A. I. Smul’skaya

Study of nanocrystals in the dynamic slip zone

Mineral composition is studied and a search to detect nanocrystals is conducted in the surface layers of slickensides formed due to dynamic slip in arkose sandstone. The infrared and Raman spectroscopy show that the slickensided layer is composed of nanocrystals of montmorillonite and anatase measuring ≈15 nm and 3 nm, respectively. The crystalline lattice of the nanocrystals of montmorillonite is stretched by ≈2.5% while the lattice of the nanocrystals of anatase is compressed by ≈0.12%. Deeper than 3 mm below the slickenside surface, the sandstone contains nanocrystals of montmorillonite, beidellite and nontronite, quartz, plagioclase, and anatase. The nanocrystals of anatase have a linear size of ≈8 nm. Their crystalline lattice is compressed by ≈0.03%. It is supposed that montmorillonite in the slickensides was formed due to hydrolytic decomposition of silicates under friction of the fault planes sliding past each other.

Effects of high pressure and temperature on the properties of nanocrystals in rocks: Evidences from Raman spectroscopy

A search is conducted to detect nanocrystals in a sample of apogranitic pseudotachylite, which is a product of extremely strong crushing of granite in a seismogenic fault. Raman spectroscopy revealed nanocrystals of quartz measuring approximately 17 to 25 nm and low-temperature albite ranging from 8 to 30 nm. The crystallographic cell in the nanocrystals is deformed. The internal stresses which might have been responsible for these deformations vary from approximately −300 (compression) to +480 (tension) MPa. It is found that after having been exposed to high pressure (1 GPa) and temperature (470–500°C for 10 minutes and 550–600°C for 16 minutes), the nanocrystals of quartz reduced in size to ≈10 nm, and the nanocrystals of albite, to 13 nm. At the same time, the level of tension in the lattice spacing of quartz increased.

Nanostructures in the deep xenolite before and after straining

A search for nanocrystals in the sample of deep rock, mantle xenolite from the kimberlitic tube, was carried out. With the use of the Raman spectroscopy method pyrope nanocrystals measuring ∼18 nm and omphacite nanocrystals measuring ∼13 nm were identified. The dimensions of the crystallographic cell in nanocrystals were increased in comparison with macrocrystals. The internal tensile stresses, which could cause these changes, were evaluated by a value of ∼1.1 GPa. The action of quasihydrostatic compression pressure with a value up to 2.5 GPa on a change in the structure and properties of nanocrystals was investigated. As a result of the compression, the sizes of pyrope nanocrystals did not change, but the dimensions of the crystallographic cell increased. The method applied did not make it possible to reliably determine the changes as a result of the pressure of the internal stresses and the sizes of the omphacite nanocrystals.

Raman spectroscopy of nanocrystals in rock

Nanocrystals were detected and identified in rocks by the method of Raman spectroscopy. The experiments showed that the Raman scattering spectra of fine-lamellar arkosic sandstone exhibit bands corresponding to lattice vibrations of anatase, α-quartz, and plagioclase. In all spectra of the rock, the bands are displaced towards high frequencies as compared with their position in spectra of single crystals and widen on the same side. These results show that, in all of the studied places of the sample, the particles of anatase, quartz, and plagioclase have nanometer sizes, namely, of the order of 10 nm in anatase and quartz and about 20 nm in plagioclase. Moreover, in different places of the sample, not only the shape and position of the bands under study but also their intensity vary, the latter being directly proportional to the concentration of nanocrystals.

IR spectroscopy of quartz nanocrystals formed during intense crushing of a heterogeneous material (granite)

The spectra of the imaginary part ɛ″(ν) of the permittivity of quartz single crystals and a heterogeneous material, i.e., pseudotachylite, formed during intense crushing of granite in the region of the seismogenic Earth’s crust fault have been calculated from IR reflection spectra. It has been found that all strong bands in the pseudotachylite spectrum ɛ″(ν) correspond to lattice vibrations in quartz nanocrystals. Bands are asymmetrically broadened due to dielectric and phonon confinements. Linear sizes of quartz nanocrystals have been estimated from the broadening as ∼70 nm. The frequency of nanocrystal lattice vibrations is higher than that of the macrocrystal, which is caused by lattice compression. The internal stresses which could cause the observed change in the frequency are ∼200 MPa.

Effect of water on the α-β phase transition in a surface quartz layer

The temperature dependence of the α-phase concentration in surface layers and in the bulk of quartz plates cut out at a distance of ∼2 mm from the natural growth surface of druses extracted at the Dodo deposit in the Polar Urals has been studied using infrared and Raman spectroscopy. It has been found that, in the bulk of the sample, the temperature dependence behaves as expected for a first-order phase transition; more specifically, below 800 K, it remains unchanged and, at high temperatures, approaches zero. In surface layers with thicknesses of ∼0.15 and ∼0.8 μm, the α-phase concentration decreases monotonically by approximately 10% with an increase in the temperature to 780 K. The temperature dependence of the α-phase concentration in the layer at a depth of ∼6 μm passes through two minima, namely, at ∼370 and ∼570 K, at which the concentration of this phase decreases by about one half. This is accompanied by an increase in the concentration of the β-phase. The revealed behavior of the α-phase concentration with an increase in the temperature has been assigned to the influence of water on crystal lattice distortions near growth dislocations. At 370 K, free water evaporates from grain boundaries, and at 570 K, the water bound by hydrogen bonds to the SiOH groups. The evaporation of water affects stresses at grain boundaries, and it is this factor that brings about a change of the α-phase concentration. It has been demonstrated that tensile stresses generated with increasing temperature in a near-surface quartz layer to ∼0.8 μm thick can reach ∼170 MPa. The stresses create microcracks, which culminate in destruction of the sample. The generation of the tensile stresses is explained by an increase in the volume of the microcrystal layer located at a depth from ∼1 to ∼8 μm from its surface as a result of the increase in the β-phase concentration in it.

Temperature-induced phase transition in quartz nanocrystals dispersed in pseudotachylite

The size and concentration of α-quartz nanocrystals dispersed in samples of pseudotachylite and the internal stresses in these nanocrystals have been determined using infrared spectroscopy in the temperature range 300–800 K. Pseudotachylite is a product of intense crushing of granite that undergoes in the Earth’s crust faults. It has been found that the size of the nanocrystals is ∼20 nm and does not depend on temperature. As the temperature increases, their concentration decreases monotonically and tends to zero at ∼650 K. This process is paralleled by a growth of the concentration of β-quartz nanocrystals. The α-quartz nanocrystal concentration regains its initial level with decreasing temperature. Thus, the α → β phase transition in quartz nanocrystals in pseudotachylite starts at temperatures lower by ∼500 K than that in the bulk of the macrocrystal (846 K), and is stretched by ∼350 K. At room temperature, the unit cell of nanocrystals is compressed by surface tension forces. These forces retard the α → β phase transition. The thermal expansion coefficient of nanocrystals is larger than that of macrocrystals, which entails a decrease of compression and a monotonic decrease of the concentration of α-quartz nanocrystals with increasing temperature.

Chuchkhur xenoliths and tectonic position of Middle Paleozoic volcano-sedimentary sequences in the Peredovoi Range, northern Caucasus

The Peredovi (Frontal) Range zone is character� ized by the most complete geological record as com� pared with other preAlpine structures of the Greater Caucasus. Its section in the wide northwestern seg� ment (Laba River basin) includes the following struc� tural units (from the base upward): Blyb metamorphic complex and its analogues; Devonian-Lower Car� boniferous to, locally, Silurian (Urup-Tokhan Com� plex) greenstone volcanosedimentary sequences; Atsgara nappe of metamorphic rocks and granitoids; and ophiolites of the Marukh nappe (figure). The neoautochthonous cover of the zone is composed of upper Visean-Permian molasses. The existence of the

Tectonic and geomechanical control of dikes and sill-like bodies: Evidence from the northwestern part of the Kola Peninsula

A study of the meticulously documented Paleoproterozoic swarms of basic dikes and sill-like bodies, as well as granite veins crosscutting Archean granite-gneiss country rocks of the Central Kola Geoblock of the Fennoscandian Shield, elucidates the question of geomechanical control of the spatial location of syntectonic sheetlike magmatic bodies intruding into heterogeneous structured geomedium. Based on structural analysis and mapping results, the succession of emplacement of several dike generations has been reconstructed and linked to structural parageneses of the corresponding deformation stages. We evaluate the effect of geomechanical and tectonic factors as well as the structural elements of enclosing strata on the places of dike localization, the character of their spatial distribution, morphology of particular bodies, and patterns of swarm systems. Geomechanical problems on the intrusion of single bodies and their communities are solved taking into account their interaction and the heterogeneity of the medium. The conditions necessary for transition of nearly vertical dikes into sills are discussed.

Structural and material records of paleoearthquakes in terrigenous rocks: Analysis and interpretation

The results of an instrumental and analytical investigation of the products of mineral and textural transformations in the tectonic slickensides and fault gouge in the near-surface terrigenous sedimentary rocks (clays, arkose sandstones, shungites) which have undergone localized deformations in fault zones of presumably seismogenic origin are presented. Based on this, several peculiarities in the behavior of a dynamic slip in the upper transition horizon from aseismic to seismogenic mode of faulting in the Earth’s crust are elucidated. The changes in the mineral phase compositions of the fault facies against the protolith composition are estimated; the parameters of the temperature regime and thermal energy balance of deformational metamorphic reactions are determined. The probable causes of instability in the faults, the mechanisms of the loss of strength, the weakening and strengthening during seismogenic dynamic slip are considered. The role of tribochemical phenomena in the course of a rock’s transformation into a fault gouge and the related energy effects are discussed. An inventory of the possible energy costs on the processes of transforming material in dynamic slip zones is compiled.