Archaean tonalitic gneiss of Finnish Lapland revisited: zircon ion-microprobe ages

We report single grain and grain-domain U–Pb zircon ages for the Tojottamanselkä tonalitic gneiss previously investigated by the whole-rock Rb–Sr, Pb–Pb and Sm–Nd methods, by conventional U–Pb zircon density/size fraction analysis and by Hf-isotopes (Kröner et al. 1981; Patchett et al. 1981; Jahn et al. 1984) and established as one of the oldest known rocks of the Baltic shield. Our data confirm the intrusive age as 3115±29 Ma (standard error), but we also found slightly older xenocrystic zircon cores with ²⁰⁷Pb/²⁰⁶Pb ages between 3161±19 and 3248±10 Ma that may either be derived from earlier phases of the tonalite melt or from pre-tonalite sialic crust. New magmatic zircon growth, probably during a metamorphic event that led to migmatization, is recorded by an age of 2836±30 Ma and may be coeval with widespread tonalite emplacement elsewhere in the northern Baltic shield at about this time.     

Titanium isotopic anomalies in hibonites from the Murchison carbonaceous chondrite

The isotopic compositions of titanium in eight grains of hibonite (CaAI₁₂O₁₉) from the carbonaceous chondrite Murchison have been determined by high precision secondary ion mass spectrometry using an ion microprobe. The titanium in the hibonites varies greatly in ⁵⁰Ti, from about ?42 to +8 permil (relative to terrestrial) with smaller (up to 4 permil), but clearly resolvable, effects in ⁴⁶Ti and ⁴⁸Ti. These results complement ion probe measurements by Faheyet al. (1985) of a 100 permil excess of ⁵⁰Ti in a hibonite grain from the carbonaceous chondrite Murray, and confirm the presence of widespread negative anomalies suggested by the results of Hutcheonet al. (1983) on hibonites from Murchison. The magnitude of these variations seems explicable only in terms of nucleogenic processes which produced extremely variable titanium isotopic abundances in the hibonite source materials. The hibonites evidently did not participate to the same extent as most material in the mixing and homogenisation processes that accompanied the formation and later evolution of the solar system. Thus, significant source materials of the hibonites may be the supernova condensates of Clayton (1978) and may support the concept of “chemical memory” (Clayton, 1978; Niemeyer and Lugmair, 1984).     

太行山—五台山区前寒武纪变质岩系同位素地质年代学研究

<正> 中国北方河北、山西两省交界地带的太行—五台山区,早前寒武纪变质岩系十分发育,分布面积达20,000平方公里,为华北地块的重要组成部分。在这一地区内,保留了早前寒武纪不同发展阶段的种种地质作用的记录。深入研究其地质历史,将对阐明华北地块的形成和演化具有重要的典型意义。作者正是基于这一目的,在该区进行了同位素地质年代学的研究。     

The Pan-African continental margin in northeastern Africa: evidence from a geochronological study of granulites at Sabaloka, Sudan

Ion microprobe zircon ages, a Nd model age and RbSr whole-rock dates are reported from the high-grade gneiss terrain at Sabaloka on the River Nile north of Khartoum, formally considered to be part of the Archaean/early Proterozoic Nile craton. The granulites, which are of both sedimentary and igneous derivation, occur as remnants in migmatites. Detrital zircon ages range from ≈ 1000 to ≈ 2650 Ma and prove the existence of Archaean to late Proterozoic continental crust in the sedimentary source region. The Nd model age for one sedimentary granulite is between 1.26 (T_CHUR) and 1.70 (T_DM) Ga and provides a mean crustal residence age for the sedimentary precursor. Igneous zircons in enderbitic gneiss crystallized at 719 ± 81 Ma ago, an age that also corresponds to severe Pb loss in the detrital zircons and which probably reflects the granulite event at Sabaloka. The RbSr data indicate isotopic homogenization at about 700 Ma ago in the granulites and severe post-granulite disturbance at ≈ 570 Ma in the migmatites. We associate this disturbance with hydration, retrograde metamorphism and anatexis that produced undeformed granites ≈ 540 Ma ago. The ≈ 700 Ma granulite event at Sabaloka suggests that this part of the Sudan belongs to the Pan-African Mozambique belt while the ancient Nile craton lay farther west. The gneisses studied here may represent the infrastructure of the ancient African continental margin onto which the juvenile arc assemblage of the Arabian-Nubian shield was accreted during intense horizontal shortening and crustal interstacking of a major collision event.     

Terrestrial,meteoritic, and lunar titanium isotopic ratios revaluated: evidence for correlated variations

Variations in the titanium isotopic ratios of various solar system materials have been reported by three laboratories. In this work we compare these data (including some new Allende inclusion values reported here) on a common basis. The data of all three groups for terrestrial samples have been found to be consistent within about ten parts per ten thousand for all ratios. The considerable disagreement in reported titanium isotopic anomalies for Allende materials has been investigated by renormalization of all published high-precision titanium isotopic ratio data to various bases in order to assess the nature, number, and loci of possible anomalies. On this basis, variations appear to be present in⁴⁶Ti and⁴⁷Ti, as well as in⁵⁰Ti. All materials reported to date can be divided into two main groups, with the only exceptions being about 10% of Allende inclusions. All but one of the materials reported to date are interpretable as mixtures of a material similar to C-1 and varying amounts (up to about 0.5% of⁵⁰Ti) of a material characterized by⁵⁰Ti/⁴⁶Ti?6 and⁴⁶Ti?⁴⁷Ti+⁴⁸Ti+⁴⁹Ti. Inclusion EK 1-4-1 requires a further component (up to about 0.5%⁴⁶Ti) characterized by⁴⁶Ti/⁴⁷Ti? 1.3and⁴⁷Ti?⁴⁸Ti+⁴⁹Ti+⁵⁰Ti.     

Investigation of the elastic characteristics of a three-storey steel structure using system identification

In this report, three different models in increasing order of complexity have been used to identify the seismic behaviour of a three-storey steel structure subjected to arbitrary forcing functions, all of which excite responses within the elastic range. All of the models are constructed using system identification. In the first model, five parameters have been used to identify the frame. Treating the system as a shear building, we assign one stiffness coefficient to each floor and introduce Rayleigh-type damping with two additional parameters. The mass, assumed to be concentrated at a floor level, is kept constant throughout the study. The parameters are established using a modified Gauss-Newton algorithm. The match between measured and predicted quantities is satisfactory when these quantities are restricted to floor accelerations or displacements. To remove the constraint imposed by assuming that the frame deforms as a shear building, a second model with eight parameters is introduced, allowing rotations of the joints as independent degrees of freedom. Six of the eight parameters are related to the stiffness characteristics of the structural members while the remaining two are related to damping as before. In constructing the eight-parameter model, we learned that it is the effective lengths of the members that change during optimization. We also found that the independent response quantities, floor accelerations and joint rotations, must be used in the cost function for the optimization algorithm to converge. The match between measured and predicted quantities for the eight-parameter model is excellent. The set of parameters derived from the minimum squared error gives a model that shows very good correlation using information on the full duration of the pulse or only a portion of it. Also the same correlation exists between the coefficients obtained from different excitations. In an effort to explain the values of the parameters associated with the girders, an additional degree of freedom, namely, the pitching motion of the shaking table, is introduced as an additional degree of freedom. The paper presents, therefore, a five-, an eight- and, finally, a nine-parameter model.     

On the relationship of thunderstorm ice hydrometeor characteristics and total lightning measurements

Satellite-borne and ground-based devices for the detection of lightning offer the opportunity to explore relationships–on all significant scales up to global–between lightning frequency, f, and other thundercloud parameters. Calculations predict that f is proportional to the product of the downflux p of solid precipitation and the upward mass flux, I, of ice crystals. This prediction has received support from limited computational studies. The physical reasons for such a relationship are explained in terms of the paramount role of ice in the electrification of thunderstorms. Herein, this prediction is subjected to further, preliminary examination through analysis of lightning and dual-polarimetric radar data collected during the STERAO experiment conducted in Northern Colorado during the summer of 1996. The analysis has yielded some highly provisional support for this flux hypothesis. Computed trends of radar derived hydrometeor fractions of solid precipitation and small ice show correlation to the total lightning frequency and raise the possibility of determining values of p and/or I from lightning measurements.It is shown that the extent to which the observed correlations between f and both solid precipitation and small ice trends are or are not strong can provide an indication as to whether the lightning activity is limited by the available concentrations of precipitating or non-precipitating ice in the upper regions of the charging zone of the thundercloud, where most of the charge transfer occurs. It is demonstrated that the most accurate determinations of precipitation rate p from measurements of lighting frequency f are likely to be for conditions where the field-growth is limited by the availability of graupel pellets. It is shown that the simultaneous time variations of f and solid precipitation trends of the type obtained in the STERAO experiment could enable us to determine the nature of the dominant glaciation process operative in the thunderclouds studied.     

'They don't like girls hanging around there': conflicts over recreational space in rural Northamptonshire

This paper provides new insights into girls' use of recreational spaces within rural areas. We draw upon data from in-depth discussion work with 10–14 year old girls undertaken in rural Northamptonshire to show how conflict between adults and children, rival groups of children, and boys and girls influences the social ownership of recreational spaces. In contrast to the rural childhood myth, we disclose geographies of anxiety, tension anddisharmony.     

Lithological and spatial controls on the distribution of quartz veins in andesite- and rhyolite-hosted epithermal Au–Ag deposits of the Hauraki Goldfield, New Zealand

Vein distributions in line samples from four epithermal Au–Ag deposits of the Hauraki Goldfield were logged and quantified by vein spacing, vein density, vein thickness and percentage of vein extension. One deposit is hosted in andesite lavas (Martha Hill), one in andesite lavas and dacite porphyry, dacitic tuffs and pyroclastic breccias (Golden Cross), and two in rhyolite lavas and rhyolitic tuffs with minor andesite lavas or andesite dikes (Ohui and Wharekirauponga). The vein systems in these deposits form fault-controlled arrays of extensional veins. Vein spacing distributions are non-fractal over two to three orders of magnitude (1 mm to 5 m), and therefore fractal dimension statistics are not applicable. The coefficient of variation (C_v) of vein spacing was used as a measure of the degree of vein clustering. Rock type has a marked influence on vein spacing distributions, with veining in rhyolite lava having lower average thickness and percentage extension, but a generally higher degree of vein clustering compared with veining in andesite lava in the same deposit. Vein spacing distributions in well-jointed lithologies, mainly andesite lava, have C_v values (0.8–1.2) that are indicative of anticlustered to weakly clustered patterns, particularly in the vein stockwork of the upper part of the Golden Cross deposit. These C_v values are consistent with field observations that joints are a major control on vein spacing. In the poorly jointed dacitic and rhyolitic rocks, the veins are weakly to strongly clustered as shown by higher C_v values (1.2–2.4), and are commonly associated with normal faults. Overall, andesite lava and dacite porphyry and pyroclastics host thicker and more persistent veins than rhyolite lava and tuff. These larger veins contain significant volumes of high-grade gold mineralisation. The higher chemical reactivity to hydrothermal fluids of andesite and dacite compared with rhyolite may have aided propagation and thickening of the veins in andesite-hosted deposits. Within an individual epithermal deposit, location close to thick veins, representing major fluid conduits, commonly overrides the effect of different lithologies. Sites that are deeper and located within or adjacent to major vein structures have higher average vein thickness, percentage extension and degree of vein clustering. Systematic collection and analysis of vein spacing, thickness and density data can be used to define trends that are useful in the exploration of gold-bearing epithermal vein deposits. Received: 25 August 1998 / Accepted: 23 December 1999