ANABRANCHING RIVERS: THEIR CAUSE,CHARACTER AND CLASSIFICATION

Anabranching rivers consist of multiple channels separated by vegetated semi-permanent alluvial islands excised from existing floodplain or formed by within-channel or deltaic accretion. These rivers occupy a wide range of environments from low to high energy, however, their existence has never been adequately explained. They occur concurrently with other types of channel pattern, although specific requirements include a flood-dominated flow regime and banks that are resistant to erosion, with some systems characterized by mechanisms to block or constrict channels, thereby triggering avulsion. The fundamental advantage of an anabranching river is that, by constructing a semi-permanent system of multiple channels, it can concentrate stream flow and maximize bed-sediment transport (work per unit area of the bed) under conditions where there is little or no opportunity to increase gradient. On the basis of stream energy, sediment size and morphological characteristics, six types of anabranching river are recognized; types 1–3 are lower energy and types 4–6 are higher energy systems. Type 1 are cohesive sediment rivers (commonly termed anastomosing) with low w/d ratio channels that exhibit little or no lateral migration. They are divisible into three subtypes based on vegetative and sedimentary environment. Type 2 are sand-dominated, island-forming rivers, and type 3 are mixed-load laterally active meandering rivers. Type 4 are sand-dominated, ridge-forming rivers characterized by long, parallel, channel-dividing ridges. Type 5 are gravel-dominated, laterally active systems that interface between meandering and braiding in mountainous regions. Type 6 are gravel-dominated, stable systems that occur as non-migrating channels in small, relatively steep basins. Anabranching rivers represent a relatively uncommon but widespread and distinctive group that, because of particular sedimentary, energy-gradient and other hydraulic conditions, operate most effectively as a system of multiple channels separated by vegetated floodplain islands or alluvial ridges.     

P-T Conditions and the Influence of Graphite on Pelitic Phase Relations in the Ballachulish Aureole, Scotland

Pressure-temperature conditions of pelites in the Ballachulishaureole, Scotland, have been determined from a calibrated petrogeneticgrid and from published geothermometers and geobarometers. Tocalibrate the mineral reactions in the grid, thermodynamic datafor appropriate end members of Ms, Chi, Qtz, And, Sil, Ky, Crn,Crd, Kfs, and Bt were derived from experimental data. This approachwas hampered by the unknown compositions of many of the mineralsused in the experiments, and by apparent inconsistency betweenthe experiments. A best compromise grid that satisfies mostof the data was obtained, which is applicable to the Ballachulishand other contact aureoles. In this grid, the first developmentof sillimanite is constrained to lie between the Richardsonet al. (1969) and Holdaway (1971) andalusite-sillimanite boundaries. A pressure estimate of 3.0 + 0.5 kb is obtained from the calibratedgrid, within 0.3 kb of estimates from geobarometry and fromtwo other independent petrological studies. Temperatures rangedfrom 560?20?C at the first development of cordierite in theassemblage Ms+Qtz+Chl+Crd+Bt to 750–800?C in Grt+Crd+Hyassemblages in pelitic screens within the igneous complex. In graphitic slates, in contrast to non-graphitic pelites, anentire andalusite-bearing subzone is developed, and initialcordierite development occurs further from the igneous contacts.The presence of graphite lowered a_H2o in the slaters, expandingthe stability field of the andalusite-bearing assemblage And+Qtz+Bt+Ms+Crdrelative to the assemblage Kfs+Qtz+Bt+Ms+Crd developed in non-graphiticunits. Initial development of cordierite in the assemblage Ms+Qtz+Chl+Crd+Btwas also promoted by reduced a_H2o in graphitic slates. The regular sequence and spacing of mineral zones in the aureolesuggests that gross equilibrium was attained during contactmetamorphism, even though the thermal metamorphic pulse is estimatedto have been less than 0.2 Ma (Buntebarth, in press). Thereis no evidence for reaction overstepping in cordierite-producingreactions.     

MIL 03443, a dunite from asteroid 4 Vesta: Evidence for its classification and cumulate origin

Abstract– The absence of dunite (>90 vol% olivine) in the howardite, eucrite, and diogenite (HED) meteorite suite, when viewed with respect to spectroscopic and petrologic evidence for olivine on Vesta, is problematic. Herein, we present petrologic, geochemical, and isotopic evidence confirming that Miller Range (MIL) 03443, containing 91 vol% olivine, should be classified with the HED clan rather than with mesosiderites. Similarities in olivine and pyroxene FeO/MnO ratios, mineral compositions, and unusual mineral inclusions between MIL 03443 and the diogenites support their formation on a common parent body. This hypothesis is bolstered by oxygen isotopic and bulk geochemical data. Beyond evidence for its reclassification, we present observations and interpretations that MIL 03443 is probably a crustal cumulate rock like the diogenites, rather than a sample of the Vestan mantle.     

TURBULENCE STRUCTURE IN A STRATIFIED BOUNDARY LAYER UNDER STABLE CONDITIONS

Turbulence structure in stably stratified boundary layers isexperimentally investigated by using a thermally stratified wind tunnel. Astably stratified flow is created by heating the wind tunnel airflow to atemperature of about 50 °C and by cooling the test-section floor to asurface temperature of about 3 °C. In order to study the effect ofbuoyancy on turbulent boundary layers for a wide range of stability, thevelocity and temperature fluctuations are measured simultaneously at adownwind position of 23.5 m from the tunnel entrance, where the boundarylayer is fully developed. The Reynolds number, Re, ranges from 3.14× 104 to 1.27 × 105, and the bulk Richardson number, Ri,ranges from 0 to 1.33. Stable stratification rapidly suppresses thefluctuations of streamwise velocity and temperature as well as the verticalvelocity fluctuation. Momentum and heat fluxes are also significantlydecreased with increasing stability and become nearly zero in the lowest partof the boundary layer with strong stability. The vertical profiles ofturbulence quantities exhibit different behaviour in three distinct stabilityregimes, the neutral flows, the stratified flows with weak stability(Ri = 0.12, 0.20) and those with strong stability (Ri= 0.39,0.47, 1.33). Of these, the two regimes of stratified flows clearly showdifferent vertical profiles of the local gradient Richardson number Ri,separated by the critical Richardson number Ri cr of about 0.25. Moreover,turbulence quantities in stable conditions are well correlated with Ri.     

Temperatures of Granulite-facies Metamorphism: Constraints from Experimental Phase Equilibria and Thermobarometry Corrected for Retrograde Exchange

This study assesses temperatures of formation of common granulitesby combining experimental constraints on the P–T stabilityof granulite-facies mineral associations with a garnet–orthopyroxene(Grt–Opx) thermobarometry scheme based on Al-solubilityin Opx, corrected for late Fe–Mg exchange. We appliedthis scheme to 414 granulites of mafic, intermediate and aluminousbulk compositions. Our findings suggest that granulites aremuch hotter than traditionally assumed and that the P–Tconditions of the amphibolite–granulite transition portrayedin current petrology textbooks are significant underestimatesby over 100°C. For aluminous and intermediate granulites,mean corrected temperatures based on our method are 890 ±17 and 841 ± 11°C, respectively (uncertainties reportedas 95% confidence limits on the mean), consistent with minimumtemperatures for orthopyroxene production by fluid-absent partialmelting in these bulk compositions. In contrast, mean temperaturesbased on Grt–Opx Fe–Mg exchange equilibria, usingthe same thermodynamic data, are 732 ± 22 and 723 ±11°C, respectively, well below the minimum temperaturesfor Opx stability. For mafic granulites, the mean correctedtemperature using our method is 816 ± 12°C, similarto the mean temperature of 793 ± 13°C from Fe–Mgexchange. Reasons for the differences between the mafic granulitesand aluminous–intermediate granulites are unclear butmay be due to the lower Al concentrations in Opx in the maficrocks and possible deficiencies in the thermodynamic modellingof these low concentrations. We discuss a number of well-knowngranulite terrains in the context of our findings, includingthe Adirondacks, the Acadian granulites of New England, theincipient charnockites of southern India and Sri Lanka, andthe Kerala Khondalite Belt. Our findings carry implicationsfor thermotectonic models of granulite formation. A computerprogram to perform our thermobarometry calculations, RCLC, isavailable from the Journal of Petrology website at http://www.petrology.oupjournals.orgor from the authors at http://www.geo.ucalgary.ca/~pattison/drm_pattison-rclc.htm. KEY WORDS: granulite-facies metamorphism; thermobarometry; garnet; orthopyroxene     

Social Democracy and Contextual Unemployment: New Zealand Labour in 1990

This paper examines the spatial implications of social-democratic policy transformation. It does so by exploring the political consequences of residential contextual unemployment engendered by a Labour government which had once been dedicated to maintaining full employment. Faced with economic crisis, as Przeworski's model predicts [Capitalism and social democracy, Cambridge University Press, Cambridge (1985)], New Zealand Labour, beginning in 1984, abandoned its traditional social-democratic statist policies, including the maintenance of full employment, and allowed unemployment to skyrocket under its stewardship. This meant subjecting its supporters to significant contextual cross-pressures, since many residential neighbourhoods became characterised by both high concentrations of Labour partisans, and by those recently unemployed by Labour's new market-liberal policies. Using original survey data merged with New Zealand census data, empirical findings show that residual partisan loyalties and a lack of a major party alternative constrained respondents' partisan disaffection, while channelling their disillusionment into both minor party voting and systemic, governmental criticisms - likely fuelling New Zealand's drive for electoral reform and a new system of proportional representation.     

Sea level control of stacked late Quaternary coastal sequences, central Great Barrier Reef

Lithofacies analysis, pollen assemblages and radiocarbon age dates of 20 stratigraphic drill holes are used to develop an evolutionary history for late Quaternary sedimentation in two coastal embayments landward of the central Great Barrier Reef. Different physiographic settings of the embayments result in two contrasting styles of sedimentary sequence: (a) an exposed, moderate energy, beach barrier-lagoon system (Wyvuri Embayment) and (b) a protected, low energy, muddy inlet fill sequence (Mutchero Inlet). Despite sharp contrast in sequence style, similar depositional cycles occur in both embayments in response to late Quaternary sea level fluctuations including: (1) a last interglacial highstand (+2 m; c. 125 000 yr bp ) beach barrier (Wyvuri); (2) an early to mid-Holocene (8000–6100 yr bp ) transgressive beach barrier-lagoon (Wyvuri) and estuarine infill (Mutchero); and (3) mid-Holocene to present highstand beach barrier (Wyvuri) and estuarine (Mutchero) progradation. Preservation of such cycles in the stratigraphic record would produce a series of vertically stacked and offset linear barrier sands surrounded by lagoonal mud and fine grained shoreface sediment juxtaposed to muddy, estuarine infills. Sea level elevations are well recorded by the upward transition from Rhizophora-dominated intertidal mangrove mud to freshwater swamp (clearly identified by pollen analysis) and by the basal contacts of beach barrier sediments which sharply overlie the upper shoreface. Transgressive sedimentation is interrupted in both embayments by a constructional beach barrier (Wyvuri) and abbreviated progradation (Mutchero) corresponding to a — 5 m pause in relative sea level rise at c. 6800 yr bp . Sea level control of fine scale coastal sedimentation patterns is beginning to be widely recognized and provides an accurate analogue for stacked ancient sequences.     

大兴安岭中生代两类流纹岩与玄武岩的成因联系

根据地质产状和地球化学特征大兴安岭中生代流纹岩类可划分为高Ti流纹岩和低Ti流纹岩。高Ti流纹岩类与亚碱性系列玄武岩类紧密伴生 ,两者在地球化学上构成连续变异系列。低Ti流纹岩类则与碱性系列玄武岩类构成地球化学双峰态。岩相学和地球化学研究表明 ,这两类流纹岩与该区同期玄武岩类有着密切的成因联系 :起源于地幔柱亏损成分的亚碱性系列玄武岩浆经过单斜辉石、斜长石、磷灰石、锆石的分离结晶 ,形成亚碱性系列低钾玄武岩→高钾玄武岩→高Ti流纹岩演化系列 ;起源于地幔柱富集成分的碱性系列玄武岩浆侵入下地壳 ,使下地壳岩石发生部分熔融 ,形成碱性系列玄武岩 -低Ti流纹岩双峰态组合     

沙尘分析及等离子空气清洁系统沙尘去除效果的评价

本研究对在中国和韩国采集的沙尘的粒径分布及质量浓度进行了测定,使用黄沙标准物质对基于静电沉降原理的等离子空气清洁系统的去除沙尘效率进行了评价。本研究所使用的黄沙采集于北京和汉城,其粒径中值在7.0~80μm之间,浓度范围为300~1462μg·m^-3。在单通道测试中,沙尘去除效果随粒径增大和流速降低而增加。系统速度为1.0m·s^-1时,去除率大于80%。在27m2的房间中,进行多通道测试时,浓度为300μg·m^-3的大气颗粒物仅用10min即可降至150μg·m^-3。因此,等离子空气清洁系统能够有效地去除沙尘及保持室内空气质量。