J. D. Fitz Gerald

Deformation of granitoids at low metamorphic grade. I: Reactions and grain size reduction

Abstract Deformation and chemical reaction in granitoids at greenschist facies conditions were investigated in samples from three shear zones: two from the Alps (Corvatsch Granodiorite and Aiguilles Rouges Granite, Switzerland) and one from Eastern Australia (Wyangala Granite). When examined by light and electron microscopy, all show similar features of deformation that progressed from coarse parent rock through to fine grained mylonite: quartz deformed plastically, while both K-feldspar and plagioclase fractured and recrystallized in conjunction with chemical change, which had, as its end point, an assemblage albite + quartz ± white mica ± CaAl-silicates. K-feldspar and calcic plagioclase (depending on fluid chemistry) from the parent rock were unstable at low metamorphic grades in the presence of aqueous fluid. Since Ca-bearing plagioclase was not stable in these environments, myrmekite did not replace K-feldspar in any of the rocks examined. Recrystallization of feldspar, which should perhaps be termed neocrystallization, was initiated predominantly at clast margins or along microfractures that are marked by fluid inclusions and twin offsets. In old feldspar grains, particularly in plagioclases, dislocations exist in wall-like structures that are commonly associated with voids, suggesting origins in microfractures. These dislocations had severely limited mobility and subgrain rotation does not appear to have contributed to recrystallization. We conclude that recrystallization mainly occurred via a classical nucleation mechanism, with minor contributions from twin boundary migration. We argue that the 450–550°C lower limit for recrystallization of feldspar, referenced throughout the literature, is not applicable to the rock systems under investigation and should be discarded as a universal limit. Microfracturing, microboudinage and reaction are seen as the prerequisites for mylonite formation from coarse grained granitic rocks at low metamorphic grades (see part II of this paper). Neocrystallization, fluid access and nucleation appear to have accompanied, or immediately followed, the first deformation increments and were primarily responsible for continued grain size reduction. Transgranular fracture was comparatively important only at the onset of deformation. Since the formation of shear zones in granitoids commonly takes place under conditions of low metamorphic grade, in the presence of aqueous fluids in the Earths crust, breakdown reactions and nucleation-recrystallization in feldspars may be important influences upon localization of deformation in granitoids.

Mineralogy and dynamics of a pyrolite lower mantle

There is a growing consensus that the Earths lower mantle possesses a bulk composition broadly similar to that of the upper mantle (known as pyrolite). But little is known about lower-mantle mineralogy and phase chemistry,, especially at depth. Here we report diamond-anvil cell experiments at pressures of 70 and 135 GPa (equivalent to depths within the Earth of about 1,500 and 2,900 km, respectively) which show that pyrolite would consist solely of magnesian-silicate perovskite (MgPv), calcium-silicate perovskite (CaPv) and magnesiowüstite (Mw). Contrary to recent speculation,, no additional phases or disproportionations were encountered and MgPv was found to be present at both pressures. Moreover, we estimate that, at ultra-high pressures where thermal expansivities are low, buoyancy forces inherent in subducted slabs because of their lithology will be of similar magnitude to those required for thermally driven upwelling. So slabs would need to be about 850 °C cooler than their surroundings if they are to sink to the base of the mantle. Furthermore, initiation of plume-like upwellings from the core–mantle boundary, long attributed to superheating, may be triggered by lithologically induced buoyancy well before thermal equilibration is attained. We estimate that ascent would commence within ∼0.5 Gyr of the slab reaching the core–mantle boundary, in which case the lowermost mantle should not be interpreted as a long-term repository for ancient slabs.

Synthesis of boron nitride nanotubes at low temperatures using reactive ball milling

Abstract Boron nitride (BN) nanotubes have been produced by thermal annealing at 1000°C of elemental boron powders which were previously ball milled in ammonia gas for 150 h at room temperature. High-energy ball milling induces nitriding reactions between the boron powder and the ammonia gas. A metastable material is formed consisting of disordered BN and nanocrystalline boron. BN nanotubes then grow out from this metastable and chemically activated structure during heat treatment in the presence of nitrogen gas. This novel process for forming BN nanotubes is distinctly different from arc discharge and laser-heating processes.

The microstructure of zircon and its influence on the age determination from Pb/U isotopic ratios measured by ion microprobe

TEM observations of a large, high-U, Sri Lankan, gem-gravel zircon suggest that certain microstructural features are associated with discordant PbU ages measured in this crystal with the SHRIMP I ion microprobe. Some of the growth bands in the rim of the crystal are optically isotropic and the microstructure of one such band (ca 100 μm wide; designated I) has been compared with the microstructure of the adjacent optically anisotropic regions (designated A). The I-band has a significantly higher U and Pb content, and its electron diffraction pattern of very diffuse rings indicates the absence of periodic atomic arrangements. A-regions consist of parallel-oriented zircon crystallites (ca 10 nm in size) in a matrix with the same characteristics as the I-band. The volume fraction of crystalline zircon in any given growth band correlates (inversely) with the measured birefringence and is taken to be an indication of the degree of radiation damage. Annealing in air at temperatures T < 900°C has little effect on the microstructures, but at T = 900°C the diffraction rings become relatively sharp and can be indexed unequivocally as zirconia which is present as randomly oriented crystallites (ca 10 nm in size), presumably in association with silica glass, in both the I-band and A-regions. However, there is no significant growth of the zircon crystallites in A-regions. After annealing at 1250°C, the zirconia crystallites in the I-band transform to baddeleyite. The crystallites are ca 100 nm in size and the silica glass phase is easily identified. However, the A-regions have recrystallized to a zircon single crystal with precipitates of baddeleyite (ca 40 nm in size) and, presumably, some silica glass. The 207Pb206Pb ages determined with the ion microprobe are essentially independent of the microstructure and average 552 Ma. However, the PbU ages are all reverse discordant (that is, greater than 552 Ma). The discordance (defined as the measured value of 206Pb238U divided by the value expected at 552 Ma) is about 1.1 in most specimens annealed up to 900°C, but as high as 3.5 in the I-band after annealing at 1250°C. It is suggested that Pb tends to concentrate in the silica glass phase which then sputters preferentially during ion bombardment in the ion microprobe, leading to an apparent excess of radiogenic Pb and hence to reverse discordance.

Deformation of granitoids at low metamorphic grade. II: Granular flow in albite-rich mylonites

Abstract The high strain deformation at low metamorphic grades of three investigated granitoids is dominated by the granular flow of albite-rich polyphase aggregates. These aggregates formed by retrograde breakdown reactions from intermediate plagioclase and K-feldspar. The predominant deformation mechanism changes in granitoids as the grain size is reduced: coarse-grained (low strain) examples are deformed by a combination of intracrystalline plasticity (quartz) and fracturing (feldspar). In the mylonites, intracrystalline plasticity of quartz plays only a minor role and the dominant deformation mechanism is a non-cataclastic granular flow of polyphase aggregates, consisting largely of albite and quartz. Deformation appears to be stable, probably because grain growth in albite-quartz mixtures is inhibited. These fine-grained aggregates are mechanically weaker than pure quartz aggregates. Thus, the change in deformation mechanism, mainly due to feldspar breakdown reactions, appears to be important for the localization of high shear strain deformation at low metamorphic grades in granitoids and other lithologies modally dominated by feldspar. The rheological behaviour of albite-dominated mineral aggregates may have two consequences for middle to upper crustal deformation of modally feldspar-dominated lithologies: (1) feldspar, in the presence of aqueous fluids, can be an important mineral controlling the rheology at low metamorphic grades, that is, below amphibolite facies P–T conditions; (2) the occurrence of granular flow of fine-grained polyphase aggregates in low-grade granitoids is probably common and calls for great care in the modelling of middle to upper crustal rheology based on flow laws for intracrystalline plasticity of single minerals such as quartz.

Phase relations, structure and crystal chemistry of some aluminous silicate perovskites

Solid solution of ∼ 25 mole% Al2O3 expands the compositional stability field of Mg,Fe silicate perovskite well beyond the limits encountered in the simple ternary system MgOFeOSiO2. Aluminous perovskites synthesised in laser-heated diamond anvil cell experiments at 55–70 GPa from starting materials on the compositional join between Mg3Al2Si3O12 and Fe3Al2Si3O12 (pyrope and almandine) can contain as much as 90 mole% of the ferrous end member. However Fe0.75 Al0.50Si0.75O3 perovskite could not be synthesised. Predictions that garnet coexists with aluminous perovskite at these pressures are unsubstantiated. These new perovskites are approximately isochemical with garnet and accommodate the full complement of Al2O3 (25 mole%) even at ∼ 70 GPa. Some contain as much as 30 mole% Al2O3, and solid solution is probably facilitated by temperature. However, there is certainly no evidence to substantiate a recent proposal that the capacity of perovskite to accommodate Al2O3 in solid solution is progressively inhibited by pressure. Magnesian silicate perovskite should therefore have no difficulty in accommodating the mantle inventory of Al2O3 in solid solution throughout the entire lower mantle pressure regime. There is no reason to expect that a new aluminous phase would be stabilised at depth within the lower mantle. Nor would exsolution of an aluminous phase at core-mantle boundary pressures be a plausible explanation for the D″ layer. Aluminous perovskites are almost always rhombohedral R3c rather than orthorhombic Pbnm, and their unit cell volumes increase by about 3% as 75 mole% of ferrous iron replaces magnesium. These new perovskites are slightly non-stoichiometric, with modest amounts of an M2(Al,Si)O5.5(M =Mg,Fe) component in solid solution. Crystal chemistry fundamentals successfully predict the site occupancy of minor and trace elements in magnesian silicate perovskite.

Dislocation generation, slip systems, and dynamic recrystallization in experimentally deformed plagioclase single crystals

Abstract Three samples of gem quality plagioclase crystals of An60 were experimentally deformed at 900 °C, 1 GPa confining pressure and strain rates of 7.5–8.7×10−7 s−1. The starting material is effectively dislocation-free so that all observed defects were introduced during the experiments. Two samples were shortened normal to one of the principal slip planes (010), corresponding to a “hard” orientation, and one sample was deformed with a Schmid factor of 0.45 for the principal slip system [001](010), corresponding to a “soft” orientation. Several slip systems were activated in the “soft” sample: dislocations of the [001](010) and 〈110〉(001) system are about equally abundant, whereas 〈110〉{111} and [101] in (131) to (242) are less common. In the “soft” sample plastic deformation is pervasive and deformation bands are abundant. In the “hard” samples the plastic deformation is concentrated in rims along the sample boundaries. Deformation bands and shear fractures are common. Twinning occurs in close association with fracturing, and the processes are clearly interrelated. Glissile dislocations of all observed slip systems are associated with fractures and deformation bands indicating that deformation bands and fractures are important sites of dislocation generation. Grain boundaries of tiny, defect-free grains in healed fracture zones have migrated subsequent to fracturing. These grains represent former fragments of the fracture process and may act as nuclei for new grains during dynamic recrystallization. Nucleation via small fragments can explain a non-host-controlled orientation of recrystallized grains in plagioclase and possibly in other silicate materials which have been plastically deformed near the semi-brittle to plastic transition.

Large volumes of anhydrous pseudotachylyte in the Woodroffe Thrust, eastern Musgrave Ranges, Australia

A mylonitic thrust zone, at least 1.5 km thick, forms a sharp contact between granulite and amphibolite facies gneisses in the eastern Musgrave Ranges, central Australia. The thrust dips gently to the south and is interpreted as an extension of the Woodroffe Thrust, which was formed about 550 Ma ago. Mylonites at the base of the thrust grade upwards into ultramylonites, which pass abruptly into a pseudotachylyte-bearing zone approximately 1 km thick, containing approximately 4% of pseudotachylyte veining. The orientation of the veins appears to be random. Pseudotachylytes occur only in the granulite facies rocks, and their precursors are felsic pyroxene and/or garnet granofelses. Rotated blocks of ultramylonite are present in some of the pseudotachylytes, and some pseudotachylyte veins have been plastically deformed, suggesting nearly contemporaneous semiductile and brittle behaviour. The matrix of the pseudotachylyte shows spectacular examples of igneous quench microstructures, especially skeletal and dendritic crystals of plagioclase and feathery pyroxene dendrites. Also present are glass devitrification microstructures (spherulites), evidence of liquid flow, and partly melted residual grains with former glassy rims showing different optical properties from those of the surrounding isotropic material. These features confirm that the pseudotachylyte formed by melting in anhydrous conditions. The matrix of the pseudotachylyte veins is less siliceous than the host rocks, owing to non-equilibrium melting of pyroxene, garnet and plagioclase. The igneous assemblages of the melt, notably the crystallization of pigeonite, are consistent with rapid cooling from very high-temperature (>1000°C). Melting and quenching is probably due to very local, short-lived rises in temperature accompanied by dilation.

Partitioning of MgO, FeO, NiO, MnO and Cr2O3 between magnesian silicate perovskite and magnesiowüstite: implications for the origin of inclusions in diamond and the composition of the lower mantle

Abstract Syngenetic mineral inclusions in diamond provide valuable information about the environment in which the diamond originally crystallized. It was earlier proposed that diamonds containing a “forbidden” inclusion assemblage of magnesiowustite (Mg number ∼ 85 and NiO ∼ wt%) plus enstatite En 94–95 may have originally formed in the lower mantle. Enstatite would therefore correspond to the retrogressive transformation product of magnesian silicate perovskite. This hypothesis has been assessed by determining the partition behaviour of MgO, FeO, NiO, MnO and Cr 2 O 3 between perovskite and magnesiowustite at lower mantle pressures (30–50 GPa). Experiments were carried out starting with synthetic olivine that was heated with an infrared laser beam in a diamond anvil high pressure cell. Run products were characterized by transmission electron microscopy and X-ray microanalysis. Perovskite (Mg number ∼ 95) and magnesiowustite (Mg number ∼ 86) are produced by the disproportionation of olivine Fo 90 , whereas Fo 85 yields perovskite plus magnesiowustite with Mg number of ∼ 94 and ∼ 78, respectively. NiO is always strongly partitioned into magnesiowustite ( K NiO mw/pv (wt%) is 6.3 ± 5.1, whilst MnO and Cr 2 O 3 show moderate preferential partitioning into magnesiowustite K MnO = 2.4 ± 1.4 and K Cr 2 O 3 = 2.1 ± 0.9. The partition behaviour of all five oxide species as observed between the enstatite and magnesiowustite inclusion assemblage in diamond is entirely consistent with equilibration at lower mantle pressures. If these rare minerals did indeed form in the lower mantle, as suggested by the above experiments, then their compositions can be used to assess various classes of lower mantle model bulk compositions. A perovskititic lower mantle would be required to be highly magnesian (Mg number ∼ 95), which is unacceptable from a geophysical perspective. A lower mantle possessing significant enrichment of FeO and SiO 2 as compared to the upper mantle, would be comprised of perovskite and magnesiowustite with Mg numbers substantially lower than those of their counterparts in diamond. However, mass-balance considerations indicate that the lower mantle could well possess a bulk composition similar to that of a depleted lithology (Mg number ∼ 92) derived from “pyrolite”. There is therefore no requirement for any profound compositional differences between the upper and lower mantle.

Dislocation nucleation and multiplication in synthetic quartz: Relevance to water weakening

Recent observations suggest that the water-related defects associated with the so-called water weakening of single crystals of “wet” synthetic quartz are high-pressure clusters of molecular water. The microstructures which evolve in these crystals during both creep and constant strain-rate experiments and by heating alone were observed by TEM and show that the clusters act as highly efficient sources of the glissile dislocations which must be nucleated before plastic flow can be induced. These microstructural observations, together with simple microdynamical concepts based on the Orowan equation, are used to rationalize the creep behaviour and all the main features of the stressstrain curves observed in “wet” synthetic quartz crystals with a wide range of bulk water-contents, without postulating any direct influence of water on dislocation glide. It is proposed, therefore, that the relatively low yield stress of “wet” synthetic quartz is primarily due to the ease with which fresh glissile dislocations are nucleated, rather than to an enhanced glide of hydrolysed dislocations as is generally assumed in most models of water weakening.