass reference material, and a CaCa or CaAr correction for plagioclase and CaCa or CaAr + REE2+ correction for rock glasses, yields the most accurate and precise 87Sr/86Sr measurements for these materials. Using the analytical and correction procedures outlined herein, spot analyses using a beam diameter of 100 μm or rastering with a 50–65 μm diameter beam can readily achieve < 100 ppm 2SE repeatability ("internal") precision for 87Sr/86Sr measurements for materials with < 1000 μg g-1 Sr. 相似文献
770.