Martyn R. Drury

Microdiamonds in a megacrystic garnet websterite pod from Bardane on the island of Fjørtoft, western Norway: Evidence for diamond formation in mantle rocks during deep continental subduction

We report the startling discovery of in situ microdiamonds in a mantle-derived peridotite lens from Bardane, Fjortoft, western Norway. Diamonds occur within multiphase solid-inclusion assemblages within spinels that are, in turn, inclusions within garnets. Euhedral inclusion-host morphologies of spinel and mineral chemistries all indicate diamond growth from infiltrating fluids at ultrahigh pressure ( P ) and moderate temperature. These results imply that the Bardane peridotite lens was present within a continental subduction zone at depths of ≥130 km. This paper not only documents the first discovery of in situ microdiamonds within the Caledonian ultrahigh-pressure terrane of western Norway, but also represents the first known global occurrence of subduction-related diamond formation within mantle rocks that have been incorporated into a major continental plate collision zone.

Morphology-dependent zeolite intergrowth structures leading to distinct internal and outer-surface molecular diffusion barriers

Zeolites play a crucial part in acid-base heterogeneous catalysis. Fundamental insight into their internal architecture is of great importance for understanding their structure-function relationships. Here, we report on a new approach correlating confocal fluorescence microscopy with focused ion beam-electron backscatter diffraction, transmission electron microscopy lamelling and diffraction, atomic force microscopy and X-ray photoelectron spectroscopy to study a wide range of coffin-shaped MFI-type zeolite crystals differing in their morphology and chemical composition. This powerful combination demonstrates a unified view on the morphology-dependent MFI-type intergrowth structures and provides evidence for the presence and nature of internal and outer-surface barriers for molecular diffusion. It has been found that internal-surface barriers originate not only from a 90 degrees mismatch in structure and pore alignment but also from small angle differences of 0.5 degrees-2 degrees for particular crystal morphologies. Furthermore, outer-surface barriers seem to be composed of a silicalite outer crust with a thickness varying from 10 to 200 nm.

Deformation processes in a peridotite shear zone: reaction-softening by an H2O-deficient, continuous net transfer reaction

The Turon de Tecouere peridotite, in the North Pyrenean Zone, is composed of protomylonites grading to a 20–40 m wide zone of ultramylonites within a 0.6 km diameter exposure. The progressive mylonitization is marked by increasing volume fractions of very fine-grained matrix that comprise up to 90% of the ultramylonite. Deformation of the fine-grained matrix took place by grain size sensitive creep, as suggested by a very fine grain size (<10 μm), lack of dislocations in matrix grains, a weak crystallographic preferred orientation, and the alignment of grain boundaries parallel to the foliation. As the percentage of fine-grained matrix increased, weakening and localization resulted from a change in the dominant deformation mechanism from dislocation creep in the porphyroclasts to grain size sensitive creep in the fine-grained matrix. Production of the matrix grains took place by the nucleation of a number of different phases at the margins of porphyroclasts, indicating that the grain size reduction resulted primarily from reaction, and not from dynamic recrystallization. The nucleation of many phases along a single porphyroclast margin can be explained by a syntectonic continuous net transfer reaction associated with the spinel- to plagioclase-lherzolite transition. This continuous net transfer reaction produced new matrix grains with the same mineralogy as the original assemblage (olivine, orthopyroxene, clinopyroxene, spinel), with new compositions, plus plagioclase. Preliminary geothermobarometry indicates that the reaction took place over a range of temperatures and pressures (750–850°C, and possibly as high as 950°C and 0.5–1.1 GPa). The presence of only small amounts of amphibole, the lack of primary fluid inclusions, and no relation between the presence of amphibole and the intensity of mylonitic deformation led Vissers et al. [Tectonophysics 279 (1997) 303–325] to conclude that the deformation took place in an H2O-deficient environment. Reaction-enhanced softening may occur in the upper mantle wherever rocks move in pressure–temperature space and cross-reaction boundaries. Reaction boundaries are often modeled as univariant (lines in pressure–temperature space), yet mantle minerals are solid solutions so that reactions are continuous (multivariant) and take place over a broader region of pressure–temperature space than end-member reactions. It is therefore likely that shear zone deformation in polymineralic rocks will involve reaction-enhanced ductility over much of pressure–temperature space in the lithospheric mantle.

Deep origin and hot melting of an Archaean orogenic peridotite massif in Norway

The buoyancy and strength of sub-continental lithospheric mantle is thought to protect the oldest continental crust (cratons) from destruction by plate tectonic processes. The exact origin of the lithosphere below cratons is controversial, but seems clearly to be a residue remaining after the extraction of large amounts of melt. Models to explain highly melt-depleted but garnet-bearing rock compositions require multi-stage processes with garnet and clinopyroxene possibly of secondary origin. Here we report on orogenic peridotites (fragments of cratonic mantle incorporated into the crust during continent-continent plate collision) from Otrøy, western Norway. We show that the peridotites underwent extensive melting during upwelling from depths of 350 kilometres or more, forming a garnet-bearing cratonic root in a single melting event. These peridotites appear to be the residue after Archaean aluminium depleted komatiite magmatism.

Mantle shear zones and their effect on lithosphere strength during continental breakup

Abstract The Erro-Tobbio lherzolite in the ophiolitic Voltri Massif of northwestern Italy includes several thrusted fragments of lithospheric mantle which were first exhumed to the ocean floor during Jurassic rifting and breakup, and at a later stage became emplaced in the Alpine collisional stack during Tertiary convergence between Africa and Europe. Coherent slices of these mantle rocks contain several sets of major shear zones generated during the Jurassic rift evolution. One such shear zone, several kilometres wide and formed at temperatures between 920 and 1040°C, is transected by up to 200 m wide, ultra-fine-grained hydrated mylonite zones formed at temperatures in the range 990-550°C. All these structures are cut by MORB-type gabbroic and basaltic dykes. The microstructures of the mylonite zones are interpreted to reflect progressive, reaction-related grainsize reduction plus localization of the deformation during the early stages of continental breakup. In view of experimental evidence that wet olivine rocks weaken considerably with decreasing grainsize, in response to a change from grainsize-insensitive dislocation creep to grainsize-sensitive creep mechanisms, it is proposed that shear localization and allied grainsize reduction may have resulted in a drastic decrease in strength of the upper mantle during rifting. In order to obtain an order of magnitude estimate of this rheological effect, we present a layered rheological model of the Piemonte-Ligurian lithosphere, based on the observed microstructures (i.e., grainsizes) and pressure-temperature data, and including appropriate rheological laws for grainsize-sensitive and -insensitive creep in wet olivine. The model calculations suggest strength values for the uppermost mantle up to four orders of magnitude lower than those expected for homogeneous deformation exclusively controlled by dislocation creep of dry olivine.

Evidence for dominant grain-boundary sliding deformation in greenschist- and amphibolite-grade polymineralic ultramylonites from the Redbank Deformed Zone, Central Australia

Abstract Microstructural and textural investigations by scanning (SEM) and transmission electron microscopy (TEM) techniques have been performed on samples taken across two quartzo-feldspathic mylonite zones from the Redbank Deformed Zone, Central Australia. One has been deformed at greenschist-facies (GS), the second at amphibolite-facies (Am), conditions. With increasing strain the rock type changes from protomylonite to mylonite to ultramylonite. The protomylonites and mylonites consist of alternating quartz and polymineralic quartz-feldspar bands. At the highest strains a homogeneous, fine-grained polymineralic ultramylonite occurs. Shear-zone geometry and microscale structures indicate that these ultramylonites experienced higher strains and were weaker than the encapsulating protomylonites and mylonites. TEM and SEM studies of the ultramylonites reveal a rectangular to square grain shape, a continuous alignment of grain and interphase boundaries across several grain diameters, a grain size (GS 0.5 μm; Am 5–11 μm) less than the equilibrium subgrain size, and open and void-containing grain and interphase boundaries. Analysis of local textures by electron back-scatter diffraction (EBSD) in the SEM showed a very weak crystallographic preferred orientation (CPO) for the quartz. The grain misorientation relationships are not consistent, with dislocation creep being the dominant deformation mechanism. All structures are of the type expected if grain-boundary sliding processes had contributed significantly to the deformation. Consequently, the deformation of such quartzo-feldspathic rocks, and by implication the rheology of the Redbank Deformed Zone, must have been controlled by the mechanical properties of these fine-grained polymineralic ultramylonites, deforming by grain-boundary sliding processes. This is in contrast to the pure quartz bands which deformed by dislocation-creep mechanisms and were less important in the rheology of the Redbank Deformed Zone.

Tomography of insulating biological and geological materials using focused ion beam (FIB) sectioning and low‐kV BSE imaging

Tomography in a focused ion beam (FIB) scanning electron microscope (SEM) is a powerful method for the characterization of three‐dimensional micro‐ and nanostructures. Although this technique can be routinely applied to conducting materials, FIB–SEM tomography of many insulators, including biological, geological and ceramic samples, is often more difficult because of charging effects that disturb the serial sectioning using the ion beam or the imaging using the electron beam. Here, we show that automatic tomography of biological and geological samples can be achieved by serial sectioning with a focused ion beam and block‐face imaging using low‐kV backscattered electrons. In addition, a new ion milling geometry is used that reduces the effects of intensity gradients that are inherent in conventional geometry used for FIB–SEM tomography.

On the role of melt-rock reaction in mantle shear zone formation in the Othris Peridotite Massif (Greece)

A 1-km-wide peridotite mylonite shear zone is exposed in the Othris peridotite massif in central Greece. The mylonites contain lenses of relatively coarse olivine crystals, which are interpreted as remnants of the tectonite microstructure in the adjacent wall rocks. Microstructure and texture analysis using light and SEM microscopy suggests that the dominant deformation mechanism in the tectonites was dislocation creep, whereas the deformation in the mylonites was probably controlled by grain-size sensitive (GSS) creep in fine-grained (<50 μm) bands consisting of a mixture of olivine and orthopyroxene. The development of the fine-grained material in the mylonites can be explained by a melt-present reaction taking place in the tectonite protolith. This reaction led to the replacement of orthopyroxene porphyroclasts by fine-grained olivine and orthopyroxene. Tectonites adjacent to the mylonite zone preserve evidence for this reaction in the form of rims of fine-grained olivine and orthopyroxene around orthopyroxene porphyroclasts. This study illustrates the significance of rheological weakening of oceanic mantle lithosphere as a result of a change from dislocation to GSS creep.

Crystallographic preferred orientations and misorientations in some olivine rocks deformed by diffusion or dislocation creep

Abstract The development of crystallographic preferred orientations (CPO) and grain misorientation distributions (MOD) in fine-grained (0.5–30 μm) olivine rocks, experimentally deformed by diffusion creep and dislocation creep has been investigated. The use of electron back-scattered diffraction (EBSD), in a scanning electron microscope (SEM), has enabled the measurement of CPO in rocks which are too fine-grained to be measured by conventional U-stage methods. Our objective is to study the influence of deformation and recrystallisation mechanisms on the CPO and MOD. The olivine rocks studied were deformed in uni-axial compression, in a gas-medium apparatus, to 17–24% strain at temperatures of 1200–1300°C and 300 MPa confining pressures. The samples show a trend of weaker CPO with lower flow stress which may be related to an increasing component of grain boundary sliding and diffusion creep. In the diffusion creep regime the CPO and MOD are weak to random, whereas in the dislocation creep regime the CPO and MOD are non-random but the MOD is principally controlled by the CPO. These results confirm the idea, based on studies from metals, that the CPO and MOD in olivine are characteristic of the deformation mechanism. Dynamic recrystallisation during dislocation creep results in the occurrence of more intermediate-angle (10–40°) grain boundaries than expected from the CPO. In local areas of complete recrystallisation the MOD is controlled by the CPO which implies that the statistical MOD retains no signature of the initial recrystallisation process. In the dislocation creep regime small grains have a weaker CPO compared to large grains. This result is consistent with predictions from deformation mechanism maps which indicate that the fine recrystallised grains deform by a combination of dislocation creep and grain boundary sliding. The grain boundaries found in the deformed olivine polycrystals are predominately high-angle boundaries with misorientations between 60 and 117°. No obvious evidence has been found for the occurrence of preferred misorientation, or special, grain boundaries.

The Porosity, Acidity, and Reactivity of Dealuminated Zeolite ZSM-5 at the Single Particle Level: The Influence of the Zeolite Architecture

A combination of atomic force microscopy (AFM), high-resolution scanning electron microscopy (HR-SEM), focused-ion-beam scanning electron microscopy (FIB-SEM), X-ray photoelectron spectroscopy (XPS), confocal fluorescence microscopy (CFM), and UV/Vis and synchrotron-based IR microspectroscopy was used to investigate the dealumination processes of zeolite ZSM-5 at the individual crystal level. It was shown that steaming has a significant impact on the porosity, acidity, and reactivity of the zeolite materials. The catalytic performance, tested by the styrene oligomerization and methanol-to-olefin reactions, led to the conclusion that mild steaming conditions resulted in greatly enhanced acidity and reactivity of dealuminated zeolite ZSM-5. Interestingly, only residual surface mesoporosity was generated in the mildly steamed ZSM-5 zeolite, leading to rapid crystal coloration and coking upon catalytic testing and indicating an enhanced deactivation of the zeolites. In contrast, harsh steaming conditions generated 5-50 nm mesopores, extensively improving the accessibility of the zeolites. However, severe dealumination decreased the strength of the Brønsted acid sites, causing a depletion of the overall acidity, which resulted in a major drop in catalytic activity.