Nick Petford

Segregation of tonalitic‐trondhjemitic melts in the continental crust: The mantle connection

Rates of melt segregation into fractures during fluid-absent partial melting above a mafic heat source are investigated numerically over a range of viscosities appropriate for tonalitic-trondhjemitic melts. Static melt segregation is dependent upon large tensile stresses (>10 MPa) set up during fluid-absent partial melting reactions that promote fracturing and vein formation in the region above the mafic intrusion. Porous flow of partial melt into veins and fractures (regions of reduced pressure) will occur provided the melt is less dense than the surrounding protolith matrix. Where vein size is much less than compaction length, maximum vein filling rates are of the order of 2.5 m/yr. Melt segregation into veins and fractures is likely to be most effective near the intrusion-protolith interface, where high temperatures result in relatively low melt viscosities (<106 Pa s) and high porosities (melt fractions). For relatively low viscosity tonalitic melts (∼104 Pa s), extraction may occur at values below the critical melt fraction, provided the melt is everywhere interconnected. Larger veins may propagate upward as dikes to feed high level plutons. These conditions for melt extraction are considered likely in tectonic settings where lower (amphibolitic) crust is being partially melted by underplating of mantle derived basalt. The model does not require the existence of large magma chambers in the source region.

An exploratory qualitative study on perceptions about mosquito bed nets in the Niger Delta: what are the barriers to sustained use?

Background: The effectiveness of malaria control programs is determined by an array of complex factors, including the acceptability and sustained use of preventative measures such as the bed net. A small-scale exploratory study was conducted in several locations in the Niger Delta region, Nigeria, to discover barriers against the use of bed nets, in the context of a current drive to scale up net use in Nigeria. Methods: A qualitative approach with a convenience sample was used. One to one interviews with mostly male adult volunteers were undertaken which explored typical living and sleeping arrangements, and perceptions about and barriers against the use of the mosquito prevention bed net. Results: Several key issues emerged from the qualitative data. Bed nets were not reported as widely used in this small sample. The reasons reported for lack of use included issues of convenience, especially net set up and dismantling; potential hazard and safety concerns; issues related to typical family composition and nature of accommodation; humid weather conditions; and perceptions of cost and effectiveness. Most barriers to net use concerned issues about everyday practical living and sleeping arrangements and perceptions about comfort. Interviewees identified were aware of malaria infection risks, but several also indicated certain beliefs that were barriers to net use. Conclusions: Successful control of malaria and scale up of insecticide-treated net coverage relies on community perceptions and practice. This small study has illuminated a number of important everyday life issues, which remain barriers to sustained net use, and has clarified further questions to be considered in net design and in future research studies. The study highlights the need for further research on the human concerns that contribute to sustained use of nets or, conversely, present significant barriers to their use.

Hydrocarbons in crystalline rocks: an introduction

Oil and gas fields in crystalline basement are discovered mostly by accident, usually when the well operator notices hydrocarbon shows and tests the well. However, as shown in this book, such reservoirs can be very prolific, especially if the basement rock is highly faulted or fractured (the Bach-Ho fractured granite reservoir, Vietnam, produced some 130,000 BQPD). The standard definition of crystalline basement by petroleum geologists is any metamorphic or igneous rock unconforroably overlain by a sedimentary sequence-. However, crystalline rocks need not be metamorphosed, nor significantly older than their sedimentary cover. Perhaps for a more appropriate definition of crystalline basement, we must again look to Landes et at. (1960): ‘the only major difference between basement rock and the overlying sedimentary rock oil deposits is that in the former case the original oil-yielding formation (source rock) cannot underlie the reservoir’. As such, further exploration involving geological, geochemical and geophysical studies may lead to a significant revision of the definition and nature of basement rocks in a particular area, with the possibility of discovering hydrocarbon source rocks located stratigraphically within rocks previously regarded as basement. Examples of where hydrocarbons have migrated into older porous metamorphic or igneous rocks to form a basement reservoir include the volcanic reservoirs of Japan, the oil fields of Mexico and the Maracaibo Basin of Venezuela (see Schutter 2003). Although still often dismissed as exotic curios, this may be a mistake. A case in point (discussed in Koning 2003) is the Suban field, southern Sumatra. Prior to its

A new method of imaging particle tracks in solid state nuclear track detectors.

Solid state nuclear track detectors are used to determine the concentration of α particles in the environment. The standard method for assessing exposed detectors involves 2D image analysis. However 3D imaging has the potential to provide additional information relating to angle as well as to differentiate clustered hit sequences and possibly energy of α particles but this could be time consuming. Here we describe a new method for rapid high‐resolution 3D imaging of solid state nuclear track detectors. A ‘LEXT’ OLS3100 confocal laser scanning microscope (Olympus Corporation, Tokyo, Japan) was used in confocal mode to successfully obtain 3D image data on four CR‐39 plastic detectors. Three‐dimensional visualization and image analysis enabled characterization of track features. This method may provide a means of rapid and detailed 3D analysis of solid state nuclear track detectors.

Controls on primary porosity and permeability development in igneous rocks

Abstract Some of the more important processes leading to the development of primary igneous porosity due to the cooling and crystallization of magma are reviewed. A distinction is made between volcanic and plutonic rocks, and crystalline and granular volcanic material. Porosity in each rock type is classified according to a proposed effective length scale and geometry into diffusive (Class D) and macroscopic flow (Class F) features. Estimated ranges in values of porosity and permeability are given for a wide selection of igneous rock types, and comparison is made with permeability variations (Δk) derived for both the continental and oceanic crust. While fracture porosity is dominant in most crystalline materials, primary porosity development may play an important role in the final (total) porosity in igneous basement. Some types of primary porosity and permeability in igneous rocks will be strongly time- and scale-dependent due to thermal effects associated with the emplacement and cooling of magmas and volcanic material. Tectonic reworking of the primary petrophysical properties of basement-forming igneous rocks may be significant in the development of regions of anisotropy and enhanced porosity.

Three-dimensional visualization of dermal skin structure using confocal laser scanning microscopy

The properties and performance of collagen‐based materials may be affected by the collagen fibre bundle pattern, orientation and weave. The aim of this study was to develop and apply methods to visualize the dermis using confocal laser scanning microscopy from thin tissue sections stained with haematoxylin and eosin. The data was processed to allow three‐dimensional (3‐D) visualization on a PC and using a 3‐D immersive technology system. The 3‐D visualization of the confocal microscope image stacks allowed the evaluation of the collagen macromolecular structure including the collagen fibre bundles. The methods developed provide a novel way of viewing complex organic structures with further potential applications in the medical field.

Thermally induced primary fracture development in tabular granitic plutons: a preliminary analysis

Abstract We present an analytical model that predicts some of the mechanical effects associated with the intrusion and subsequent cooling of a rectangular intrusion emplaced at a uniform temperature into elastic continental crust. Assuming an idealized geometry and initial conditions, we recover the temperature field and subsequent strain field as a function of both position and time. The strain field is particularly relevant as it provides information on the primary (cooling-related) fracture formation pattern and direction within and immediately surrounding the pluton. We find a large strain jump across the pluton-country rock contact, implying that fracture formation should be maximized at the edges and corners of the intrusion. The direction of the fractures is predominantly vertical within the pluton centre, but becomes progressively more inclined towards the pluton margin and into the adjacent country rock. Fracture orientation may depend critically on the geometry of the intrusion, in particular the ratio of the longest to shortest dimension L1/L2.

Physical geology of high-level magmatic systems: introduction

Despite their wide occurrence and structural importance for the development of the upper continental crust, the physical geology of high-level dykes, sills and laccoliths (so-called minor intrusions) has not received the level of detailed attention that it deserves. Factors determining the final emplacement level of subvolcanic intrusions are complex, and depend upon a range of physical parameters, including magma driving pressure, the local (and regional) stress field, and the physical properties (viscosity and density) of the intruding material (Breitkreuz et al. 2002). SiO2-poor magmas rise through tabloid or ring-shaped dykes, acting as feeder systems for Hawaiian to strombolian eruptions or for their phreatomagmatic to subaquatic equivalents. The ascent of silica-rich magmas leads to explosive eruptions, extrusion of lava or emplacement of subvolcanic stocks and laccoliths. The main reason for this variation in emplacement style appears to be the initial volatile content of the rising magma (e.g. Eichelberger et al. 1986). Despite this, and as shown in this volume, the resulting emplacement geometries are surprisingly limited in range, suggesting that interactions between magma pressures and local (and regional) stress fields act to minimize the degree of freedom available for space creation, irrespective of initial composition. Interaction between magmas and sediments is an important process in high-level intrusive complexes, and a number of papers address this topic. In the field, the distinction between subvolcanic intrusions and lavas, and even some high-grade rheomorphic ignimbrites, is not always clear cut, especially in the case of ancient units exposed in limited outcrop or in drill

Image-based modelling of lateral magma flow: the Basement Sill, Antarctica

The McMurdo Dry Valleys magmatic system, Antarctica, provides a world-class example of pervasive lateral magma flow on a continental scale. The lowermost intrusion (Basement Sill) offers detailed sections through the now frozen particle microstructure of a congested magma slurry. We simulated the flow regime in two and three dimensions using numerical models built on a finite-element mesh derived from field data. The model captures the flow behaviour of the Basement Sill magma over a viscosity range of 1–104 Pa s where the higher end (greater than or equal to 102 Pa s) corresponds to a magmatic slurry with crystal fractions varying between 30 and 70%. A novel feature of the model is the discovery of transient, low viscosity (less than or equal to 50 Pa s) high Reynolds number eddies formed along undulating contacts at the floor and roof of the intrusion. Numerical tracing of particle orbits implies crystals trapped in eddies segregate according to their mass density. Recovered shear strain rates (10−3–10−5 s−1) at viscosities equating to high particle concentrations (around more than 40%) in the Sill interior point to shear-thinning as an explanation for some types of magmatic layering there. Model transport rates for the Sill magmas imply a maximum emplacement time of ca 105 years, consistent with geochemical evidence for long-range lateral flow. It is a theoretically possibility that fast-flowing magma on a continental scale will be susceptible to planetary-scale rotational forces.

High resolution 3D confocal microscope imaging of volcanic ash particles

We present initial results from a novel high resolution confocal microscopy study of the 3D surface structure of volcanic ash particles from two recent explosive basaltic eruptions, Eyjafjallajökull (2010) and Grimsvötn (2011), in Iceland. The majority of particles imaged are less than 100μm in size and include PM10s, known to be harmful to humans if inhaled. Previous studies have mainly used 2D microscopy to examine volcanic particles. The aim of this study was to test the potential of 3D laser scanning confocal microscopy as a reliable analysis tool for these materials and if so to what degree high resolution surface and volume data could be obtained that would further aid in their classification. First results obtained using an Olympus LEXT scanning confocal microscope with a ×50 and ×100 objective lens are highly encouraging. They reveal a range of discrete particle types characterised by sharp or concave edges consistent with explosive formation and sudden rupture of magma. Initial surface area/volume ratios are given that may prove useful in subsequent modelling of damage to aircraft engines and human tissue where inhalation has occurred.