The zircon-thorite mineral group in metasomatized granite,Ririwai, Nigeria 1. Geochemistry and metastable solid solution of thorite and coffinite

Summary At Ririwai, Nigeria, a biotite granite has suffered extensive post-magmatic metasomatism to produce albitized, microclinized, and greisenized rocks; the latter two lithologies form the wallrocks adjacent to tin-zinc vein-style mineralization. All the rock-types carry accessory amounts of thorite, locally accompanied by trace amounts of paragenetically late coffinite and xenotime. Petrographical and electron-microprobe data are presented for these minerals. In the biotite granite, thorite and coffinite range to high Zr contents (25.1 wt% ZrO₂ in thorite and 11.7 wt% in coffinite), exceeding the highest values previously reported in both minerals. In the lode wallrocks, yttrium contents range up to 13.9 wt% Y₂O₃ in thorite and 15.9 wt% in coffinite. From stoichiometric considerations, both minerals are inferred to contain OH. Petrographically, the thorites of the lode wallrocks are distinctive, and they are interpreted as having grown from the metasomatizing fluids. The albitized rocks and some of the lode wallrocks contain the thorites with the highest U contents, including one example of continuous zoning to coffinite. The unusual mineral compositions are interpreted as metastable solid-solution ranges formed from fluids that transported U, Th, Y and, over short distances, Zr.

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Die Zirkon-Thorit Mineral Gruppe in metasomatisiertem Granit, Ririwai, Nigeria. 1. Geochemie und metastable feste Lösungen des Thorit und Coffinit

Zusammenfassung In Ririwai, Nigeria, wurde ein Biotit-Granit durch extensive Metasomatose in albitisierte, mikroklinisierte und vergreisete Gesteine umgewandelt. In Mikroklin-Gesteinen und Greisen sitzt eine Zinn-Zink-Gangvererzung! Sämtliche Gesteinstypen führen Thorit als akzessorischen Gemengteil, stellenweise mit Spuren von spätgebildetem Coffinit und Xenotim. Petrographische und Mikrosonden-Daten dieser Minerale werden präsentiert. Im Biotit-Granit erreichten die Zr-Gehalte im Thorit und Coffinit Werte, wie sie bisher noch nicht bekannt waren (25.1 Gew.-% ZrO₂ im Thorit und 11.7 Gew.-% im Coffmit). In den Nebengesteinen reichen die Yttrium-Gehalte im Thorit bis 13.9 Gew.-% Y₂O₃, und im Coffinit bis 15.9 Gew.-%. Aus stöchiometrischen Überlegungen kann vermutet werden, daß beide Minerale OH enthalten. Petrographisch sind die Thorite aus den Nebengesteinen eindeutig definiert, und sie dürften aus metasomatischen Fluida entstanden sein. Die albitisierten Gesteine und auch einige Nebengesteine führen Thorite mit den höchsten U-Gehalten, darunter ein Beispiel mit einem kontinuierlichen Zonarbau bis zum Coffinit. Die ungewöhnlichen Mineralzusammensetzungen werden als metastabile feste Lösungen interpretiert, die aus Fluiden gebildet worden sind, welche U, Th, Y und über kurze Distanzen auch Zr transportiert haben.

     

Plastic ingestion by petrels breeding in Antarctica

Plastic particles were found to be common pollutants in stomachs of Wilson's Storm Petrels and Cape Petrels breeding on the Antarctic continent. Highest incidence of plastics was found in chicks of Wilson's Storm Petrels that had died before fledging. Few or no plastics were found in Snow Petrels and Antarctic Petrels. Evidence suggests that most plastics originate from wintering areas outside the Antarctic, and that relatively few plastics are available in Antarctic waters. Possible hazards of plastic levels observed in Wilson's Storm Petrels are discussed.     

A REVIEW OF PHYSICAL GEOGRAPHY IN SOUTH ASIA

The development of physical geography in South Asia has generally followed the British and European trends. Diverse themes from denudational chronology to palaeoflood geomorphology have attracted the attention of geomorphologists, particularly in the last 25 years. In addition, geographers’ interest in climatology and biogeography became pronounced during the 1980s. Major methodological shifts in the late 1960s in the west have affected geographical studies in South Asia. These shifts include the use of methods of quantification, remote sensing data, and the increased importance of field and laboratory studies. The second notable aspect in recent times is the accelerating interest amongst physical geographers in environmental studies and the applied aspects of the discipline as well as an increased interaction with scientists in cognate fields. An attempt is also made to identify the key research themes that will interest physical geographers in the near future.     

Slope-induced changes in channel character along a gravel-bed stream: The Allt Dubhaig,Scotland

Changes in channel character along a small river in the Scottish Highlands are described using measurements in seven reaches over a 3 km length with no significant tributaries but a decline in slope from 0.02 to 0.00015 because of local baselevel control. This decline in slope is associated with rapid downstream fining of the gravel bed followed by an abrupt transition to a sand bed. The channel pattern alters progressively rather than abruptly, in the sequence (1) near-braided, (2) meandering with active point-bar chutes, (3) meandering with active outer-bank talweg, (4) stable equiwidth sinuous. The changes in channel pattern and hydraulic geometry are predicted better by rational approaches based on critical shear stress or other physical concepts than by purely empirical discriminant or trend equations. Measurements in five reaches confirm a downstream decrease in shear stress and the amount and calibre of bedload. It is argued that the downstream changes in channel character in this stream are induced by profile concavity inherited from deglacial conditions, are typical of many streams in mountainous areas and can be understood in terms of slope-induced changes in hydraulic properties.     

Robustness of variograms and conditioning of kriging matrices

Current ideas of robustness in geostatistics concentrate upon estimation of the experimental variogram. However, predictive algorithms can be very sensitive to small perturbations in data or in the variogram model as well. To quantify this notion of robustness, nearness of variogram models is defined. Closeness of two variogram models is reflected in the sensitivity of their corresponding kriging estimators. The condition number of kriging matrices is shown to play a central role. Various examples are given. The ideas are used to analyze more complex universal kriging systems.Research performed while on leave at Centre de Geóstatistique et de Morphologie Mathématique, Fontainebleau.     

Catastrophic events reveal the dynamic nature of salt-marsh vegetation in Southern California

Recent hydrological disturbances, including flooding, dry-season streamflow, and drought, greatly altered coastal wetland habitats in sourthern California. At Tijuana Estuary, a six-year study of salt-marsh vegetation patterns during these rare conditions documented substantial temporal variability in plant growth and distribution. Important to cordgrass (Spartina foliosa Trin.) dynamics were the amount and timing of streamflows, which reduced soil salinity and alleviated stresses on plant growth. Poorest growing conditions occurred in 1984 when both river and tidal flows were lacking; soils had low moisture and extreme salinities (avg.=104‰ in September). Plant stress was documented in 1984 as high mortality (62% fewer stems than in 1983) and reduced height (19% less than in 1983). Cordgrass height was greatest in 1980 following winter flooding (20% increase over 1979); densities were greatest in 1983 with summer freshwater influxes (60% increase over 1982). A carbon allocation model is proposed to explain the varied responses.     

Changing patterns of population distribution in Zimbabwe

This paper critically evaluates the causes and consequences of changes in population distribution in Zimbabwe during the colonial period and since independence in 1980. Five main aspects of population geography are examined. Firstly, by way on background on contemporary patterns of settlement and land use, the history of tenure policies is outlined. Secondly, the distribution of the African population as revealed in the 1982 census is described and major changes between the census years of 1962, 1969 and 1982 are discussed. Thirdly, changing patterns of settlement and land use within the peasant farming areas (Communal Lands) are examined in the context of increasing population pressures. Fourthly, trends in the growth of the main urban centres and the process of urbanisation of the African population are described. Fifthly, post-independence development policies directed at effecting changes in the distribution of population are discussed with particular reference to the land resettlement programme. The present distribution of population is seen as the product of interacting political, economic and environmental factors, knowledge of which is essential in devising appropriate development strategies to achieve a more equable balance of population and resources.