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

Interpretation of porosity and LWD resistivity from the Nankai accretionary wedge in light of clay physicochemical properties: Evidence for erosion and local overpressuring

International audienceIn this study, we used porosity to assess the compaction state of the Nankai accretionary wedge sediments and any implications for stress and pore pressure. However, hydrous minerals affect porosity measurements, and accounting for them is essential toward defining the interstitial porosity truly representative of the compaction state. The water content of sediments was measured in core samples and estimated from logging data using a resistivity model for shale. We used the cation exchange capacity to correct the porosity data for the amount of water bound to clay minerals and to correct the porosity estimates for the surface conductivity of hydrous minerals. The results indicate that several apparent porosity anomalies are significantly reduced by this correction, implying that they are in part artifacts from hydrous minerals. The correction also improves the fit of porosity estimated from logging-while-drilling (LWD) resistivity data to porosity measured on cores. Low overall porosities at the toe of the accretionary wedge and in the splay fault area are best explained by erosion, and we estimated the quantity of sediments eroded within the splay fault area by comparing porosity-effective stress relationships of the sediments to a reference curve. Additionally, a comparison of LWD data with core data (resistivity and P wave velocity) obtained at Site C0001 landward of the mega-splay fault area, suggested a contribution from the fracture porosity to in situ properties on the formation

Strong sediments at the deformation front, and weak sediments at the rear of the Nankai accretionary prism, revealed by triaxial deformation experiments

Nineteen whole-round core samples from the Nankai accretionary prism (IODP Expeditions 315, 316, and 333) from a depth range of 28–128 m below sea floor were experimentally deformed in a triaxial cell under consolidated and undrained conditions at confining pressures of 400–1000 kPa, room temperature, axial displacement rates of 0.01–9.0 mm/min, and up to axially compressive strains of ∼64%. Despite great similarities in composition and grain size distribution of the silty clay samples, two distinct “rheological groups” are distinguished: The first group shows deviatoric peak stress after only a few percent of compressional strain (10%), or does not weaken at all. This is characteristic of structurally strong material. The strong samples tend to be overconsolidated and are all from the drillsites at the accretionary prism toe, while the weak and normally consolidated samples come from the immediate hanging wall of a megasplay fault further upslope. Sediments from the incoming plate are also structurally weak. The observed differences in mechanical behavior may hold a key for understanding strain localization and brittle faulting within the uniform silty and clayey sedimentary sequence of the Nankai accretionary prism

Normal grain growth of quartz by experiment and discussion on the effect of grain size reduction by deformation in natural conditions

International audienceGrain size is an important factor that contributes to rheology. Grain size increases or decreases by grain growth or grain size reduction, in which the latter includes fracturing, dynamic recrystallization, dissolution-precipitation, and reaction. These two opposite mechanisms would occur in nature. Quartz is a major constituent of the Earth's crust. However, little is known about its grain growth parameters. In this study, we experimentally determined grain growth laws of quartz aggregates. We performed experiments using a piston cylinder. We prepared two quartz samples; novaculite as a quartz aggregate whose grain size is ~3 µm and natural quartz powder whose grain size is ~2 µm. We enclosed the two samples with added water of up to 10 wt% in a platinum capsule. Experimental conditions were under pressure of 1.0-2.5 GPa, temperature of 800-1100 °C, and durations of 6-240 hours. Normal grain growth occurred in these two samples, but we did not see differences in grain growth due to differences in amounts of added water. The powder sample showed porosities of ~10 vol %, which caused a slightly slower grain growth rate than that of the novaculite. We obtained the grain size exponents of ~3 and water fugacity exponents of ~2 for the two samples, and activation energies of 50 and 60 kJ/mol for the powder and novaculite, respectively. We applied our grain growth laws to crustal conditions assuming different initial grain sizes. We calculated grain growth rates with time and discussed contribution of plastic strain given under different strain rates which could cause grain size reduction by dynamic recrystallization. The strain-time relationship shows that strain is negligible until ~1000 years when strain rate is -12/sec. This means that the contribution of dynamic recrystallization is negligible. In the meantime, grain size can increase to a few tens of µm under the middle crustal condition (at 400 °C where pressure and water fugacity are calculated with a geological temperature and pressure gradients of 25 °C/km and 27 MPa/km, respectively) and to ~a few hundred μm under the lower crustal condition (at 600 °C). Our results indicate that there can be natural conditions where grain growth can be dominant even plastic deformation is operating

Normal grain growth of quartz by experiment and discussion on the effect of grain size reduction by deformation in natural conditions

International audienceGrain size is an important factor that contributes to rheology. Grain size increases or decreases by grain growth or grain size reduction, in which the latter includes fracturing, dynamic recrystallization, dissolution-precipitation, and reaction. These two opposite mechanisms would occur in nature. Quartz is a major constituent of the Earth's crust. However, little is known about its grain growth parameters. In this study, we experimentally determined grain growth laws of quartz aggregates. We performed experiments using a piston cylinder. We prepared two quartz samples; novaculite as a quartz aggregate whose grain size is ~3 µm and natural quartz powder whose grain size is ~2 µm. We enclosed the two samples with added water of up to 10 wt% in a platinum capsule. Experimental conditions were under pressure of 1.0-2.5 GPa, temperature of 800-1100 °C, and durations of 6-240 hours. Normal grain growth occurred in these two samples, but we did not see differences in grain growth due to differences in amounts of added water. The powder sample showed porosities of ~10 vol %, which caused a slightly slower grain growth rate than that of the novaculite. We obtained the grain size exponents of ~3 and water fugacity exponents of ~2 for the two samples, and activation energies of 50 and 60 kJ/mol for the powder and novaculite, respectively. We applied our grain growth laws to crustal conditions assuming different initial grain sizes. We calculated grain growth rates with time and discussed contribution of plastic strain given under different strain rates which could cause grain size reduction by dynamic recrystallization. The strain-time relationship shows that strain is negligible until ~1000 years when strain rate is -12/sec. This means that the contribution of dynamic recrystallization is negligible. In the meantime, grain size can increase to a few tens of µm under the middle crustal condition (at 400 °C where pressure and water fugacity are calculated with a geological temperature and pressure gradients of 25 °C/km and 27 MPa/km, respectively) and to ~a few hundred μm under the lower crustal condition (at 600 °C). Our results indicate that there can be natural conditions where grain growth can be dominant even plastic deformation is operating