Syn-kinematic hydration reactions, grain size reduction, and dissolution-precipitation creep in experimentally deformed plagioclase-pyroxene mixtures

Source at https://doi.org/10.5194/se-9-985-2018 .It is widely observed that mafic rocks are able to accommodate high strains by viscous flow. Yet, a number of questions concerning the exact nature of the involved deformation mechanisms continue to be debated. In this contribution, rock deformation experiments on four different water-added plagioclase–pyroxene mixtures are presented:(i) plagioclase(An60–70)–clinopyroxene–orthopyroxene,(ii) plagioclase(An60)–diopside,(iii) plagioclase(An60)–enstatite,and iv) plagioclase(An01)–enstatite. Samples were deformed in general shear at strain rates of 3×10−5 to 3×10−6 s−1, 800°C, and confining pressure of 1.0 or 1.5GPa. Results indicate that dissolution–precipitation creep (DPC) and grain boundary sliding (GBS) are the dominant deformation mechanisms and operate simultaneously. Coinciding with sample deformation, syn-kinematic mineral reactions yield abundant nucleation of new grains; the resulting intense gray size reduction is considered crucial for the activity of DPC and GBS. In high strain zones dominated by plagioclase, a weak, nonrandom, and geometrically consistent crystallographic preferred orientation (CPO) is observed. Usually, a CPO is considered a consequence of dislocation creep, but the experiments presented here demonstrate that a CPO can develop during DPC and GBS. This study provides new evidence for the importance of DPC and GBS in mid-crustal shear zones within mafic rocks, which has important implications for understanding and modeling mid-crustal rheology and flow

The south-western Black Forest and the Upper Rhine Graben Main Border Fault: thermal history and hydrothermal fluid flow

The thermal history of the south-westernmost Black Forest (Germany) and the adjacent Upper Rhine Graben were constrained by a combination of apatite and zircon fission-track (FT) and microstructural analyses. After intrusion of Palaeozoic granitic plutons in the Black Forest, the thermal regime of the studied area re-equilibrated during the Late Permian and the Mesozoic, interrupted by enhanced hydrothermal activity during the Jurassic. At the eastern flank of the Upper Rhine Graben along the Main Border Fault the analysed samples show microstructural characteristics related to repeated tectonic and hydrothermal activities. The integration of microstructural observations of the cataclastic fault gouge with the FT data identifies the existence of repeated tectonic-related fluid flow events characterised by different thermal conditions. The older took place during the Variscan and/or Mesozoic time at temperatures lower than 280°C, whereas the younger was probably contemporary with the Cenozoic rifting of the Upper Rhine Graben at temperatures not higher than 150°

Experimental grain growth of quartz aggregates under wet conditions and its application to deformation in nature

Grain growth of quartz was investigated using two quartz samples (powder and novaculite) with water under pressure and temperature conditions of 1.0–2.5&thinsp;GPa and 800–1100&thinsp;∘C. The compacted powder preserved a substantial porosity, which caused a slower grain growth than in the novaculite. We assumed a grain growth law of dn-d0n=k0fH2Orexp⁡(-Q/RT)t with grain size d (µm) at time t (seconds), initial grain size d0 (µm), growth exponent n, a constant k0 (µmn&thinsp;MPa−r&thinsp;s−1), water fugacity fH2O (MPa) with the exponent r, activation energy Q (kJ&thinsp;mol−1), gas constant R, and temperature T in Kelvin. The parameters we obtained were n=2.5±0.4, k0=10-8.8±1.4, r=2.3±0.3, and Q=48±34 for the powder and n=2.9±0.4, k0=10-5.8±2.0, r=1.9±0.3, and Q=60±49 for the novaculite. The grain growth parameters obtained for the powder may be of limited use because of the high porosity of the powder with respect to crystalline rocks (novaculite), even if the differences between powder and novaculite vanish when grain sizes reach ∼70&thinsp;µm. Extrapolation of the grain growth laws to natural conditions indicates that the contribution of grain growth to plastic deformation in the middle crust may be small. However, grain growth might become important for deformation in the lower crust when the strain rate is &lt;&thinsp;10−12&thinsp;s−1.</p

Anticlockwise metamorphic pressure–temperature paths and nappe stacking in the Reisa Nappe Complex in the Scandinavian Caledonides, northern Norway: evidence for weakening of lower continental crust before and during continental collision

This study investigates the tectonostratigraphy and metamorphic and tectonic evolution of the Caledonian Reisa Nappe Complex (RNC; from bottom to top: Vaddas, Kåfjord, and Nordmannvik nappes) in northern Troms, Norway. Structural data, phase equilibrium modelling, and U-Pb zircon and titanite geochronology are used to constrain the timing and pressure–temperature (P–T) conditions of deformation and metamorphism during nappe stacking that facilitated crustal thickening during continental collision. Five samples taken from different parts of the RNC reveal an anticlockwise P–T path attributed to the effects of early Silurian heating (D1) followed by thrusting (D2). At ca. 439&thinsp;Ma during D1 the Nordmannvik Nappe reached the highest metamorphic conditions at ca. 780&thinsp;∘C and ∼9–11&thinsp;kbar inducing kyanite-grade partial melting. At the same time the Kåfjord Nappe was at higher, colder, levels of the crust ca. 600&thinsp;∘C, 6–7&thinsp;kbar and the Vaddas Nappe was intruded by gabbro at &gt;&thinsp;650&thinsp;∘C and ca. 6–9&thinsp;kbar. The subsequent D2 shearing occurred at increasing pressure and decreasing temperatures ca. 700&thinsp;∘C and 9–11&thinsp;kbar in the partially molten Nordmannvik Nappe, ca. 600&thinsp;∘C and 9–10&thinsp;kbar in the Kåfjord Nappe, and ca. 640&thinsp;∘C and 12–13&thinsp;kbar in the Vaddas Nappe. Multistage titanite growth in the Nordmannvik Nappe records this evolution through D1 and D2 between ca. 440 and 427&thinsp;Ma, while titanite growth along the lower RNC boundary records D2 shearing at 432±6&thinsp;Ma. It emerges that early Silurian heating (ca. 440&thinsp;Ma) probably resulted from large-scale magma underplating and initiated partial melting that weakened the lower crust, which facilitated dismembering of the crust into individual thrust slices (nappe units). This tectonic style contrasts with subduction of mechanically strong continental crust to great depths as seen in, for example, the Western Gneiss Region further south.</p

Locally Resolved Stress‐State in Samples During Experimental Deformation: Insights Into the Effect of Stress on Mineral Reactions

Understanding conditions in the Earth's interior requires data derived from laboratory experiments. Such experiments provide important insights into the conditions under which mineral reactions take place as well as processes that control the localization of deformation in the deep Earth. We performed Griggs‐type general shear experiments in combination with numerical models, based on continuum mechanics, to quantify the effect of evolving sample geometry of the experimental assembly. The investigated system is constituted by CaCO3 and the experimental conditions are near the calcite‐aragonite phase transition. All experimental samples show a heterogeneous distribution of the two CaCO3 polymorphs after deformation. This distribution is interpreted to result from local stress variations. These variations are in agreement with the observed phase‐transition patterns and grain‐size gradients across the experimental sample. The comparison of the mechanical models with the sample provides insights into the distribution of local mechanical parameters during deformation. Our results show that, despite the use of homogeneous sample material (here calcite), stress variations develop due to the experimental geometry. The comparison of experiments and numerical models indicates that aragonite formation is primarily controlled by the spatial distribution of mechanical parameters. Furthermore, we monitor the maximum pressure and σ1 that is experienced in every part of our model domain for a given amount of time. We document that local pressure (mean stress) values are responsible for the transformation. Therefore, if the role of stress as a thermodynamic potential is investigated in similar experiments, an accurate description of the state of stress is required.Plain Language Summary: To understand processes in the Earth's interior, we can simulate the extreme conditions via laboratory experiments by compressing and heating millimeter‐sized samples. Such experiments provide important insights into mineral reactions and processes that control deformation in the Earth. We performed rock deformation experiments close to calcite‐aragonite phase (CaCO3) transition. Deforming the sample leads to stress variations due to the experimental geometry. These variations are documented by locally occurring phase transition and variation in the grain‐size. We performed computer simulations of the deforming sample to quantify, for the first time, the effect of sample geometry on the distribution of mechanical variables, such as stress, pressure, or deformation, inside the sample. The new findings document that any mechanical variable cannot be treated as homogeneous within the sample because the variations can be significant. Deforming the sample leads to stress concentrations. By comparing the experimental observations and simulation results, we show that locally high pressure triggers the phase transition to aragonite, the high‐pressure polymorph. This has important consequences for further thermodynamic interpretations of systems under stress, where the role of deformation, pressure, or maximum principal stress on mineral reactions is investigated.Key Points: Heterogeneous stress distribution in deformation experiments is investigated by numerical models, locally resolving mechanical variables. Resolving the mechanical variables in experiments suggests a link between local pressure (mean stress) variations and phase transition. Thermodynamic interpretations of deformed samples require a detailed understanding of local mechanical parameters.ETH Zürich Foundation http://dx.doi.org/10.13039/501100012652https://doi.org/10.5281/zenodo.697476

The effect of muscovite on the microstructural evolution and rheology of quartzite in general shear

International audienceWe conducted general shear experiments on synthetic mixtures of quartz and muscovite aggregates at 800 • C and 1.5 GPa with 0.1 wt% H 2 O added in the Griggs apparatus to investigate the role of muscovite content on the microstructural evolution and rheological properties of quartz aggregates. Muscovite content varied between 0, 5, 10 and 25% muscovite. Mechanically, the sample strengths decrease with an increase in muscovite content. At high strains muscovite grains align sub-parallel to the shear plane with C ′-bands commonly observed in the muscovite-bearing samples. The presence of muscovite has significant influence on the amount of dynamic recrystallization in quartz; at high strains the pure quartz sample completely recrystallizes while only ~5% of the quartz in the 25% muscovite sample is dynamically recrystallized at similar strains. The presence of muscovite also has a significant influence on crystallographic preferred orientations (CPO) and grain shape preferred orientations (SPO) of quartz at high strains. At high strains, muscovite is interpretated to deform primarily by basal glide and dissolution-precipitation creep. Finally, our mechanical results fit well with rheological mixing models, where we estimate aggregates with 25% muscovite may deform 1-2 orders of magnitude faster than pure quartz aggregates at conditions near the brittle-ductile transition