Stephen J. Covey-Crump

Thermally-induced grain growth of calcite marbles on Naxos Island, Greece

The island of Naxos is composed of an elliptically shaped structural and thermal dome of Miocene age. Peak metamorphic temperatures within the central migmatite complex exceeded 700° C, decreasing to about 300° C at the most distant exposures on the island. Equigranular calcite marbles which outcrop together with metapelites and metabasites over the whole island show a systematic pattern of increasing grain-size towards the central migmatite complex, with a significant discontinuity in the pattern corresponding approximately with the 500° C isotherm. The microstructures and grain-size distributions in the marbles are consistent with normal grain-growth. The variation of grain-size with peak temperature attained can be explained equally well by the assumptions that (a) a maximum grainsize had developed, particularly at higher temperatures, or that (b) the grain-size had been frozen-in by a combination of cooling and coarsening, both of which combine to reduce the rate of grain-growth.The grain-size data do not impose strong constraints on the mechanism of transport of heat responsible for the metamorphism, whether by conduction or by advection, but the 500° C discontinuity indicates that the tectonothermal history of the migmatitic core and its envelope of metasediments were different.

Accurate quantification of the modal mineralogy of rocks when image analysis is difficult

Abstract The volume proportions of the mineral phases in two strongly deformed olivine-orthopyroxene rocks have been quantified by whole-pattern stripping of fixed geometry X-ray powder diffraction data. The results were compared with the phase proportions as determined by Rietveld refinement of time-of-flight neutron powder diffraction data, and were shown to be in excellent agreement. The X-ray technique not only provides a very rapid and cost-effective method of determining phase proportions, but it also circumvents several of the problems associated with obtaining this information by image analysis. Moreover, the technique is particularly advantageous in strongly textured rocks or in rocks that contain significant residual strains. As such it offers a powerful technique for analysing the mineralogical composition of fine-grained and/or deformed experimental run products, which makes it of considerable potential for monitoring in situ the progress of mineral reactions during laboratory experiments.

Using neutron diffraction measurements to characterize the mechanical properties of polymineralic rocks

Abstract Conventional experiments designed to investigate the mechanical properties of polycrystalline geological materials are generally restricted to measurements of whole-rock properties. However, when comparing the measurements with theoretical models, it is frequently essential to understand how the deformation is accommodated at the grain-scale. This is particularly true for polymineralic rocks because in this case most theories express the whole-rock properties as some function of the properties of their constituent minerals, and hence the contribution which each phase makes to those properties must be measured if the theories are to be fully assessed. The penetrating nature of neutrons offers a method of addressing this problem. By performing deformation experiments in the neutron beam-line and collecting neutron diffraction patterns at different applied loads, the lattice parameters of all the mineral phases present may be determined as a function of load. The elastic strain experienced by each phase is then easily determined. Moreover, the strain in different lattice directions is also obtained. From this information a wide range of problems relevant for the characterization of the elastic and plastic deformation behaviour of polymineralic geological materials can be explored. An experimental technique for carrying out such experiments is described, and its validity is demonstrated by showing that the results obtained from deforming an elastically isotropic olivine + magnesiowustite sample agree, to within very tight bounds, with the behaviour predicted by theory for elastically isotropic composites.

Strain partitioning during the elastic deformation of an olivine + magnesiowustite aggregate

In order to measure directly the extent of elastic strain partitioning between the phases in a two phase material, room temperature uniaxial compression experiments have been performed on an olivine + magnesiowustite sample within the neutron beam at the ISIS neutron spallation source, Rutherford Appleton Laboratory, U.K. Neutron diffraction data were collected at fourteen different applied loads. At each load the olivine and magnesiowustite lattice parameters in directions parallel and normal to the direction of applied load, were extracted from the diffraction data, and the axial and radial elastic strains experienced by each phase were calculated from these parameters. The observed strain partitioning is in close agreement with that predicted by the widely used Hashin-Shtrikman analysis of the elastic properties of composites, confirming both the validity of that analysis, and the potential which the experimental technique has for providing insight into the controls on the mechanical properties of polymineralic materials.

A new apparatus for measuring mechanical properties at moderate confining pressures in a neutron beamline

A simple pressure vessel suitable for use at room temperature has been developed which allows neutron diffraction data to be collected from cylindrical samples of up to 10 mm diameter, at confining pressures of up to 160 MPa, whilst they are also being deformed in compression by the application of a uniaxially symmetric load. The vessel has been commissioned on the ENGIN-X beamline at the ISIS neutron facility (Rutherford Appleton Laboratory, Chilton, UK). The commissioning results show that neutron diffraction data of quality equivalent to that obtained using an identical experiment geometry at room pressure can be acquired using the pressure vessel with only about a factor of two increase in count times.

Using neutron diffraction to investigate the elastic properties of anisotropic rocks: Results from an olivine + orthopyroxene mylonite

[1] The penetrating power of neutrons means that neutron diffraction may be employed to determine the lattice parameters of mineral phases within samples over 1 cm in size. We have exploited this fact to obtain high-resolution measurements of the elastic strains experienced by olivine and orthopyroxene during room temperature, uniaxial deformation experiments performed in a neutron beam-line, on a mylonite from the Oman ophiolite. Specimens were loaded in three orthogonal directions with respect to the macroscopic fabric of the rock. In each case the average olivine and orthopyroxene strains were the same, even though the elastic stiffnesses of the two phases were different. In comparison with the strains expected from the single-crystal elastic stiffness tensors, the average strains in the different lattice directions for each phase were found to be greater in the stiffer directions, and lesser in the more compliant directions. The overall effect of this tendency toward strain homogenization was for the measured elastic anisotropy of the rock to be significantly lesser than that given by Voigt/Reuss averaging of the single-crystal elastic properties according to the lattice preferred orientations of each phase. The technique used to derive these conclusions potentially provides an important experimental method for the quantitative examination of nonfracture-related controls on the elastic anisotropy of geological materials.

Neutron Diffraction and the Mechanical Behavior of Geological Materials

Establishing methods to monitor how deformation is accommodated at the grain scale within samples during mechanical tests has a key part to play in advancing our understanding of the mechanical properties of polycrystalline materials. This information is essential, both for testing the assumptions and approximations used in theoretical analyses designed to predict these properties from the properties of their constituent grains, as well as for using such analyses to interrogate the results of deformation experiments. Conventional deformation experiments (particularly those on nonporous materials) generally provide only whole sample properties, and information about how the deformation has been accommodated within the sample is usually restricted to that which can be recovered from an analysis of the experimental run products. However, the penetrating power of neutrons means that if the deformation experiment is performed on a neutron beam line, the deformation behavior within the sample can be monitored as it is being deformed from diffraction patterns collated at different stages of the deformation. Over the past 20 years, numerous studies on engineering materials have exploited this strategy. In the following contribution, parallel work on geological materials is reviewed with the aim of illustrating the potential of the approach for examining matters of mechanical property characterization that are of particular interest in the earth sciences.

Geomechanical characterization of mud volcanoes using P-wave velocity datasets

Abstract Mud volcanoes occur in many petroliferous basins and are associated with significant drilling hazards. To illustrate the type of information that can be extracted from limited petrophysical datasets in such geomechanically complex settings, we use P-wave velocity data to calculate the mechanical properties and stresses on a two-dimensional vertical section across a mud volcano in the Azeri-Chirag-Guneshly field, South Caspian Basin. We find that: (1) the values of the properties and stresses calculated in this way have realistic magnitudes; (2) the calculated pore fluid pressures show spatial variations around the mud volcano that potentially highlight areas of fluid recharging after the most recent eruption; and (3) the information obtained is sufficient to provide helpful indications of the width of the drilling window. Although calculations of this kind may be readily improved with more sophisticated petrophysical datasets, the simplicity of this approach makes it attractive for reconnaissance surveys designed to identify targets worthy of further investigation in developing our understanding of mud volcano geomechanics, or which could be used to help formulate drilling strategies.

Variation of the exponential and power law creep parameters with strain for Carrara Marble deformed at 120° to 400°C

Reliable extrapolation of flow laws for geological materials from laboratory to natural conditions depends upon a sound micromechanical interpretation of the values of the material parameters in those flow laws. In practice, such interpretations require the experimentally determined values of the material parameters to be close to those applying during steady state deformation and/or at given material structure (mechanical state). Using an especially detailed mechanical data set for Carrara marble and a description of flow properties incorporating an explicit structure dependence, the variation of the constant strain and constant structure values of the material parameters in the exponential and power law creep equations is determined as a function of strain. The large variation that is observed underscores the need both for the acquisition of mechanical data to large strains and for further work investigating the structure dependence of flow properties, if flow law extrapolation is to be performed with confidence.

E. H. Rutter: a biography

Ernest Henry Rutter was born in January 1946 in Sunderland. His father, George, was a sergeant in the army and then worked for the Post Office and his mother, Irene, worked in retail. From an early age, Ernie showed aptitude for all things scientific and especially practical science. At 11 he was selected for a technical high school where his interest in science and electronics was encouraged. Indeed, his chemistry experiments led to an explosion in the garden shed and partial defoliation of his neighbour’s garden. During his studies for university entrance he was enthused by electronics and constructed his own television from constituent parts. With this knowledge, he worked in his spare time as a television repair man in Sunderland. During his teenage years he also developed a love of classical music, regularly attending concerts at Newcastle City Hall. He gained entrance to Imperial College, London to study Geology in 1964. He was interested in all things geological and in fieldwork in particular. He retained his enjoyment of practical science, building his own cathodoluminescence microscope in the undergraduate petrology laboratory. He conducted his undergraduate thesis work on the basic igneous rocks of Løkken in Norway, having persuaded a mining company that their exploration would be aided by knowledge of the structure of the area. The results from his undergraduate thesis work on massive sulphide deposits were published in 1967 and 1969. In his final year, he was particularly intrigued by the fluid mechanics of how ammonites propel themselves through water, and considered the possibility of a palaeontological career, but chose to undertake a PhD in the field of Structural Geology and Rock Mechanics instead. Palaeontology’s loss was Structural Geology’s gain. Ernie graduated from Imperial College in 1967 with a First Class Honours degree. At that time, Structural Geology at Imperial College was a subject in the ascendancy. At this time he was very influenced by a publication by Professor Derek Flinn (of the University of Liverpool), who introduced concepts of deformation mechanisms based on material science literature to geologists (Flinn, 1965 in The Controls of Metamorphism Volume). He was greatly influenced and encouraged at Imperial College by Professors Neville Price and John Ramsay. They recognized the potential for Structural Geology of quantifying the mechanics of rocks under pressure and temperature which, at that time, was largely an area of research dominated by universities in the USA. When Neville Price gained Natural Environment Research Council funding to establish a Rock Deformation Laboratory at Imperial College, Ernie was appointed as a graduate student to lead this endeavour. Ernie’s hands-on, practical approach and knowledge of electronics, in addition to his geological skills, made him an ideal candidate, although it is hard to imagine a graduate student being placed in such a position today! The designs and drawing for the first deformation apparatus used in the Rock Deformation Laboratory were provided by Professor Hugh Heard from the Lawrence–Berkeley Laboratory in California. At this time, Ernie established his long-running working relationship with Mr Robert Holloway, who was appointed as the mechanical technician for the new laboratory. Together they constructed six Heard-type apparatuses and Ernie ran his first experiments on the deformation of carbonate rocks. Despite the enormous task of initiating a laboratory from scratch, Ernie coped admirably and was appointed to the academic staff in 1969, a year before the end of his PhD. Ernie had to juggle preparing lectures and teaching with writing up his thesis at the same time. Around this time the first publications from the laboratory appeared. What followed was an extraordinary contribution to the field of Rock Deformation spanning almost every imaginable aspect from high-temperature plastic deformation to brittle rock mechanics. Ernie was also a key factor in an immensely successful taught Master’s programme in Structural