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Origin of fine-scale sheeted granites by incremental injection of magma into active shear zones: examples from the Pilbara Craton, NW Australia 总被引：1，自引：0，他引：1
In the Archaean Pilbara Craton of Western Australia, three zones of heterogeneous centimetre- to metre-scale sheeted granites are interpreted to represent high-level, syn-magmatic shear zones. Evidence for the syn-magmatic nature of the shear zones include imbricated and asymmetrically rotated metre-scale orthogneiss xenoliths that are enveloped by leucogranite sheets that show no significant internal strain. At another locality, granite sheets have a strong shape-preferred alignment of K-feldspar, suggesting magmatic flow, while the asymmetric recrystallisation of the grain boundaries indicates that non-coaxial deformation continued acting upon the sheets under sub-solidus conditions. Elsewhere, randomly oriented centimetre-wide leucogranite dykes are realigned at a shear zone boundary to form semi-continuous, layer-parallel sheets within a magma-dominated, dextral shear zone.
It is proposed that the granite sheets formed by the incremental injection of magmas into active shear zones. Magma was sheared during laminar flow to produce the sheets that are aligned sub-parallel to the shear zone boundary. Individual sheets are fed by individual dykes, with up to 1000s of discrete injections in an individual shear zone. The sheets often lack microstructural evidence for magmatic flow, either because the crystal content of the magma was too low to record internal strain, or because of later recrystallisation. 相似文献
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Understanding species distributions, and how they change in space and time, is vital when prioritising conservation or management initiatives. We assessed the distribution and density patterns of common dolphins (Delphinus sp.), bottlenose dolphins (Tursiops truncatus) and Bryde’s whales (Balaenoptera edeni) in the Hauraki Gulf, New Zealand. Dedicated boat-based surveys were conducted in the inner Hauraki Gulf (IHG) and off Great Barrier Island (GBI) during 2010–2012. Generalised linear models were used to investigate temporal changes in relative densities and kernel density estimation was implemented to examine spatial trends. Common dolphins were widely distributed during all seasons, with higher densities observed during winter and spring in the IHG but during autumn off GBI. There was inter-annual variation in Bryde’s whale distribution, with high densities recorded off GBI in 2011. Bottlenose dolphins were infrequently sighted in the IHG but regularly encountered off GBI, with the highest densities during spring and summer. 相似文献
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Alison R. Pawley John R. Holloway Paul F. McMillan 《Earth and Planetary Science Letters》1992,110(1-4):213-225
The solubility of CO2CO fluids in a mid-ocean ridge basalt (morb) has been measured at 1200°C, 500–1500 bar, and oxygen fugacities between NNO and NNO-4. High oxygen fugacities, and thus CO2-rich fluids, were produced by using a starting material equilibrated at NNO, and Ag2C2O4 as the fluid source. Low oxygen fugacities were achieved by using graphite capsules, and MgCO3 as the fluid source. These graphite-saturated fluids have the lowest possibleC/O2CO ratio for a given pressure and temperature.
Experiments were run in a rapid-quench internally heated pressure vessel. Fluid compositions were measured using a simple vacuum technique and by Raman spectroscopy of fluid inclusions. The two techniques yielded comparable results. Fourier transform micro-infrared spectroscopy was used to identify and measure concentrations of dissolved volatiles in double-polished wafers of the quenched glasses. Carbonate was the only carbon-bearing species identified. Raman spectroscopic analysis of inclusion-free areas of glass confirmed the absence of dissolved molecular CO2, CO and carbon. The measured concentrations of dissolved CO2 in the glasses were proportional to the fugacity of CO2 during the experiments, calculated from the measured fluid compositions. The data were fit to the equationXCO2melt(ppm)= 0.492 fCO2 (bar).
The insolubility of CO, compared to CO2, may be related to the fact that dissolution of CO requires reduction of another species in the melt, whereas dissolution of CO2 does not. Due to the fact that CO will be an important component of natural CO fluids at low pressures and low oxygen fugacities, equilibrium dissolved CO2 contents will be less than calculated assuming pure CO2 fluids, but as theC/O2CO ratio in a pure CO fluid at fixed pressure and temperature is a direct function of oxygen fugacity, measurement of the oxygen fugacity of quenched glasses or trapped fluids in natural samples should allow saturation concentrations to be calculated. Dissolved CO2 contents of somemorb are less than expected if they were in equilibrium with pure CO2. These samples must, therefore, have been more reduced than average if they were fluid-saturated. Together with results from other studies of CO2 and H2O solubilities in basalt, the results of this study provide a comprehensive framework for modelling CO2 solution inmorb. 相似文献
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Della K. Murton Steven M. Pawley Julian B. Murton 《Proceedings of the Geologists' Association. Geologists' Association》2009,120(4):209-222
The sedimentary sequence through the Hemingbrough Formation exposed at two sites in the central part of the Vale of York, south of the Escrick moraine ridge, is described and used to reconstruct the palaeoenvironmental history of Glacial Lake Humber. Interbedded wave ripples and laminated silts and clays at both sites indicate that Lake Humber was characterised by fluctuating water levels, often no deeper than wave base. Optically stimulated luminescence ages of 21.0 ± 1.9, 21.9 ± 2.0, and 24.1 ± 2.2 kyr returned from two wave-rippled sandy beds within the glaciolacustrine sequence at Hemingbrough, c. 10 km south of the Escrick moraine ridge, provide the first direct chronological determination for the low-level phase of Lake Humber. As these beds are principally attributed to glacial meltwater emanating from the Vale of York ice lobe of the British Ice Sheet, when its margin was at or near the Escrick moraine ridge, this corroborates the interpretation that this ridge marks the LGM ice limit. 相似文献
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The hydroxyl stretching frequencies of four phyllosilicates have been measured at high pressures and temperatures using an externally heated diamond-anvil cell and synchrotron infrared spectroscopy. Spectra were measured up to 26, 31, 21 and 8 GPa at room temperature for samples of talc, pyrophyllite, muscovite and 10-Å phase, respectively. Spectra were also measured in the range 273–500 K at ambient pressure for all samples and at 8–9 GPa for talc and pyrophyllite. The frequency of the Mg3OH band in talc increases with pressure due to the absence of hydrogen bonding. The different orientation of the hydroxyl group in pyrophyllite and muscovite leads to hydrogen bonding and a decrease in the frequency of the Al2OH band with pressure. 10-Å phase is approximately equivalent to talc with the addition of interlayer H2O. In a spectrum of a sample synthesised for 143 h, two hydroxyl stretching bands are clearly resolved on compression. One is the same as the Mg3OH band in talc, indicating the presence of intra-layer hydroxyl in a talc-like environment with no hydrogen bonding. The other, which separates from the talc-like band at 1 GPa, is associated with intra-layer hydroxyl that is hydrogen bonded to interlayer H2O. There are equivalent bands in high-pressure spectra of a sample of deuterated 10-Å phase, synthesised for 400 h. This sample shows a greater extent of hydrogen bonding at ambient pressure than the 143 h sample. For all of the phases studied, increasing temperature leads to a decrease in frequency for every hydroxyl stretching vibration, both at low and high pressures. The shifts in frequency with temperature are an order of magnitude greater than the shifts with pressure when normalised to previously measured structural parameters. 相似文献
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Pyroxenes of general stoichiometry Mg(Ge x Si1?x )O3 were encountered in attempts to synthesise Ge-substituted talcs at 0.2 GPa, 650–700 °C. Orthopyroxenes (Pbca) of compositions x = 0.21, 0.30, and 0.34 were identified, and also a P21/c clinopyroxene of composition x = 0.63, and C2/c clinopyroxenes of compositions x = 0.91 and 1. End-member clinoenstatite MgSiO3-P21/c synthesised at 16 GPa, 1300 °C and transformed from C2/c was also included in the study. Crystal structure refinements using single-crystal XRD data showed that unit-cell parameters vary linearly with Si–Ge for the Pbca and P21/c pyroxenes, both of which have two symmetrically non-equivalent tetrahedral chains. Refinement of Si–Ge occupancies at tetrahedral sites showed that the two chains of all primitive pyroxenes have very different compositions, with XGe(TB) ? XGe(TA). This difference arises from the greater flexibility of the B-chain to rotate in response to tetrahedral expansion due to increasing Ge content. The TA-M2 shared polyhedral edge imposes significant constraints on the flexibility of the A-chain, which can accommodate much less Ge than the B-chain. Linear trends of cell parameters, site occupancies, and structural parameters for the primitive pyroxenes, when extrapolated to published data for MgGeO3–Pbca, extend across the entire Si–Ge join. 相似文献
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Alison R. Pawley Nicholas J. Chinnery Simon M. Clark Michael J. Walter 《Contributions to Mineralogy and Petrology》2011,162(6):1279-1289
The 10-Å phase (TAP) is a hydrous magnesium silicate that forms from the reaction of talc with H2O at high pressures. Its high-pressure, low-temperature stability means that it could be a storage site for H2O in subduction zones. We have determined the position of the TAP dehydration reaction, TAP = enstatite + coesite + H2O, in phase-equilibrium experiments from 5.0 to 7.1 GPa. Because previous studies had suggested that the composition of TAP is a function of synthesis duration, we used a TAP sample that was synthesised for 392 h. Over the pressure interval of our experiments, the dehydration reaction is isothermal, occurring at a temperature of ~690°C. It is coincident, within experimental uncertainty, with the position of the dehydration reaction of TAP synthesised in short experiments (up to 46 h). Above 7.5 GPa, TAP breaks down to enstatite + stishovite + H2O. This reaction has a negative dP/dT and terminates at an invariant point involving the 3.65-Å phase at ~9.5 GPa, 500°C. The zero volume change implied by the isothermal reaction TAP = enstatite + coesite + H2O was used to calculate the interlayer H2O content of TAP along the reaction. A best-fit H2O content of 1 H2O pfu was obtained. This H2O content is independent of TAP synthesis conditions, suggesting that variations in previously measured H2O contents of TAP occur during quenching and decompression of the samples. The stability of TAP in the Earth is probably limited to cold subduction zones, but in these, it could persist to 300 km depth. 相似文献
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Effect of water on the fluorine and chlorine partitioning behavior between olivine and silicate melt
Bastian?JoachimEmail authorView author&#;s OrcID profile André?Stechern Thomas?Ludwig Jürgen?Konzett Alison?Pawley Lorraine?Ruzié-Hamilton Patricia?L.?Clay Ray?Burgess Christopher?J.?Ballentine 《Contributions to Mineralogy and Petrology》2017,172(4):15
Halogens show a range from moderate (F) to highly (Cl, Br, I) volatile and incompatible behavior, which makes them excellent tracers for volatile transport processes in the Earth’s mantle. Experimentally determined fluorine and chlorine partitioning data between mantle minerals and silicate melt enable us to estimate Mid Ocean Ridge Basalt (MORB) and Ocean Island Basalt (OIB) source region concentrations for these elements. This study investigates the effect of varying small amounts of water on the fluorine and chlorine partitioning behavior at 1280?°C and 0.3 GPa between olivine and silicate melt in the Fe-free CMAS+F–Cl–Br–I–H2O model system. Results show that, within the uncertainty of the analyses, water has no effect on the chlorine partitioning behavior for bulk water contents ranging from 0.03 (2) wt% H2O (DCl ol/melt = 1.6?±?0.9 × 10?4) to 0.33 (6) wt% H2O (DCl ol/melt = 2.2?±?1.1 × 10?4). Consequently, with the effect of pressure being negligible in the uppermost mantle (Joachim et al. Chem Geol 416:65–78, 2015), temperature is the only parameter that needs to be considered for the determination of chlorine partition coefficients between olivine and melt at least in the simplified iron-free CMAS+F–Cl–Br–I–H2O system. In contrast, the fluorine partition coefficient increases linearly in this range and may be described at 1280?°C and 0.3 GPa with (R 2?=?0.99): \(D_{F}^{\text{ol/melt}}\ =\ 3.6\pm 0.4\ \times \ {{10}^{-3}}\ \times \ {{X}_{{{\text{H}}_{\text{2}}}\text{O}}}\left( \text{wt }\!\!\%\!\!\text{ } \right)\ +\ 6\ \pm \ 0.4\times \,{{10}^{-4}}\). The observed fluorine partitioning behavior supports the theory suggested by Crépisson et al. (Earth Planet Sci Lett 390:287–295, 2014) that fluorine and water are incorporated as clumped OH/F defects in the olivine structure. Results of this study further suggest that fluorine concentration estimates in OIB source regions are at least 10% lower than previously expected (Joachim et al. Chem Geol 416:65–78, 2015), implying that consideration of the effect of water on the fluorine partitioning behavior between Earth’s mantle minerals and silicate melt is vital for a correct estimation of fluorine abundances in OIB source regions. Estimates for MORB source fluorine concentrations as well as chlorine abundances in both mantle source regions are within uncertainty not affected by the presence of water. 相似文献