The Biot-Gassmann theory of poroelasticity forms the basis of most in­vestigations of wave propagation in fluid saturated rock. In recent years the need to incorporate the concept of squirt flow into the theoretical framework has been recognised. Microstructural models which contain squirt flow give inconsistent predictions which contradict rigorous results from poroelastic theory. I derive a microstructural poroelastic model which incorporates the squirt flow mechanism. The model is consistent in its limiting forms with the stan­dard results of poroelasticity and effective medium theory. An important feature of the model is that it is relatively independent of assumptions about the aspect ratio spectrum. I describe how the various parameters which occur in my model may be derived or estimated from experimental data, and proceed to a preliminary calibration of the model using published resonant bar data. Although I show that the data can be fit satisfactorily, significant ambiguity remains in the interpretation of the results. A number of ultrasonic tests of P- and S- velocity, in rock similar to the resonant bar, as a function of both effective stress and pore fluid type show results which are at variance with the predictions of published poroelastic theories. I demonstrate that the anomaly can be explained with reference to physical effects predicted by my model. Moreover, the requirement to explain the ultrasonic results places constraints upon the modelling of the resonant bar data, removing much of the ambiguity from the analysis. I present a calibration which gives a consistent qualitative explanation of both the resonant bar and ultrasonic data. The calibrated model makes a number of predictions concerning the ef­fect of changing pore fluid viscosity, sample permeability and frequency. In principle experiments could be carried out to test these predictions.

No description available.

Field work was carried out to record the layering phenomena exhibited by the two syenitic bodies. In both cases the layering developed close to the edge of the intrusion. The rhythmic layers are c.20cm thick and have a thin, c.3cm, melanocratic base. Over a short distance this grades into leucocratic syenite which has the same modal composition as homogeneous syenite above and below the layered series. Cumulus phase are pyroxene, olivine, feldspar, apatite and opaque oxides and sulphides. The occurrence of the Nunarssuit layered syenite is cyclic. In each cycle the bases of layers become more melanocratic up section. Each cycle is capped by a thick melanocratic layer. Two fully developed cycles are present. A third cycle starts to develop but then layering fades and the syenite becomes essentially homogeneous. Troughs, rich in mafic phases, and unconformities between layers are present in both layered series. Slumps and slump breccias are present in Nunarssuit. Samples from across individual layers, groups of adjacent layers, distantly spaced layers, slump structures and breccias were collected. Electron probe, synchrotron XRF-microprobe and XRF whole-rock analysis were used to determine mineral and rock chemistry. δ<SUP>18</SUP>O values were determined for Nunarssuit samples. Grain size analysis, cathodoluminescence studies, and SEM and electron probe BSE image analysis were used for textural analysis. The composition of the magmas from which the syenites crystallised was estimated and its physical and chemical properties determined. The samples from the Nunarssuit layered syenite become more magnesian up section. In West Kûngnât the samples become more ferroan up section. In both intrusions samples from trough layers are more magnesian than samples from adjacent syenite. Fluid dynamic calculations indicate that the crystallising magma convected turbulently. Crystals would have been kept in suspension in the body of the magma but may have settled in stagnant boundary layers along the bottom and sides of the chamber. Non-linear analysis indicates that the layer producing mechanism was chaotic. Textures and mineral chemistry were modified after crystallisation due to interaction with fluids. At high temperatures, below the solidus, small grains of olivine and pyroxene were absorbed by larger grains of the same mineral species. At temperatures as low as c.450°C fluids reacted with the syenites and: 1) pyroxenes were partially altered to amphiboles and pyroxene rims were enriched in the acmite component, 2) olivines were altered to biotites, 3) feldspars exsolved and, 4) apatites and zircons lost some or all of their original zoning.

No description available.

This thesis presents a detailed petrographic and geochemical study of several Dalradian metacarbonate and calc-silicate rock units from Scotland and Northern Ireland, which were metamorphosed at greenschist to middle amphibolite facies conditions during the Ordovician Grampian Orogeny. The study aimed: a) to determine the extent to which metamorphic fluid-rock interaction has modified original sedimentary/diagenetic chemical signatures, b) to relate original signatures to the pre-metamorphic history of the carbonate rocks, and c) to elucidate and quantify the nature and mechanisms of metamorphic fluid-rock interaction. Conventional bulk carbonate d¹⁸O, d¹³C and ⁸⁷Sr/⁸⁶Sr ratios were determined at a range of sample scales, including whole-rock samples from outcrops, small-sale samples from within hand specimens, and mini-core samples from a drilled profile. Oxygen isotope and trace element compositions of two metalimestones were studied on grain and subgrain sales by Secondary Ion Mass Spectrometry (SIMS), aided by cathodoluminescence imaging. This work elucidated fluid infiltration pathways and mechanisms, the extent of altered domains, timescales and mechanisms of isotope exchange, and the true scale of isotope equilibration. The work also aided interpretation of the bulk carbonate data. Petrographical and microchemical analysis of calc-silicate rocks from Glen Rinnes, NE Grampian Highlands, indicate varied and complex buffering of the fluid phase. The lack of diopside, the presence of plagioclase + calcite and zoisite breakdown textures indicate local buffering of the fluid phase to more XCO₂-rich compositions along unvariant equilibria and to invariant points. This implies local buffering of fluid composition by the rock, mitigating against infiltration of very water-rich fluids. Stable and strontium isotope data show that pervasive metamorphic fluid infiltration of the limestones did not commonly occur and primary/near-primary d¹³C and ⁸⁷Sr/⁸⁶Sr ratios have been widely preserved. ⁸⁷Sr/⁸⁶Sr ratios of metadolostones are markedly different to those in metalimestones and probably relate to dolomitisation.

Detailed investigations of the petrology and geochemistry of the Jagersfontein mantle peridotite xenolith suite have yielded new refinements to models of sub-cratonic mantle petrogenesis. The Jagersfontein peridotitic xenoliths show three distinct xenolith types as recognised by Winterburn (1987): a coarse low-temperature, a coarse medium-temperature, and a deformed high-temperature suite geothermobarometry, using the orthopyroxene-clinopyroxene solvus thermometer, and aluminium in orthopyroxene in equilibrium with garnet barometer of Brey and Kohler (1990), has been used to refine the depth ranges of these suites, and yields a geotherm lacking an inflection at high temperatures. The general major-minor-trace element geochemistry of garnets shows variations related to depth, whilst complex heterogeneities in the composition of single garnets yield evidence of changing geochemistry with time. By combining these types of information a model of geochemical evolution, involving melt metasomatism, is developed in both space and time for the sub-cratonic mantle. Garnets from coarse low-temperature xenoliths are G9 (1herzolitic) and plot is confined region near the base (low Ca-Cr) of the G9 trend on the Cr<SUB>2</SUB>O<SUB>3</SUB>-CaO diagram. Garnets from coarse-medium temperature xenoliths are G10 (harzburgitic) and plot in variable Ca and Cr to the low-Ca side of the G9 trend Garnets from deformed xenoliths are also from 1herzolite assemblages and define the G9 trend, with the garnets from the lowest pressure deformed samples plotting at the high Ca-Cr end of the G9 trend. With increasing depth deformed xenolith garnets plot at progressively lower Ca-Cr, with the deepest samples plotting at the base of the trend. Some garnets in the deformed, and coarse medium-temperature suites possess preserved major and/or trace element heterogeneities formed by metasomatism which diffusion constraints indicate to have occurred within a few million years prior to kimberlite eruption. Geometries of these heterogeneities give clear evidence that peridotite containing these garnets was infiltrated by a melt which caused growth of new garnet, as well as cracking/annealing, and chemical exchange.

No description available.

The island of Reunion lies in the western Indian Ocean, 700 km. east of Madagascar and approximately 1200 km. west of the mid-Indian Ocean Ridge. With the islands of Mauritius 170 km. R. and Rodriguez 800 km. S., it forms the geographic group known as the Mascarene Islands. Reunion appears to have developed away from the mid-Indian Ocean Ridge and independently from the other Mascarene Islands and the Mascarene Plateau to the north-east. The island is built of two volcanoes; Piton des Neiges (3,069 m.) an extinct volcano forms the north-western two-thirds of the island, while the active volcano of Piton de la Fournaise (2,631 m.) occupies the south-eastern part. Deep erosion on Piton des Neiges has exposed an older series of faulted, altered rocks termed the Older Oceanite Series, unconformably underlying mainly fresh, gently dipping lavas of the Younger Oceanite Series. Both series consist of predominantly olivine basalts and constitute the main shield-building phase of Piton des Neiges, which is overlain by a capping of more varied products ranging from basalt to trachyte, constituting the differentiated series. The Older Oceanite Series extrusive rocks, consisting of breccias and subordinate 'lobate' lavas, were erupted into a shallow submarine environment at depths of less than 500 m. However, much of the magma was intruded at shallow levels as parallel sheet swarms and dyke networks from fissures trending in a mid - S to N.N.L. - S.S.W. direction. Following and partly contemporaneous with this activity, there was rapid uplift of at least 1900 m. in the late Pliocene/Pleistocene, associated with vertical movement along rejuvenated, N.S. - S.W. trending, late Cretaceous transform fault lines, passing north and south of Reunion. Rapid uplift was succeeded by deposition of the Younger Oceanite Series malta on to faulted platforms of the older rocks. In spite of their different environments of eruption, the Older and Younger unite Series are believed to have been originally virtually indistinguishable in their range of compositions and mineralogy and can thus be treated as a whole in any petrogenetic model. Studies of trapped silicate melt inclusions in olivine reveal that primititmas ancestral to the Oceanite Series were probably picritic liquids. Their K, 2-rich character could be explained by previous high-pressure crystal fractionation e.g. eclogite, or more probably by small amounts of partl-:1 meltin E 3: mantle material containing minor amounts of a o, Ti-rich phase, possibly biogopite. The picritic, primitive magmas accumulated at depths of circa 15 km. and nuer, in a system of small magma chambers and ramifying channels. Crystallise-ti rid fractionation of olivine with small amounts of chromite, immiscible Ni-Fe nlphide, clinopyroxene and plagioclase formed on eruption, a suite of transitior boleiitic olivine basalts, containing associated dunite and occasional wehrlite and ga bbro inclusions. A limited development of hydrous conditions in the subvolcanic zone led to crystallisation of amphibole gabbros, which were subsequently triated as inclusions in basalts during explosive eruption. Periodically, small quantities of main migrated into the volcanic superstructure and differentiated to form relatively evolved gabbroic bodies. Eventually, the main magma supply was diverted south-east from Piton des Neiges to Piton de la Fournaise. A large body of magma has trapped about 50 - 1' below the surface of Piton des Neiges and underwent olivine-clinopyroxene fractition to produce a magma parental to the Differentiated Series eruptives. During this period of quiescence, the Older Oceanite Series and nasal part S the lounger Oceanite Series near to the volcanic centre were metamorphosed in the zeolite and lower greenschist faciee. Zones of zeolitisation were developing around the centre of the volcano. The distribution of these was dependent on the burial proximity to volcanic conduits and intrusive centres and the circulating friction of hot fluids. The onset of metamorphism may have been linked with the establishment of a large magma chamber below Piton des Neiges.

No description available.

No description available.