Reconstruction and visualisation of melt topology in veined microdioritic enclaves

A number of xenocrystic microdioritic enclaves within the Ross of Mull Granite contain narrow veins of monzonitic material that have formed through the selective partial fusion of granitic magma mechanically incorporated into the microdiorite magma. Field evidence suggests that, as melting of the granitic material ensued, a monzonitic melt formed and exfiltrated from the surrounding microdioritic matrix resulting in Saffman–Taylor instabilities and the development of viscous fingering phenomena. The monzonitic melt network is now preserved as monzonitic veins. Mechanical serial sectioning through a representative veined microdioritic enclave shows that: (a) the topology of the veins is complex but crudely polyhedral, (b) there is porosity (monzonitic melt fraction) variation throughout the length of the veined enclave and (c) that the monzonitic veins are interconnected in three dimensions. Using image analysis software, the total porosity of individual veined enclave sections has been estimated, along with the variation in porosity with depth. Three dimensional rendering and animation software has been used to reconstruct and visualise the monzonitic melt network using Apple QuickTime™ software. The above method provides a cheap and powerful way of visualising remnant macroscopic transport properties (such as connectivity and melt topology) in partially molten igneous rocks.Electronic supplementary material Electronic supplementary material is available for this article at and accessible for authorised users.     

The simulation of fabric development during plastic deformation and its application to quartzite: the influence of deformation history

The effect of deformation history on the development of crystallographic preferred orientation in quartzities has been simulated using a computer program based on the Taylor-Bishop-Hill analysis. Model quartzities with different combinations of glide systems have been subjected to various coaxial and non-coaxial deformation histories. It is possible to obtain information from the fabrics that develop during simple histories; for example, the location of the axis of extension is generally associated with a pole free area on a c-axis plot, and progressive axial shortening, plane strain and axial shortening produce characteristic fabrics. In progressive simple shear the fabric skeleton becomes asymmetric relative to the sense of shear and a-axes preferentially align in the flow plane parallel to the flow direction. However, this example illustrates that the fabric orientation and characteristics are controlled by the kinematic framework and bear only an indirect relationship to the finite strain accumulated to that point in the history.The imprint of the closing stages of deformation limits to some degree the use of crystallographic fabrics as a tool for structural geologists, but in favourable circumstances data can be obtained concerning characteristics of the deformation history, on the scale of the hand-specimen, for the last part of this history.     

Vorticity and non-coaxiality in progressive deformations

A measure of the non-coaxiality involved in progressive deformation histories is proposed in the form of the kinematical vorticity number, W_k. This number is a measure of the relative effects of rotation of material lines (relative to the instantaneous stretching axes) and of stretching of these material lines. As such, W_k, is a measure of the instantaneous degree of non-coaxiality. A detailed example is first presented in the form of a progressive simple shearing in which the shear plane rotates relative to an external coodinate system. This is followed by examples of more complicated deformation histories. Three specific types of progressive, isochoric (constant volume) deformation histories are recognized. Those for which 0 ≤ W_k < 1 correspond to deformation histories where no line that has been extended is shortened in future increments; W_k > 0 is a special case of these corresponding to a coaxial history. Histories with W_k > 0 are non-coaxial. Those histories with W_k = 1 correspond to progressive simple shearing. Those histories with 1 < W_k < ∞ are pulsating and lines that have been extended may be shortened in future increments.     

Metamorphic olivine in ultramafic rocks from Western Australia

Textural relationships plus associated mineralogy are compatible with a metamorphic origin for some olivines in altered ultramafics. Upper amphibolite facies PT conditions are indicated.     

Some problems in digital heat-flow measurement

The most accurate method of digitizing a resistance requires that a bridge be brought to null balance by means of digital steps. Satisfactory circuits of reasonable complexity require irregular electrical dissipation in the sensors, and a relatively wide frequency bandwidth in the detector. This combination places considerably greater demands on sensor quality than do conventional electromechanical recorders, and this has been strikingly confirmed during the initial use at sea of a newly developed digital resistance recorder with a sensitivity of 10^?5. Mounted on a coring tool, with miniature outrigger sensors to measure geothermal gradients in the pelagic sediment, the recorder produces sensor readings with considerable scatter even when the sensor probes are buried in sediment. These contrast sharply to the steady reading of the calibration arm. An additional symptom with some sensors is the shunning of certain numbers in the low decades of the digital readings. The reading scatter varies sharply among different sensor types and can be satisfactorily explained by the production of a high level of white noise when the sensors are biased with excitation current. No adequate explanation has been found for the preferential reading of certain numbers, and it is necessary to speculate about obscure contact effects in those sensors.     

Studies of oceanic tectonics utilizing detailed surveying and stratigraphy

The sea floor off the coasts of Washington and southern British Columbia consists of two major features: the Cascadia Basin and the chain of seamounts to the west of it, called the Cobb Rise. The Basin is magnetically featureless and appears to be a syncline deeply filled by sediment. Clues to the tectonic history of the region are therefore more likely to be found on and beyond the Cobb Rise, which forms the western boundary of the Basin and interrupts the westward flow of terrigenous sediment. A program of detailed topographic and magnetic surveving, complemented by heat-flow coring and seismic profiling, is underway to attempt to determine the structure and history of the abyssal hills on the seaward side of Cobb Rise. Knowledge of the stratigraphy and the topography of the basement-sediment interface is expected to reveal the approximate age of the sea floor in this area and possibly the nature of the basement itself. The magnetic and geothermal data may provide direct evidence of the subcrustal structure associated with Cobb Rise, particularly if the transition zone between the high heat flow on the rise and the low heat flow west of it can be studied in detail. A small seamount has been discovered at 47°N, 132°W and has been intensively surveyed as the nucleus of the future survey area. It rises 630 meters above the surrounding bumpy plain and has extremely steep faces to the south and east. Heat flow near the seamount is somewhat below the oceanic average and the several long gravity cores obtained in the area show some correlable layering in the pelagic ooze.     

Mg isotope heterogeneity in the Allende meteorite measured by UV laser ablation-MC-ICPMS and comparisons with O isotopes

First results from a new UV laser ablation MC-ICPMS method for measuring Mg isotope ratios in situ in meteoritical materials show that there are mass-dependent variations in δ²⁵Mg and δ²⁶Mg up to 1.5 ‰ per amu in chondrules and 0.3‰ per amu in a CAI from the Allende meteorite. In both cases the mass-dependent fractionation is associated with alteration. Comparisons with laser ablation O isotope data indicate that incorporation of pre-existing grains of forsterite with distinct Mg and O isotopic compositions and post-formation alteration both contributed to the variability in Mg isotope ratios in the chondrules, resulting in a correlation between high δ²⁵Mg and low Δ¹⁷O. The laser ablation analyses of the CAI show that high-precision determinations of both δ²⁵Mg and δ²⁶Mg can be used to discriminate features of the ²⁶Al-²⁶Mg isotope system that are relevant to chronology from those that result from element mobility.     

The effect of internal gas pressurization on volcanic edifice stability: evolution towards a critical state

Results from simple physical and numerical models investigating the effects of increased internal pore‐fluid pressures of a Mohr–Coulomb volcanic edifice are presented. Physical experiments make use of a heap built from angular sand on top of a stiff substrate of variable angle, with the provision for injection of internal fluid (gas) pressures into the base. The resulting failure geometries arising from internal pressurization of the model appear similar to some natural examples of sector collapse. Two‐dimensional limit equilibrium models analysing 42 500 possible failure surfaces were run with internal pressures (P₀) in the range 5–35 MPa, and show that the potential critical failure surface migrates to increasingly deeper levels with increasing internal pressure. Although internal pressurization alone is unlikely to reduce the factor of safety (F_S) below unity, the edifice is driven towards a state of criticality that will render in susceptible to any internal or external perturbations.     

Embedding the reforms in New Zealand schooling: After neo-liberalism?

Two decades of reforms to the state in New Zealand have altered policy, its making and the ways in which it is realised in micro settings. This paper uses the example of schooling to examine the rationality of these reforms, their spatial logic and what they mean for our understanding of the national state. It examines the development and practices of the Education Review Office (ERO), the body established under the reforms to evaluate and audit the performance of schools in the new national education `system'. The paper interprets neo-liberalism as a governmentality, and argues that the development of new managerial technologies of remote control such as contract and audit constitute a spatial model of control. The paper suggests that this model encourages, and relies for its efficacy upon, the cultivation of neo-liberal subjectivities. It argues that although the political projects working through the reforms have shifted, the altered rationality of the state and the models of control erected to secure it define an enduring and neo-liberalising social transformation. The shift to the `Third Way' in New Zealand's political and social economy is underpinned by neo-liberalising processes, which continue to reorganise social and economic spaces. This revised version was published online in July 2006 with corrections to the Cover Date.