Showing papers by "Open University published in 1981"

The Theory of Commodity Price Stabilization: A Study in the Economics of Risk

Abstract: This book is an important contribution to the debate on the new international economic order. The theory of commodity price stabilization is a subject of continuing and active policy concern to the developed and less developed countries.

The Oman Ophiolite as a Cretaceous Arc-Basin Complex: Evidence and Implications

Abstract: Geological and geochemical evidence suggest that the Oman ophiolite is a fragment of a submarine arc-basin complex formed above a short-lived subduction zone in the mid-Cretaceous. Detailed studies of the lava stratigraphy and the intrusive relationships of dykes, sills and high-level plutons provide further evidence for the magmatic and tectonic development of the complex in question. Four consecutive events can be recognized to have taken place before emplacement: (1) eruption of basalts of island arc affinity onto pre-existing (Triassic) oceanic crust; (2) creation of new oceanic crust by back-arc spreading; (3) intrusion of magma into this back-arc oceanic crust accompanied by eruption of basalts and andesites from discrete volcanic centres; (4) further intrusion of magma accompanied by uplift and eruption of basalts and rhyolites in submarine graben. A combined structural and geochemical analysis of the dyke swarm indicates that extension took place in approximately a N-S (ridge) and an ESE-WNW (leaky transform) direction relative to an inferred direction of subduction to the NE, and that a small but significant proportion of the sheeted dykes were injected during the 'arc' rather than the earlier 'back-arc spreading' episode. These various observations can be explained in terms of the progressive response of a non-isotropic lithosphere to the stresses induced during subduction.

Petrogenetic modelling of in situ fractional crystallization in the zoned Loch Doon pluton, Scotland

Abstract: The late Caledonian Loch Doon granitic intrusion ranges in composition from hypersthene diorite at the margin, through quartz diorite, granodiorite and granite to cordierite microgranite at its core. Petrogenetic modelling of trace element variations and least squares analysis of major elements indicate that two distinct magmas are involved, each magma controlled by crystallization of plag-opx-cpx-bio. Late stage rocks related to the second magma include the cordierite microgranites and aplites, which are interpreted as the final residue which crystallized rapidly after a build up and loss of volatiles. Analyses of whole rocks and minerals for REE's and other elements of moderate-high ionic potential indicate that these elements are strongly controlled by minor phase crystallization; apatite, zircon, sphene and allanite are dominant at intermediate compositions but other accessory minerals such as monazite and xenotime may also become important at acid compositions. It is probable that within each magma the mechanisms of crystal settling and filter pressing operated, the former being initially dominant, and the latter becoming more important with increasing degree of fractional crystallization.

The Construction of Shapes

Abstract: An algorithm for shape rule application is presented.A shape rule α → β applies to a labelled shape γ whenever there is a transformation τ that makes α a subshape of γ. In this case, a new labelled shape can be obtained by replacing the occurrence of τ(α) in 7 with τ(β). The algorithm required for this process is developed in this paper. This algorithm determines all possible distinct transformations under which a given shape rule applies to a given labelled shape and the corresponding labelled shapes resulting from such applications. The definitions and notations given for labelled shapes and shape grammars by Stiny (1980) are used.

Self-esteem and delinquency revisited

Abstract: A recent investigation by Rosenberg and Rosenberg used longitudinal data from the Youth in Transition study to explore the causal relationships between delinquency and self-esteem. The present study is based on the same sample of young men in high school and extends Rosenberg and Rosenberg's analysis, first by using the same cross-lagged correlation methods applied over a longer time period, and then by employing a “causal modeling” approach using the LISREL computer program. Each of the analyses was carried out using the total sample as well as two subsamples, the highest and lowest quartiles in initial self-esteem. The causal modeling analyses attempted (a) to take careful account of the actual periods referenced by the measures of delinquency and self-esteem, (b) to control socioeconomic status and ability, and (c) to extend the model to demonstrate ways in which participation in teenage social life and current educational attainment might also influence and be influenced by self-esteem. The analyses suggest that self-esteem plays little part in influencing the teenage behaviors and orientations that follow in time. Consistent with Kaplan's prediction, among young men who enter high school with low self-esteem, the effects of delinquent behavior tend primarily to be self-enhancing.

A Unified Approach to Design of Experiments

Abstract: SUMMARY The four main aspects of design-model, randomization, treatment allocation, analysisare discussed in a unified framework which shows their interdependence. Some common assumptions are examined. The approach presented is applicable to a wide variety of situations in design, and it is pointed out how the approach may be modified to deal with other situations.

Old model Nd ages in Namibian Pan-African rocks

Abstract: The Nd-isotope analytical technique is a powerful tool for studying many geological processes: particularly the evolution of the upper mantle as seen through the isotope and trace element geochemistry of mantle-derived volcanic rocks1–3, and magmatic processes along destructive plate margins4,5. However, previous studies of continental crust6–8 have been largely restricted to Archaean examples and concerned primarily with dating their time of derivation from the upper mantle. In the present study we investigated the Nd- and Sr-isotope characteristics of rocks involved in a relatively young (800–450 Myr) orogenic event. Such events represent critical stages in the evolution of most of the Earth's continental crust—new material is added from the mantle, and pre-existing crust is remobilized by erosion and sedimentation, deformation and magmatic activity. Nd- and Sr-isotopes were used to outline the range in age and geochemical characteristics of rock sequences and provinces within both the upper mantle and the pre-existing crust which were sampled during the orogeny, and hence provided the major components of what is now a stable segment of continental crust.

Parasites and the fossil record

Abstract: Parasitologists are fond of alluding to the possible antiquity of their chosen group. They may even place the origin in a specific period of geological time and trace its co-evolution, or lack thereof, with the hosts. These speculations usually make only passing comments on the actual fossil record, but a critical examination suggests that this area can throw much light on both the origins and evolution of parasitism. This review is concerned primarily with the fossil record and its immediate implications. It does not appraise parasite evolution in the light of evidence from sources, often of excellent quality, such as comparative anatomy and physiology or ecology. As some parasitologists may not be entirely familiar with the geological column and time scale, Table 1 depicts these and indicates some records of parasitism mentioned here, together with the major events in this history of life.

Plagiogranites from the ocean crust and ophiolites

Abstract: Rock types similar to those found in ophiolites have been recovered from the floors of ocean basins1–3. This is critical evidence for the theory that ophiolites represent the ocean crust4,5. Aumento6 has described diorites, quartz diorites and trondhjemites dredged from the Mid-Atlantic Ridge (MAR) near 45° N, and noted that these were the first rocks analogous to the low-K acid differentiates in ophiolites (collectively referred to as ‘oceanic plagiogranites’7,8) to be recognized amongst those recovered from the modern ocean floor. Only a few plagiogranites from the ocean floor (ocean-crust plagiogranites) have since been described; these were dredged from widely spaced localities on the MAR9–11, the Amani Plateau and the Kyushu-Palau Ridge (north-west Pacific)12. However, the Kyushu-Palau Ridge could be the product of destructive plate-margin magmatism13, as are the potassic granites from the Aves Ridge (Caribbean)14. Plagiogranite xenoliths also occur in extrusives on Surtsey and Askja (Iceland)15. That only some of the petrological compositions of ophiolite plagiogranites are represented in the ocean crust is not surprising in view of the small sample of ocean-crust plagiogranites. It is less obvious, however, if ophiolites represent the ocean crust, why certain acidic rock types from the oceans have not been found in ophiolites. I show here that the difference between the two groups of plagiogranites is evidence that ophiolites do not represent all kinds of ocean crust and that a greater range of magma compositions and/or conditions of magmatic differentiation is represented by the ocean crust.

Andean andesites and crustal growth

Abstract: Over the last 200 Ma, the ensialic Andean plate margin has been characterized by calc-alkaline magmatism. The early (Mesozoic), activity was dominantly of basaltic volcanism while the Cainozoic volcanism was of intermediate, calc-alkaline character. The restriction of Recent volcanism to parts of the Andes underlain by thick wedges of asthenospheric mantle, and the Sr and Nd isotopic relations, indicate that the calc-alkaline parental magmas are derived from the asthenospheric mantle. There is no unequivocal geochemical and geophysical evidence that continental crust or sediment has contributed to the mantle source for Andean magmatism. The chemical compositions of the calc-alkaline volcanic rocks of the active volcanic zones are controlled by fractional crystallization, whereas O-Sr isotopic relations reflect crustal interaction of mantle-derived parental magma with the sialic basement of the Andes. The variable extent of fractional crystallization, partial melting, and mixing of crustal contaminant are related to the variable thickness and age of crust in the different volcanic provinces. Calc-alkaline magmatism was largely responsible for post-Mesozoic crustal growth in the Andes and would have depleted the underlying mantle unless balanced by circulation within the asthenospheric mantle wedge. In terms of net growth of the South American continent, it is not certain where the balance lies between growth by magmatic addition and shrinking by erosion.

The application of spinel-bearing metapelites to P/T determinations: An example from South India

Abstract: Metasedimentary migmatites from the Archean charnockitic terrain of South India contain the five phase equilibrium assemblage spinel-cordierite-garnet-corundum-sillimanite. The assemblages is a result of anatexis which has generated a silica-deficient anhydrous restite. Peak metamorphic conditions are defined by the intersection of two divariant reactions in the A12O3-SiO2-FeO-MgO system at which the five phases coexist. These reactions are univariant and their intersection invariant if the Fe/Mg ratio of at least one femic phase is fixed.

A Nonequilibrium Entropy for Dynamical Systems

Abstract: It is proposed to define entropy for nonequilibrium ensembles using a method of coarse graining which partitions phase space into sets which typically have zero measure. These are chosen by considering the totality of future possibilities for observation on the system. It is shown that this entropy is necessarily a nondecreasing function of the timet. There is no contradiction with the reversibility of the laws of motion because this method of coarse graining is asymmetric under time reversal. Under suitable conditions (which are stated explicitly) this entropy approaches the equilibrium entropy ast→+∞ and the fine-grained entropy ast→−∞. In particular, the conditions can always be satisfied if the system is aK-system, as in the Sinai billiard models. Some theorems are given which give information about whether it is possible to generate the partition used here for coarse graining from time translates of a finite partition, and at the same time elucidate the connection between our concept of entropy and the entropy invariant of Kolmogorov and Sinai.

Space and Time in Granite Plutonism

Abstract: Granitic suites formed in the last 2500 Ma are either calc-alkaline or, less commonly, alkalic. The space-time trends of granitoid rocks from modern compressional arcs include variations towards more silicic and potassic products. But even the most ‘mature’ magmas of these arcs have Fe/Mg ratios near to 1, in contrast with the strongly iron-enriched alkalic suites that characterize zones of crustal extension. Both calc-alkaline and alkalic suites are mainly mantle-derived; they evolved, respectively, from tholeiitic and alkali basalt parental magmas which have been subject, in most examples, to a two-stage fractionation history. Other factors that may influence the evolution of these parental magmas include, first, the addition of variable, but minor, amounts of crustal melt and, secondly, the persistence of refractory mineral phases in the zone of melting. Contributions from subducted continental detritus are thought to be of minor significance. A long-term trend, over the entire history of the crust from more sodic to more potassic calc-alkaline magmatism may reflect the declining importance of subducted ocean lithosphere in magma generation and the increasing role of the overlying volatile-fluxed mantle wedge. This trend and, in particular, the widespread development of alkalic granite suites in mid-Proterozoic times, may be linked to the declining vigour of tectonic and associated magmatic processes during the Earth’s history. These alkalic suites may indicate a period when the continental lithosphere had become stable enough to resist fragmentation, leading to intracontinental ‘rift and swell’ zones of magmatism. In some cases, new plate cycles, with more normal calcalkaline igneous rocks, seem to have occurred in the same zones. Although much of the evidence favours the operation of plate tectonic and subduction processes in magma generation during Archaean and early Proterozoic times, this link cannot be taken as proved except for the last ca . 1000 Ma.