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191.
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193.
Tony D. Peterson 《Contributions to Mineralogy and Petrology》1989,102(3):336-346
Two fractionation trends in sodic alkaline ultramafic liquids have been predicted from experiments in subsystems of the join Di-Ak-Ne-Lc-Qz. The products of these trends are equated with contrasting suites of peralkaline nephelinites from two nephelinite-carbonatite volcanos of the south Gregory Rift, Shombole (southern Kenya) and Oldoinyo L'engai (northern Tanzania). In both trends, peralkalinity is interpreted to result from fractional crystallization of aluminous clinopyroxene. The Shombole trend has olivine nephelinite as its parental magma, and the differentiation products are mildly peralkaline [(Na+K)/Al1.15] nephelinites. It is the most common lineage observed in nephelinite-carbonatite centres. The Oldoinyo L'engai trend has melilitite or olivine-melilite nephelinite as its parental magma, and produces extremely peralkaline [(Na+K)/Al=1.4–2.3] wollastonite- and combeite- (Na2 Ca2Si3O9) bearing nephelinites. The presence of a reaction relation between wollastonite and liquid to produce combeite, indicated by corroded wollastonite phenocrysts armoured by combeite in some nephelinites from Oldoinyo L'engai, is confirmed by melting experiments. Combeite nephelinites from Oldoinyo L'engai were erupted simultaneously with natrocarbonatite ash, and are very similar in composition to silicate liquids that have been shown by experiment to be immiscible with natrocarbonatite. Because the L'engai trend is rarely expressed at extrusive centres (combeite has been recorded at only three localities), and combeite nephelinites are highly evolved magmas, it is unlikely that natrocarbonatite is primary to other carbonatite types. It is proposed that carbonatite liquid is exsolved at crustal pressures from a wide range of nephelinitic liquids: Mg-rich carbonatite from primitive, olivine-bearing alkaline ultramafic liquids, Ca-rich carbonatite from olivine-free nephelinites of low peralkalinity, and natrocarbonatite from strongly peralkaline combeite nephelinites. 相似文献
195.
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197.
Tony D. Peterson 《Contributions to Mineralogy and Petrology》1996,124(3-4):395-405
Log-normal size distributions are of the form n=noe−L/α, where n=number density, L=crystal length, and α is a constant. A method for measuring three-dimensional log-normal crystal
or grain size distributions (CSDs) in thin section has been deduced from computer experiments, in which 2D sections were cut
through assemblages of 3D solids. The size ranges and distributions studied were appropriate for igneous microphenocryst to
megacryst populations. Conversion from 2D to 3D is based on an exact correction for spheres of uniform diameter. Cumulate
numbers of polygons with length≥L (N2D) are converted to N3D by the equation:
ln(N3D)=ln(N2D/[L·S])+ln(γ)−β/[L·S]
The number density is then obtained as n=−dN/dL. The parameters S and γ correct the measured lengths and no (no=number density at L=0) respectively, and are functions of crystal shape. The parameter β is a weak function of the degree
of spatial orientation of the crystals. Highly symmetrical shapes such as cubes, octahedra, and elongated prisms can be accurately
measured when randomly oriented; however, rectangular solids with a≠b≠c cannot be accurately measured because they produce
bimodal length distributions in cross section. Strongly oriented textures (trachytic or lineated) can be accurately measured
regardless of crystal shape. New CSD data from alkaline rocks and a kimberlite give examples of CSDs modified by megacryst
retention, xenocryst addition, phenocryst accumulation, and groundmass nucleation.
Received: 25 November 1994 / Accepted: 14 November 1995 相似文献
198.
Tony Greer Ian Douglas Kawi Bidin Waidi Shun Jadda Suhaimi 《Singapore journal of tropical geography》1996,16(1):1-21
Resource development in the form of cyclical commercial logging activities results in a short period of often severe land disturbance followed by a prolonged phase of recovery. The monitoring of catchment sediment yield gives some indication of the gross erosion processes within the catchment as a result of the disturbance and may also effectively measure any ameliorative processes. Studies in Ulu Segama, Sabah, East Malaysia, recorded significant increases in stream suspended sediment loads as a result of logging. Stream loads were derived from a combination of daily sample measurements and storm event sampling using automated liquid samplers. Measured loads were then compared to computed loads from sediment discharge rating sets derived from the actual samples. Application of these ratings in the disturbed catchment initially underestimated sediment loads; however, a progressive overestimation of loads occurred as the catchment vegetation recovered. When using sediment rating curves as a tool to measure catchment erosion rates in disturbed environments, considerable caution has to be used. Vegetation recovery is reflected in the rapid recovery of stream water quality, making it necessary to continually review the rating. 相似文献
199.
Tony Gore 《GeoJournal》2008,72(1-2):59-73
Contemporary capitalist development facilitates the large-scale geographical reorganization of economic activity, involving both spatial clustering and decentralization. In the European Union the resulting regional disparities have provoked concerns about growing inequality on the one hand and poor competitiveness on the other. The concept of ‘territorial cohesion’ has been adopted to address such issues, with the need for co-operation across local, regional and national boundaries encouraged as a means of constructing more effective economic zones. This then raises the question whether the European Union’s own Structural Funds programme has been able to contribute to such collaborative working. Evidence from South Yorkshire/Sheffield in England and the Central Valleys/Cardiff in Wales suggests that any contribution is likely to be modest. Both areas were covered by Objective 1 programmes between 2000 and 2006, but differed markedly in the extent to which collaborative governance structures and processes developed. Key factors were the extent to which moves in this direction were already under way, and the extent to which management and decision-making were devolved from the centre to local and sub-regional actors. 相似文献
200.
The rate of sandstone weathering in the semi-arid climate of the Hunter Valley, New South Wales, Australia has been estimated
from observations of gravestone weathering in the area. The gravestone data points to two distinct stages in the weathering
process. The first stage covering the first century of exposure is characterised by a relatively low recession rate of 0.5 mm/100 years.
This is followed by a second stage in which the rate of weathering increases sharply to ca 2.5 mm/100 years. The non-linear
nature of the weathering trends over time suggests that during the first century of exposure, structural changes took place
within the sandstone material, which lay the foundation for accelerated weathering after further exposure. Laboratory trials
were also conducted to identify the effectiveness of different weathering processes in the decay of sandstone in this region.
Of the four processes examined, only the freeze–thaw cycle produced a significant degree of mass loss and is therefore most
likely a strong contributor to the weathering of sandstone in this region. 相似文献