40 research outputs found
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 plagioclaseâpyroxene mixtures are presented:(i) plagioclase(An60â70)âclinopyroxeneâorthopyroxene,(ii) plagioclase(An60)âdiopside,(iii) plagioclase(An60)âenstatite,and iv) plagioclase(An01)âenstatite. Samples were deformed in general shear at strain rates of 3Ă10â5 to 3Ă10â6 sâ1, 800°C, and confining pressure of 1.0 or 1.5GPa. Results indicate that dissolutionâprecipitation 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 1.0â2.5 GPa and 800â1100 âC. 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=k0fH2OrexpâĄ(-Q/RT)t
with grain size d (”m) at time t (seconds), initial grain size
d0 (”m), growth exponent n, a constant k0 (”mn MPaâr sâ1), water fugacity
fH2O (MPa) with the exponent r,
activation energy Q (kJ molâ1),
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 âŒ70 ”m. 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 < 10â12 sâ1.</p
Anticlockwise metamorphic pressureâtemperature 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, KĂ„fjord, 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 pressureâtemperature
(PâT) 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
PâT path attributed to the effects of early Silurian heating (D1)
followed by thrusting (D2). At ca. 439 Ma during D1 the
Nordmannvik Nappe reached the highest metamorphic conditions at
ca. 780 âC and âŒ9â11 kbar inducing kyanite-grade partial
melting. At the same time the KĂ„fjord Nappe was at higher, colder, levels
of the crust ca. 600 âC, 6â7 kbar and the Vaddas Nappe was
intruded by gabbro at > 650 âC and ca. 6â9 kbar. The
subsequent D2 shearing occurred at increasing pressure and decreasing
temperatures ca. 700 âC and 9â11 kbar in the partially molten
Nordmannvik Nappe, ca. 600 âC and 9â10 kbar in the KĂ„fjord
Nappe, and ca. 640 âC and 12â13 kbar in the Vaddas Nappe.
Multistage titanite growth in the Nordmannvik Nappe records this evolution
through D1 and D2 between ca. 440 and 427 Ma, while titanite
growth along the lower RNC boundary records D2 shearing at 432±6 Ma. It emerges that early Silurian heating (ca. 440 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 StressâState 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 Griggsâtype 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 calciteâaragonite 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 phaseâtransition patterns and grainâsize 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 Ï1 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 millimeterâsized samples. Such experiments provide important insights into mineral reactions and processes that control deformation in the Earth. We performed rock deformation experiments close to calciteâaragonite 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 grainâsize. 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 highâpressure 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 ZĂŒrich Foundation
http://dx.doi.org/10.13039/501100012652https://doi.org/10.5281/zenodo.697476
Quartz grain size reduction in a granitoid rock and the transition from dislocation to diffusion creep
Semi-brittle deformation of granitoid gouges in shear experiments at elevated pressures and temperatures
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 âą 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 âČ-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