Water Redistribution in Vein Quartz Under Progressive Deformation (During Plastic Deformation): μFTIR and EBSD Study (Western Transbaikalia, Russia)

Abstract

Water distribution in the structure of vein quartz formed as a result of successive plastic deformations associated with dislocation slip and subsequent recrystallization was estimated using infrared microspectroscopic (μFTIR) mapping. Water contained in quartz demonstrates a broad absorption band in the IR range at 2800–3750 cm⁻¹, which indicates its molecular state and suggests the presence of water bearing water inclusions. In addition to water, the presence of an absorption band located at 2341 cm⁻¹ seems to be due to the presence of carbon dioxide in a molecular state. A necessary step before assessing the distribution of volatile components in the quartz structure was to calibrate the boundaries obtained by calculating the intensity ratios of the peaks at 1118 and 1160 cm⁻¹ in the reflectance spectrum and using electron back scatter diffraction (EBSD). A variety of fluid distributions in different elements of the structure was observed. At medium temperatures and medium strain rates, dislocation mass transfer is effective during dislocation slip. At low strain rates and elevated temperatures, the contribution of diffusion creep gradually increases, which facilitates the interaction of volatile components with migrating boundaries. It was found that in the process of successive rearrangements, migration of fluid components occurs within the main elements of the structure due to the redistribution of dislocations between defects of different scale levels. Redistribution of fluid from fluid inclusions as a result of plastic deformations in the quartz structure is one of the ways of relaxing intracrystalline stresses during strengthening of the structure.

1. Introduction

The crystal structure of quartz consists in spirals of SiO₄ tetrahedrons connected through vertices. Rotation around the c-axis by 60° and translation of one third of the unit cell height along the axis leads to coincidence with the former position of the tetrahedrons [1] and provides the presence of a third-order screw axis. Because of external influences and disruptions to natural growth processes, the crystal lattice can undergo changes with the formation of point, linear and volumetric defects. The appearance of defects is accompanied by disordering in the arrangement of atoms, and changes in the parameters of the crystal cell, and leads to an increase in the internal energy and entropy of the quartz crystal. During the transition from brittleness to plasticity under conditions of dynamo-metamorphism, plastic deformations contribute to the transformation, redistribution and evolution of defects during the relaxation of applied stresses.

Disruption of the crystal structure of a mineral as a result of structure defects leads to the formation of fluid inclusions that contain molecular water. The crystal structure is characterized by a broad absorption band in the region of ~3400 cm⁻¹, consisting of a superposition of molecular H₂O bands and few discrete absorption bands [2,3,4]. Under the influence of stress and temperature changes, the shapes of fluid inclusions are modified. Cracks are formed in adjacent areas and then heal with the formation of new vacuoles [5,6,7,8]. At this moment, molecular water is redistributed from larger inclusions to smaller ones in those parts of the crystals that were previously free of inclusions [9]. In this case, a close spatial connection between newly formed submicroscopic water bubbles and dislocations is observed [10,11].

The interaction between fluid inclusions and dislocations [12,13] during their slip leads to water recirculation between inclusions and the dislocation core [14,15,16,17,18]. OH defects alter some macroscopic physicochemical properties of quartz, including its rheology and piezoelectric characteristics [19,20,21,22]. They also provide pathways for diffusive hydrogen flow through the quartz, by which the fluid inclusions contained in the quartz can be modified [23,24,25]. This may lead to an increase in water saturation of quartz and as a consequence facilitate its plastic deformation through hydrolytic weakening [7,26,27,28,29,30,31,32].

According to the results of numerous experiments, the crystal chemistry of the interaction of the quartz crystalline structure with water during the redistribution of fluid is associated with deformation processes. The recorded increase in the size of fluid inclusions during heating of quartz for 112 h at low deformation rates indicates the processes of water redistribution during deformations and demonstrates the dependence of the strength of single quartz crystals in relation to the creep process on the nature and distribution of extended defects and inclusion groups [20,33,34,35]. When quartz deforms under creep conditions, efficient matter transport along grain boundaries is observed [36,37,38]. In quartz from naturally deformed metamorphic rocks, researchers have observed an interconnected porosity network formed by nanometer-scale open grain boundaries, cavities and depressions, which allows fluid circulation and can influence the physical properties of rocks [39,40]. In this regard, we recorded three types of voids at grain boundaries: (1) open pores of approximately 40–500 nm width disposed parallel to grain boundaries; (2) cavities of variable shape up to one micrometer in size along open grain boundaries; (3) nanometer-sized cone-shaped depressions where dislocation lines intersect with open grain boundaries [40]. The boundaries migrating during quartz recrystallization promote drag, redistribution and removal of the fluid phase [41,42,43,44]. Meanwhile, the large difference in the wetting characteristics of CO₂ and brines implies that carbon dioxide-rich fluid will collect at grain boundaries and water will redistribute laterally.

Thus, the evolution of defects in the deformation structure of quartz caused by transformations occurring under dynamo-metamorphic conditions is accompanied by redistribution of fluid from fluid inclusions [45]. Analysis of the behavior of this fluid during successive structural rearrangements shows the influence of deformation processes on fluid migration and demonstrates the role of fluid as a major factor controlling metamorphic reactions [46,47,48]. However, most experiments designed to address this problem are one-act experiments and do not reflect the nature of sequential interactions between the deformation structure and fluid from fluid inclusions. This is due to the variety of options of deformation structure formation under different mechanisms. In this paper, we present the results of IR mapping measurements to show how water is distributed as a result of successive rearrangements of the quartz structure associated with dislocation slip processes and subsequent recrystallization.

2. Object of Research

The quartz of the Krivoy brook vein system (Krivoy zone), located within the Baikal-Vitimskaya folded system in the eastern part of the Sayan-Baikal folded area, has been studied by [49,50]. Quartz veins are localized among the two-feldspar granites of the Bambukoy complex of the Late Riphean age. Granites are subject to intensive schistation, fracturing and metasomatosis (albitization, sericitization). The formation of quartz veins is associated with two deformation stages. The first stage is expressed by the development of a northwest-trend thrust system. The second stage is associated with the superimposed shear-fault component of latitudinal (~240∠80 NW) and northeast-trend (~263∠76 SW) [51].

2.1. Structure of the Vein System

According to the features of orientation in the structure of the vein system, three types of veins are distinguished (Figure 1). The predominant position is occupied by powerful main veins of southwestern dip (in Figure 1, highlighted in blue). Closely associated with them are veins dipping at low-medium angle of meridional and east-northeast strike, which are limited in strike (highlighted in pale pink in Figure 1). The third type of vein bodies is represented by dipping at high-medium angle south-east and north-west trending veins up to 10 cm thick (highlighted in pink in Figure 1).

Based on the analysis of the structural position of veins (Figure 1e), it can be concluded that the development of quartz veins of the Krivoy zone is controlled by the rear zones of thrust deformation and is complicated by left-lateral latitudinal strike-slip deformations (~173∠76 SW) and northeastern strike-slip deformations (~330∠80 SW). Southwest-dipping main veins (~242∠30 SW) are associated with the development of fault deformations during the formation of the thrust system. Low-angle dipping veins, which are limited along strike (~310∠68 NW), and high-angle dipping to the southeast veins (~332∠78 NW, ~170∠70 SW) are associated with the process of stretching and the development of two generations of separation cracks during the formation of flank upwash shear deformations.

2.2. Diversity of Microstructures and Structural Elements

The previously conducted structural and crystallographic analysis of quartz demonstrates a significant deformation heterogeneity, expressed in the manifestation of signs of dynamo-metamorphism and the appearance of various microstructure types. Detailed results of the systematization and classification of deformation structures of vein quartz based on petrographic and electron microscopic studies are presented in [52]. Optical microscopic studies have made it possible to identify five main types of quartz microstructures: undeformed protogranular microstructure of type A, weakly deformed fragmentary microstructure of types B and C, deformed pseudo-porphyroblastic microstructure of type D, granular microstructure of type E (Figure 2). In the spatial distribution of these structures, no clear patterns have been established relative to the contacts of veins and structural elements of the host rocks (layering, cleavage, shale). Quartz microstructures alternate each other in the vein space, determining the block-cellular structure of vein bodies, where isolated areas of weakly deformed aggregates are separated from each other by zones of increased deformation.

Typical microstructure elements are subgrains, deformation bands, mechanical Dauphiné twins and recrystallization grains (Table 1). The undeformed protogranular microstructure of type A is characterized by large grains bounded by large-angle boundaries of the general type (>65°) (Figure 2a). Within these grains are fixed Dauphiné twins, which were shown by the EBSD method. Quartz grains in which subblocks and deformation bands are recorded are characterized by a fragmentary microstructure (Figure 2b). Quartz of the pseudo-porphyroblastic microstructure demonstrates the presence of areas of development of groups of recrystallized grains confined to deformation bands (Figure 2e) or located in the body of the parent grain and at their boundaries (Figure 2c,d). When the stage of the polarizing microscope is rotated, such subblocks go out in random order. Medium-sized (from 1 mm) isometric weakly deformed grains with smooth, wavy boundaries (Figure 2f) characterize the granular quartz microstructure. Such grains may form groups surrounded by quartz of pseudo-porphyroblastic microstructure or be linearly confined to deformation bands.

The analysis of structural heterogeneity of vein quartz aggregates allowed us to establish that the five types of microstructures were formed in the course of successive structural rearrangements reflecting the stages of deformation transformations. These types of structural rearrangements, and types and stages of deformations are described and detailed in [45]. The diagram of deformation mechanisms of quartz (Figure 3) schematically represents the sequential process of rearrangement of quartz aggregates: black color indicates large-angle boundaries, green—small-angle boundaries, red—Dauphiné twin boundaries.

The transformations of the first stage (I) are linked by the effects of strains arising from synkinematic crystallization. This favors the initiation of dislocation slip at medium velocities and temperatures of 300–400 °C, and the formation of Dauphiné twins. In the course of this, quartz with deformation Dauphiné twins (type A) and subsequent or simultaneous formation of subblock boundaries and deformation bands (types B and C) is formed in all vein systems.

Transformations of the second deformation stage (II) are marked by the beginning of the process of dynamic recrystallization under conditions of low strain rates at increasing temperature (up to ~600 °C). The rearrangement of aggregates in this case leads to recrystallization associated with dislocation creep and the formation of a pseudo-porphyroblastic microstructure (type D). Successive microstructure transformations within the three modes of dynamic recrystallization initiate the scattering of twin boundaries and subsequent static recrystallization under diffusive creep conditions. As a result, the previously formed twins partially unfold. This leads to the formation of transparent quartz of the grain microstructure (type E).

We performed IR mapping of the described quartz aggregates and compared them with orientation maps obtained by the EBSD method in order to characterize the distribution of water in grains and boundaries of different types.

3. Materials and Methods

3.1. Sample Selection

To study the quartz samples, transparent-polished plates were prepared and sawed off perpendicularly to the vein strike. The quartz surface was prepared by mechanical polishing and ion etching on a multifunctional device, SEMPrep2 (Technoorg Linda Co., Ltd., Budapest, Hungary). Carbon sputtering of the samples for EBSD analysis was carried out on an EMITECH K450X machine. The sputtering layer was 30 nm.

3.2. EBSD Analisis

Samples of vein quartz were studied using the reflected electron diffraction (EBSD) method on the scanning electron microscope Tescan Mira 3 LMU (Tescan, Brno, Czech Republic) on the equipment of the Tomsk Regional Collective Use Centre of the National Research Tomsk State University. The scanning electron microscope was equipped with a Nordlys F device (Oxford Instruments, Abingdon, UK) for the EBSD diffraction method.

Imaging was performed at an accelerating voltage of 20 kV, beam current ~0.8 nA, working distance of about 15 mm, and sample inclination of 70°. EBSD imaging was performed on a rectangular grid by moving the electron beam with a regular step of 2 or 4 μm (at least 10 pixels per grain). The scanning area was 1 mm².

The results were processed using Oxford Instruments AZtec licence software (version 4.2). The obtained EBSD data were analyzed using HKL Channel 5 software (Oxford Instruments, UK). When dividing the boundaries into small- and large-angle boundaries, a disorientation angle value of 10 degrees was taken [56,57]. The overall indexing for trigonal quartz ranged from 80 to 90%. In order to identify Dauphiné twins, boundary trace analysis was used in the processing of the EBSD data. From the crystallographic analysis, 3 main characteristics of the boundaries can be extracted: crystallographic orientations of adjacent grains <hkul>, the magnitude of the disorientation angle (ѳ), and the crystallographic orientation of the rotation axis <uvw>. When interpreting the orientation maps obtained by the EBSD method, the color corresponds to the orientation on the inverse pole figure (IPF) map.

3.3. μFTIR Analysis

Reflectance and absorption spectra of prepared quartz plates were collected using a Mikran-3 infrared microscope and a Simex-801 spectrophotometer. The aperture size of the imaging was 25–30 μm. The signal was recorded using a liquid nitrogen-cooled MCT detector. The spectral resolution was 4 cm⁻¹, and the signal was averaged over 3–6 scans. The reflectance spectra were measured in the 650–4000 cm⁻¹ spectral region. The absorbance spectra were measured in the 2000–4000 cm⁻¹ spectral region due to the sample thickness.

The infrared radiation incident on the sample is depolarized, but the reflected beam acquires polarization. The different orientations of the grains are best observed in the reflectance spectra. The authors in [58] demonstrated that the ratio of the peak at 1118 cm⁻¹ to the dip at 1160 cm⁻¹ varies significantly depending on the angle between the c-plane and the incident beam. Thus, by IR reflection spectra, it is possible to estimate the disorientation of grains relative to each other and to register the grain boundaries.

Thus, it is possible to determine grain orientation and grain boundaries by measuring reflectance spectra. Subsequently, without moving the sample, absorption spectra can be measured to determine the distribution of water and gas–liquid inclusions within the sample.

The sample for such measurements should be prepared accordingly. Firstly, its surfaces should be polished to optimize the measurement of the reflection spectra. On the other hand, the sample should be a plane-parallel plate with a thickness of 100–200 μm to ensure optimal measurement of the absorption spectra in the absorption region 4000–2200 cm⁻¹ where bands related to the stretching vibration of CO₂, OH and organic groups are located.

3.4. Calibration of Methods by Misorientation Angle

In this study, the determination of the grain boundary position and c-axis orientation of quartz were compared using EBSD analysis and μFTIR analysis. After mapping the quartz plate using reflectance μFT-IR, the ratio of the intensities of the peak at 1118 cm⁻¹ and the dip at 1160 cm⁻¹ (Ref1118/Ref1160) at each point was calculated, as was the variation in the c-axis from perpendicular to the imaged plane following the procedure described in [58]. The peak at 1118 cm⁻¹ is attributed to the asymmetrical stretching vibration of Si-O. The reflectance spectra are shown in Figure 4. It is clearly seen that the ratio between the peak at 1118 cm⁻¹ and the dip at 1160 cm⁻¹ differs for grains whose orientations do not match. Details of the methodology are described in [58]. In this article, it was pointed out that the second relationship between the peak at 1180 cm⁻¹ and the dip at 1160 cm⁻¹ also depends on grain orientation. However, this relationship is much less sensitive in comparison to the ratio (Ref1118/Ref1160). The same section was then imaged using EBSD, where the deviation in the c-axis from perpendicular to the sample plane was also determined using a straight pole figure [0001]. The results obtained showed good convergence, allowing the use of reflectance μFT-IR together with the technique of [58] to roughly determine the grain boundaries and c-axis orientation of quartz. However, reflectance spectroscopy only provides information on the c-axis orientation of the quartz crystal perpendicular to the imaging plane, without considering the direction of deflection and rotation around the c-axis. Is thus is impossible to obtain the true orientation of the crystal based only on the reflectance μFT-IR data. Reflectance μFTIR is not of any help in order to identify Dauphiné-type twinning, which is expressed as a 60° rotation about the c-axis. The true orientation of the crystal can be obtained by EBSD, where the orientation of the crystal is determined by the three Euler angles. It can be concluded that reflectance μFT-IR is suitable for grain identification, but for analyzing deformation microstructures, it is necessary to use this method in combination with EBSD.

4. Results

We have performed IR mapping of quartz aggregates of different microstructures and analyzed the distribution of water and carbon dioxide within selected structural elements reflecting different plastic deformation conditions.

4.1. Type A Microstructure

Quartz of the protogranular microstructure is characterized by large grains with homogeneous internal texture and insignificant blockiness expressed by the appearance of low-angle boundaries. Within the grains, the reflected electron diffraction method reveals continuous boundaries of Dauphiné twins, which form two main orientations corresponding to orientations close to the angles of the standard inverse stereographic triangle of the <1010> type (Figure 5a). Grains with a closed, sinuous boundary (highlighted in green) are located in the matrix of the mother grain (highlighted in purple). They are separated from each other by a twin boundary with a disorientation angle of 60°. The map of intensity ratios of peaks 1118 and 1160 cm⁻¹ in FTIR reflection spectra (Figure 5b) shows two orientations, the boundary between which resembles the twin boundary in shape, but does not always coincide with its contours. The distribution of water in the structure of the Dauphiné twins is not regular (Figure 5c,d). The areas containing water have an isometric shape and are concentrated both near the boundaries and inside the grain. More water-containing regions are observed in one of the twins than in the other. The character of carbon dioxide distribution in quartz of protogranular microstructure is also not regular. Several isometric-shaped regions are observed, which do not cross the twin boundary (Figure 5e). The absorption spectra in the region 2000–4000 cm⁻¹ are given in Figure 5f. The peak at 2350 cm⁻¹ corresponds to the CO₂ asymmetric stretching mode, but the wide band at about 3400 cm⁻¹ corresponds to the H-O-H stretching modes.

4.2. Type B + C Microstructure

In quartz of fragmentary microstructure, a large number of small-angle boundaries formed as a result of dislocation slip under the conditions of the first stage of deformation are observed (Figure 6a). The small-angle boundaries confine elongated subblocks up to 0.06 mm wide. From IR mapping data, the distribution of molecular water (3400 cm⁻¹) and carbon dioxide (2350 cm⁻¹) (Figure 6f) in the fragmentary microstructure of quartz follows the trend of low-angle boundaries. The regions with increased and decreased water and carbon dioxide contents have an elongated shape and almost coincide with the shape of the subblocks. The regions containing water and carbon dioxide occupy the same position and overlap (Figure 6e). However, water has a wider distribution compared to carbon dioxide.

4.3. Type D Microstructure

In the studied fragment of a quartz vein of pseudo-porphyroblastic microstructure, two domains separated by a bay-shaped large-angle (>65°) migrating boundary are recorded (Figure 7a). To the left of the boundary, a large grain with a primary protogranular microstructure is noted. On the right is a grain that has undergone recrystallization processes and contains a newly formed recrystallization grain with a different orientation from the primary grains. The disruption of the continuity of the twin boundary of this grain (highlighted in red) indicates its formation during successive structural rearrangements and serves as evidence of the recrystallization process [45].

IR mapping of this quartz fragment demonstrates a diverse distribution of volatile components in grains with different microstructures. Grains with protogranular microstructure are characterized by the same position of regions with elevated water and carbon dioxide contents. Such regions have predominantly isometric shape. In the domain on the other side of the migrating large-angle boundary, the separation in space of water and carbon dioxide phases is fixed. In Figure 6d in this domain, the distribution of regions with increased carbon dioxide content orthogonal to the migrating boundary is observed. The point-like nature of its distribution indicates its preferential localization along mobile dislocations or subgrain boundaries due to its lower wetting and infiltration ability compared to water [59]. Water continues to occupy wide areas, but its intensity drops markedly near the migrating boundary (Figure 7c). As a result, the newly formed recrystallization grain is almost free of water and carbon dioxide (Figure 7e).

4.4. Type E Microstructure

Weakly deformed grains with smooth, wavy, large-angle boundaries (Figure 8a) characterize the granular microstructure of quartz. Such grains were formed during static recrystallization under diffusive creep conditions. They are characterized by ubiquitous water distribution and local point distribution of carbon dioxide. Figure 8c shows the absence of significant water contents within the grain boundary. A characteristic feature is the similar pattern of the grain boundary and the boundary of the water-free region. This indicates water migration processes associated with boundary rearrangement. The areas of carbon dioxide distribution in granular quartz (Figure 8d) have a much smaller, local distribution area and accumulate near low-angle disorientations. This is clearly seen when comparing the disorientation map obtained by IR reflectance mapping (Figure 8b) and the carbon dioxide distribution map. In addition, carbon dioxide actively accumulates around and within the large-angle grain boundary (Figure 8e).

5. Discussion

Observed transformations in the quartz structure associated with stress relaxation during plastic deformation reflect the evolution of defects, which are accompanied by mass transfer and fluid redistribution. The migration of fluid components during structural rearrangements with successive complexity and hardening, and its relationship with plastic deformation, can be observed. According to the data obtained by FT-IR analysis, a broad absorption band at ~3400 cm⁻¹ was recorded in all quartz samples, which corresponds to molecular water in fluid inclusions [60,61]. The stretching mode of water strictly probes vibrations along the O-H bond. However, the influence of neighboring hydrogen bonds alters the intramolecular O-H bond and consequently the infrared absorption spectrum. The shape of the liquid water band is described by several Gaussian bands. The interpretation of the complicated shape of the water stretching vibration bands depends on the cooperative hydrogen bonding arising from a network of hydrogen bonds in the liquid [62]. On the other hand, the shape of the wide absorption band around 3400 cm⁻¹ depends on the concentration of NaCl or other salts dissolved in water inclusions [63,64]. This explains the fact that the shapes of the 3400 cm⁻¹ bands in different samples are slightly different. In the future, after calibration with inclusions of known salinity, infrared determination of salinity in inclusions could be possible. Additionally, the absorption bands of Al-OH-bearing defects could be located at approximately 3313 and 3379 cm⁻¹ [3]. However, the FWHM of these bands is much lower than that of molecular water because the OH− groups are located in anionic positions. On the other hand, for molecular water, the broadened bands are typical due to the varying bond lengths between O-H. Furthermore, the samples do not contain a sufficient amount of Al impurities, which was not detected using SEM or Electron Paramagnetic Resonance after sample irradiation.

On this basis, the process of fluid redistribution in the quartz crystal structure will be considered in close connection with fluid inclusions [45]. The sequential scheme of migration of fluid components in the quartz deformation structure during the development of two stages of plastic deformation was taken into consideration.

During the first stage of plastic deformation under dislocation slip conditions (Figure 3), depending on the initial orientation of the quartz grain relative to the normal stress axes, subblock and fragmentary structures or boundaries of Dauphiné twins are formed [53,65]. As a result, quartz crystals containing Dauphiné twins (protogranular microstructure) retain regions with insignificant deformation manifestations and inhomogeneous distribution of fluid inclusions. Within such a structure, water-carbonic fluid is concentrated in inclusions of isometric shape, and is practically not involved in deformation. An interesting feature is that the regions containing volatile components do not cross the boundaries of the Dauphiné twins (Figure 5). In addition, one of the twins contains a much larger number of fluid inclusions. The detected twin boundary is not traced by areas containing water or carbon dioxide, despite being a high-scale defect [66,67].

During the ongoing dislocation slip on the walls of the vacuoles of fluid inclusions trapped in quartz, dislocations are formed, due to which the fluid inclusions act as stress concentrators, which is confirmed by experiments [11]. The process of redistribution of volatile components under conditions of low temperatures and medium deformation rates as a result of dislocation slip is recorded during FTIR mapping of the subblock and fragmentary microstructure of quartz (Figure 6). Activation of slip planes in quartz leads to local deformation in the region around inclusions, movement of point defects [10], cracking of inclusions and their subsequent healing [68]. Due to this, the shape of the vacuoles of fluid inclusions changes and they are pulled into linear directions [69]. Figure 6 shows the correspondence between the elongated shape of the regions occupied by volatile components in fluid inclusions and the preferred orientation of structural elements. As a result of local deformations, excessive internal fluid pressure inside the inclusion vacuole provokes fracturing with the formation of many small newly formed inclusions. According to experimental data [69,70], such secondary inclusions contain more salt and gas, while water participates in crack healing around the inclusions [68] and is transported via dislocation mass transfer [12,13,66] to larger-scale defects. This study confirms the experimental results by recording more localized regions of carbon dioxide distribution compared to water (Figure 6), suggesting residual carbon dioxide accumulation at the site of secondary inclusions and water migration.

The second stage of deformation of quartz veins occurs under conditions of temperature increase at low strain rates (Figure 3). Structural rearrangements of quartz aggregates are carried out due to the development of the dynamic recrystallization process as a result of creep. At this stage, the main mechanism of fluid migration in the quartz structure is the movement of migrating boundaries [43]. High temperatures at low strain rates increase the diffusion gradient and the contribution of diffusive mass transfer [71,72], which influences further redistribution of volatile components. The processes of slow boundary migration contribute to the intensive development of low-angle boundaries, and the appearance of subgrains and recrystallization grains [54]. The diffusing fluid is involved in the recrystallization process by redistributing from higher pressure regions to lower pressure regions due to the boundary migrating during recrystallization [38,73]. Areas without volatile components gradually appear in the quartz structure. When comparing the orientation map of quartz of pseudo-porphyroblastic microstructure (type D) and the water distribution map, areas represented by subgrains are limited by low-angle boundaries, where no water and carbon dioxide contents are.

Due to the difference in wetting characteristics, water and carbon dioxide migrate differently in the deformation structure of quartz, which leads to their separation in space. The hindered diffusion of carbon dioxide [74] leads to its minimal mobility and promotes the fixation of grain boundaries during the process of phase transfer along mobile dislocations or subgrain boundaries through the creep mechanism. Water, in turn, redistributes laterally, having a greater wetting and infiltration capacity than carbon dioxide [59]. Thus, the formed recrystallization grains of pseudo-porphyroblastic microstructure (less than 1 mm in size) are almost completely free from fluid inclusions containing volatile components (Figure 7).

Further temperature increase and recrystallization contributes to stress equalization in deformed quartz as a result of annealing under diffusive creep conditions [75]. This leads to the reconstitution of the crystal structure and its subsequent equilibration, which is accompanied by the disappearance of dislocations and growth of new grains with a diffuse texture (grain microstructure, type E) more than 1 mm. Intensive processes of dissolution and precipitation of grain boundary material occur [76], affecting water redistribution within the grain boundaries, its diffusion across grain boundaries and its release. As a result, a characteristic water-free region within the grain boundary that follows the contours of the boundary (Figure 8) can be observed. Similar evidence for some water release along grain boundaries during recrystallization was obtained in IR mapping of quartz from granites [77]. At this stage, towards the end of the cycle of structural rearrangements, a significant increase in the areas occupied by water within the grain (Figure 8) is observed in contrast to the initial stages (Figure 5). This indicates a gradual redistribution of fluid into regions not initially occupied by fluid inclusions [9,14]. According to previous studies, quartz grains formed at this stage contain a large number of water–salt fluid inclusions, which are small and elongated in shape, orthogonal to the few grain boundaries [45]. This allows us to observe the processes of final migration of volatile components to grain boundaries and their destruction [31].

Figure 9a shows a map of quartz grain orientations, within which areas with different types of microstructure are observed, which records the fragmentary nature of structural rearrangements. In the lower part of the fragment, grains of pseudo-porphyroblastic microstructure (type D) are presented, and in the upper part, two large grains formed during static recrystallization (type E) can be seen. The obtained maps of the distribution of volatile components in the deformation structure of quartz for this fragment demonstrate the same patterns of fluid redistribution described before. Intensive migration of fluid is recorded during dynamic recrystallization, and expressed in the gradual release of space for new subgrains (type D) (Figure 9b,c). In this case, migration processes can be described by the mechanism of volume diffusion as a result of creep, where the diffusion flow occurs toward or from the grain boundary and depends on the prevailing tensile or compressive stresses, respectively [73]. Subsequently, during grain coarsening during static recrystallization (type E), active migration processes are observed within the grain boundaries, which determines the Koble creep (diffusion along grain boundaries) [53]. Based on this, it can be argued that the main factor influencing the redistribution of fluid during recrystallization under creep conditions is the pressure.

The simultaneous occurrence of the described processes demonstrates the migrating, wave-like character of deformation with the sequential involvement of structural concentrators of the same type in different parts of tectonic zones, which is energetically more favorable. At the same time, the process of structural evolution acquires a heterogeneous, migrating character in both space and time.

It is a well-known fact in crystal lattice mechanics that pores or inclusions are preferential areas for dislocation nucleation during plastic deformation. At the initial stages of deformation, dislocations are formed around the inclusions. Then, due to the evolution of dislocations, the inclusions change shape until slip planes are activated. Activation of the slip plane leads to redistribution of the aqueous phase inside the grain. Lateral redistribution of the aqueous phase promotes local hydrolytic weakening and an increase in the diffusion gradient [30,32,38]. Hindered diffusion around carbon dioxide [74] probably contributes to the pinning of grain boundaries. Thus, the processes of fluid redistribution in the quartz structure accompany the migration of dislocations between defects of different scale levels during plastic deformations and are subject to the general evolution of defects. Fluid migration during plastic deformations is one of the ways of relaxing intracrystalline stresses.

6. Conclusions

The spatial distribution of volatile components in the deformation structure of vein quartz from the Krivoy zone in the Sayan-Baikal folded region using IR mapping was studied. The distribution of water and carbon dioxide in quartz aggregates of protogranular, fragmentary, pseudo-porphyroblastic and grainular microstructures formed during two stages of plastic deformations was also studied. The stages of successive structural rearrangements are associated with the processes of dislocation slip and subsequent recrystallization under creep conditions. According to IR mapping, a broad absorption band at ~3400 cm⁻¹ and a band at 2350 cm⁻¹ are observed in all quartz samples, which correspond to molecular water and carbon dioxide, respectively. In the studied quartz, water and carbon dioxide are non-uniformly distributed in the form of fluid inclusions. The distribution features of fluid components within quartz aggregates formed at different stages of deformation indicate their confinement to microstructural elements such as subblocks, deformation bands and recrystallization grains. During structural rearrangements of quartz aggregates, fluid is redistributed from fluid inclusions between the selected elements due to dislocation migration between defects of different scale levels. The water-carbon dioxide fluid captured during grain growth is concentrated in isometric fluid inclusions that are preserved in the quartz of the protogranular microstructure. At the initial stages of structural transformations under conditions of dislocation slip, fluid inclusions change their shape and size due to the action of applied stress forces, stretching along regular linear directions. Continuing deformations promote the activation of slip planes and the migration of a certain amount of the aqueous phase due to dislocation mass transfer. Fluid behavior during subsequent dynamic and static recrystallization under creep conditions is mainly determined by pressure redistribution and diffusion processes. Fluid migration occurs during the interaction of migrating boundaries with fluid inclusions. Due to this, the space of subgrains and recrystallization grains is freed from volume defects represented by fluid inclusions. At the final stages of deformation, during the establishment of structural equilibrium, diffusion processes within the grain boundaries acquire greater significance. The described redistribution of fluid from fluid inclusions in quartz is determined by the evolution of defects in the crystal structure as a result of imposed deformations.

Author Contributions

Conceptualization, E.N.K. and A.A.B.; methodology, R.Y.S. and E.N.M.; software, R.Y.S. and A.A.B.; validation, P.A.T. and D.V.L.; formal analysis, R.Y.S. and E.N.M.; investigation, E.N.K., A.A.B. and R.Y.S.; resources, E.N.K. and P.A.T.; data curation, A.A.B.; writing—original draft preparation, E.N.K.; writing—review and editing, A.A.B.; visualization, A.A.B.; supervision, P.A.T. and D.V.L.; project administration, P.A.T.; funding acquisition, P.A.T. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

The data presented in this study are contained within this article.

Acknowledgments

Investigations were carried out using the equipment center for collective use “Analytical Center Geochemistry of Natural Systems” NR TSU. The infrared spectroscopic measurements were performed in the Isotope-geochemical research center for Collective Use (A. P. Vinogradov Institute of Geochemistry of the Siberian Branch of the Russian Academy of Sciences).

Conflicts of Interest

The authors declare no conflicts of interest.

Footnotes

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Figures and Table

Figure 1. The structure of the vein systems (the Krivoy zone): (a) Plan view; (b) S–N profile; (c) Position of quartz veins in an outcrop; (d) Main vein of the southwestern dipping; (e) Stereographic projection of the distribution of structural elements and veins.

Figure 2. Types of microstructure in vein quartz from Krivoy zone: (a) protogranular; (b) subblock and fragmentary; (c–e) pseudo-porphyroblastic; (f) granular.

Figure 3. Deformation evolution of vein quartz in a modified chart of quartz deformation mechanisms. Strain rates (in s-1) are along y axis. The sketch illustrates changes in quartz aggregates. Colors show high-angle (black), low-angle (green), and Dauphiné twin (red) boundaries [45]. Mechanisms of dynamic recrystallization by [53,54,55]: BLG—Bulging recrystallization, SGR—subgrain rotation recrystallization, GBM—grain boundary migration. Types of microstructure in vein quartz: A—protogranular, B—sub-block, C—fragmentary, D—pseudo-porphyroblastic, E—granular.

Figure 4. Example of reflection spectra of two grains with different orientations related to incident beam. The arrows indicate the peak at 1118 and the dip at 1160 cm−1.

Figure 5. Features of the distribution of water and carbon dioxide in quartz of protogranular microstructure: (a) the processed EBSD band contrast + inverse pole figure map with the color-coded boundaries—the symbols shown here are color-coded for the boundaries to indicate that the angle of boundaries misorientation will be used in the next figures; (b) Map of the intensity ratio of peaks 1118 and 1160 cm−1 in the reflection spectrum; (c) Map of the distribution areas of water and carbon dioxide within the selected elements of the microstructure—the symbols shown here indicate that the composition of the component will be used in the next figures; (d) Map of the distribution of H2O based on the area of peak ~3400 cm−1; (e) Map of the distribution of CO2 based on the area of peak 2350 cm−1; (f) Absorption spectra of H2O and CO2 (the numbers correspond to the regions in d and e).

Figure 6. Features of the distribution of water and carbon dioxide in quartz of fragmentary microstructure: (a) the processed EBSD band contrast + inverse pole figure map with the color-coded boundaries; (b) Map of the intensity ratio of peaks 1118 and 1160 cm−1 in the reflection spectrum; (c) Map of the distribution areas of water and carbon dioxide within the selected elements of the microstructure; (d) Map of the distribution of H2O based on the area of peak ~3400 cm−1; (e) Map of the distribution of CO2 based on the area of peak 2350 cm−1; (f) Absorption spectra of H2O and CO2 (the numbers correspond to the regions in d and e).

Figure 7. Features of the distribution of water and carbon dioxide in quartz of pseudo-porphyroblast microstructure: (a) the processed EBSD band contrast + inverse pole figure map with the color-coded boundaries; (b) Map of the intensity ratio of peaks 1118 and 1160 cm−1 in the reflection spectrum; (c) Map of the distribution areas of water and carbon dioxide within the selected elements of the microstructure; (d) Map of the distribution of H2O based on the area of peak ~3400 cm−1; (e) Map of the distribution of CO2 based on the area of peak 2350 cm−1; (f) Absorption spectra of H2O and CO2 (the numbers correspond to the regions in (d,e)).

Figure 8. Features of the distribution of water and carbon dioxide in quartz of grain microstructure: (a) the processed EBSD band contrast + inverse pole figure map with the color-coded boundaries; (b) Map of the intensity ratio of peaks 1118 and 1160 cm−1 in the reflection spectrum; (c) Map of the distribution areas of water and carbon dioxide within the selected elements of the microstructure; (d) Map of the distribution of H2O based on the area of peak ~3400 cm−1; (e) Map of the distribution of CO2 based on the area of peak 2350 cm−1; (f) Absorption spectra of H2O and CO2 (the numbers correspond to the regions in (d) and (e)).

Figure 9. Features of the distribution of water and carbon dioxide in quartz: (a) the processed EBSD band contrast + inverse pole figure map with the color-coded boundaries; (b) Map of the distribution of H2O based on the area of peak ~3400 cm−1; (c) Map of the distribution of CO2 based on the area of peak 2350 cm−1; (d) Map of the distribution areas of water and carbon dioxide within the selected elements of the microstructure.

Table 1

Typical elements of vein quartz structure and their characteristics [52].

Characteristic Angles of Misorientation	Type of Substructure	Characteristic Size, mm	The Morphology of the Boundary	Prevailing Regimes and MechanismsDeformation
<10°	Subgrain	0.005–0.3	Sinuous	Dislocation glide: High stress, moderate temperatures
30°	Deformation bands	0.5–1.5	Straight-line	Dislocation glide with subsequent twinning
55–65°	Dauphine twin boundaries	(a) Continuous(b) Grains about 0.05	Sinuous	(a) Strain(b) Temperature
>10°	Recrystallization grains with boundaries of general type	(a) Grains 0.005–0.1(b) Grains >1	Smooth	Dislocation and diffusion:(a) Strain + Fluid(b) Temperature + Fluid

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