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Strain localization and fabric development in polycrystalline anorthite + melt by water diffusion in an axial deformation experiment
© The Author(s) 2018
- Received: 31 March 2017
- Accepted: 26 December 2017
- Published: 4 January 2018
- Anhydrous polycrystalline anorthite
- Water diffusion
- Griggs-type deformation apparatus
- Plastic deformation
The deformation of feldspar can control the rheology of the lower-middle crust because of its dominance (e.g., Rutter and Brodie 1988; Kohlstedt et al. 1995; Bürgmann and Dresen 2008). The deformation mechanisms of feldspar and its microstructures have been investigated through analyses of naturally deformed samples (e.g., Olsen and Kohlstedt 1985; Kruhl 1987; Prior and Wheeler 1999; Jiang et al. 2000; Kruse et al. 2001; Menegon et al. 2008; Raimbourg et al. 2008; Fukuda et al. 2012; Fukuda and Okudaira 2013; Getsinger et al. 2013; Menegon et al. 2015 and references therein) and experimental studies (e.g., Tullis and Yund 1987, 1991; Dimanov et al. 1998, 1999, 2000; Heidelbach et al. 2000; Stünitz and Tullis 2001; Rybacki and Dresen 2004; Rybacki et al. 2006, 2008, 2010 and references therein). The flow laws of feldspar were constructed for both dry feldspar and feldspar with water, which reduces the strength of a rock (Dimanov et al. 1999; Rybacki and Dresen 2004; Rybacki et al. 2006).
Many tomographic observations suggest that low seismic velocity zones or electrically high-conductivity zones beneath active faults are related to an enriched distribution of pressurized water (e.g., Ogawa et al. 2001; Okada et al. 2006; Wannamaker et al. 2009; Nakajima et al. 2010; Becken et al. 2011 and references therein). In these zones, it is assumed that water is continuously supplied to rocks, thereby enhancing the strain, strain rate, and reaction mechanisms and causing plastic deformation in regions supplied with water; these regions therefore evolve in both time and space depending on the water supply.
As for the effects of supplied water on rock deformation, several deformation experiments have been conducted on the addition of water into quartz and feldspar as major constituents of the crust (Kronenberg and Tullis 1984; Den Brok and Spiers 1991; Den Brok et al. 1994; Post et al. 1996; Post and Tullis 1998; Stünitz et al. 2003; Vernooij et al. 2006; Chernak et al. 2009). These studies revealed that added water reduces the strengths of samples via the enhancement of dislocation creep, diffusion creep, reaction creep, and/or solution–precipitation creep, which are also associated with the development of microstructures. However, these studies did not focus on or observe the time-dependent development of microstructures or a water distribution.
In this study, we performed water-added deformation experiments on a dry anorthite aggregate and externally added water to a sample during deformation. We accordingly discuss the development of microstructures via water diffusion and reactions under differential stresses. The microstructures are compared with those from an experiment without water. In addition, we analyze the water distribution and discuss the diffusion of water. We also discuss the evolution of the strain rate of a rock mass controlled by water diffusion at various scales and times that are assumed in nature.
The confining pressure and temperature were carefully raised in the stability field of anorthite and water (Matthews and Goldsmith 1984). The temperature and pressure were raised as follows: from room temperature to 100 °C at 200 MPa, to 200 °C at 400 MPa, to 300 °C at 800 MPa, and finally to 900 °C at 1 GPa. The rate of pressure increase was ca. 100 MPa/h and was achieved via a hand pump. The rate of temperature increase was ca. 50 °C/min. After reaching 1 GPa and 900 °C, an axial compression test was immediately performed with a strain rate of 10−4.8 s−1 for both experiments. This procedure requires ca. 1 h for the piston movement before deformation to detect the sample hit point (pre-hit). Then, the samples were deformed for 6 h for 32% total strain in the dry experiment and 4 h for 18% total strain in the wet experiment. In the wet experiment, water diffused into the sample within a total time of 5 h. The force and displacement data were converted into differential stress and strain values, respectively, according to changes in the sample diameters, corrections for apparatus distortion, and friction within the sample assembly. The stress resolution of the apparatus has been estimated to be ± 30 MPa in recent papers (Holyoke and Kronenberg 2010; Kido et al. 2016).
The recovered samples were longitudinally cut into halves (i.e., along the axial displacement direction). Thin sections were made from one of those halves for microstructural observations. The other half (ca. 100 μm) was used for IR spectroscopy analyses. The residual sample pieces in these procedures were used to observe broken surfaces via an SEM.
To measure the crystallographic orientation of the anorthite, we used a Hitachi S-3400N scanning electron microscope equipped with an EBSD analyzer provided by HKL Technology. The analyses were performed for highly polished thin sections with an accelerating voltage of 20 kV, a stage current of 1 nA, and a working distance of 18 mm for specimens that were tilted 70°. The EBSD data were analyzed by the HKL Technology Channel 5 software. An index of Kikuchi patterns with a mean angular deviation of < 1° was accepted as orientation data.
The transmitted IR spectra were measured using a Fourier transform IR micro-spectrometer (Nicolet iS10, Thermo Scientific). Mapping measurements using an auto XY-stage were taken for the sample recovered from experiment (2). First, a measurement was taken along the direction of axial compression around the upper and lower sample boundaries with a rectangular aperture of 25 × 50 μm (height × width) and the same step size. Next, a measurement was taken in the direction normal to axial compression apart from the sample boundary with an aperture size of 50 × 25 μm and the same step size. Each spectrum was averaged over 100 scans with a frequency resolution of 4 cm−1 under atmospheric conditions. The background was corrected after each acquisition of 25 spectra. The absorption coefficients for the water absorption bands in the feldspar groups were determined to estimate the water contents following Beran (1987) and Johnson and Rossman (2003), whose methods have been used in, e.g., Dimanov et al. (1998) and Rybacki et al. (2006) for a hot-pressed anorthite aggregate and Fukuda et al. (2012) for natural K-feldspar aggregates. However, in this study, the water contents were calculated from a water-stretching band using the calibration of Paterson (1982), which is independent of the mineral species because the water absorption bands measured in this study include glass and reaction products in addition to anorthite. This calibration approach includes the integral molar absorption coefficient of hydrogen I (ℓ/mol H·cm2) and the mean wavenumber νmean with I = 150γ[3780 − νmean], where γ is an orientation factor of 1/3 assuming an isotropic OH distribution, any beam direction and no polarization. Our results show that the νmean values depend on the type of water among the different minerals. The absorption coefficient units of ℓ/mol H·cm2 were converted into ℓ/mol H2O·cm2 through multiplication by 2. The water contents were calculated using the Beer–Lambert law and expressed as wt ppm H2O using the molar mass of H2O and the density of either anorthite or zoisite as the reaction product. The mapping results are displayed as color-contoured images of the calculated water contents.
These minerals were observed during the deformation experiment for An54 and An60 in Stünitz and Tullis (2001). Kyanite grains were not observed in the SEM images, similar to the observations by Matthews and Goldsmith (1984). The melt phase in the above reaction would be associated with SiO2 materials, which may be amorphous or crystalline (Fig. 6b).
The strain in the bulging part can be calculated from the initial length of the sample and the width of the bulging section. We used a relatively linear sample segment in Fig. 5, where the deformation appears to have been accommodated only by bulging. The difference between the initial sample length (10.2 mm) and the linear segment in Fig. 5 (9.0 mm) gives the initial length of the bulging part (1.2 mm). The strain calculated from the initial sample length and the linear segment in the deformed sample is consistent with a total permanent strain of 12% in the mechanical data, which also uses the initial sample length (Fig. 3). The length of the bulging part in the deformed sample corresponds to the width (0.7 mm) of the transparent portion around the top and bottom sample core boundaries. The calculated strain in the bulging part determined from the initial length of the bulging segment and the length of the transparent portion in the deformed sample is ca. 40%, which is approximately four times the permanent strain of the sample in the mechanical data (Fig. 3).
Diffusion of water
Enhancement and localization of plastic deformation by water
Previous studies that uniformly deformed samples and observed steady states of stress have confirmed that strength reduction can be achieved via added water (e.g., Kronenberg and Tullis 1984; Post and Tullis 1998; Chernak et al. 2009). In contrast, in our water-added experiment, the calculated strain in the bulging part (Fig. 5) was ~ 40%, which is approximately four times the permanent strain of 12% when assuming homogenous deformation in the mechanical data (Fig. 3). The compressive flow strength of a laminated rock composed of two phases is directional (e.g., Ji et al. 2000). For a layer composite with interface planes normal to the compressive axis, those two phases are subjected to equal stresses (and different strain rates; Reuss bound). The higher strain in the bulging part indicates that the strain rate at the same deformation time was four times higher than the bulk sample and that deformation was localized by water diffusion, implying that the effective viscosity (~ stress/strain rate) of the water-diffused part was lower than that of the initial dry part. This would be due to the limited width of the water-diffused part (~ 250 μm both upward and downward in the two cores with total lengths of ~ 9 mm; Fig. 5). In contrast, cataclasis dominated in the dry sample (Fig. 4). The strengths between the dry and wet experiments were not different (Fig. 3). Consequently, the strengths of the bulging and dry parts in the water-added sample were not different, and the measured sample bulk strength may not have been different from that of the dry sample.
In the water-added sample, zoisite grains up to 50 μm are observed as a reaction product (Fig. 6). They separately exist, and each grain is fractured. Therefore, zoisite does not directly contribute to the deformation, as was also documented in Stünitz and Tullis (2001). The SiO2 domain is partially connected (Fig. 6). According to Spiess et al. (2012), such a domain, which is different from the initial glass with high silica and aluminum contents in our case (Fig. 1), might have been formed by dissolution and precipitation in open spaces. The precipitated grains might have been caused by the reaction in Eq. (1), but they could also have been due to the glass initially included in the sample (Fig. 1). We did not observe a segregation of the glass similar to Dimanov et al. (1998, 2000), who performed axial deformation experiments on homogeneously wet plagioclase aggregates with melt reaching up to 12 vol%. Diffused water, which can cause a reaction in anorthite, is incorporated in the original glass in our sample. The IR spectra for the water-added sample show that the band due to glass has a maximum at 3540 cm−1 around the sample boundary (Fig. 9) and especially the side of the sample column (Fig. 10). The incorporation of water into the melt as well as the inclusion of water along the anorthite grain boundaries contributes to the strength of the sample, although the overall quantitative effect is difficult to estimate in this study. The water contents along the side of the sample column reach up to 1000 ppm H2O, indicating the presence of saturated water trapped in the glass and along the anorthite grain boundaries.
The profile of the water concentration (Fig. 11) is similar to that of the zoisite distribution (Fig. 7). IR spectra from the upper and lower sample boundaries show that water is dominantly trapped within the zoisite (Fig. 9). Assuming a 2.0 wt% H2O stoichiometric amount of water in a zoisite crystal structure (e.g., Hurlbut 1969), the representative zoisite area fractions of 18.3% (maximum at 25 μm from the upper and lower sample boundaries), 12.6% (50 μm), 3.5% (75 μm), and 0.9% (200 μm) shown in Fig. 7 can be converted into 3700, 2520, 700, and 180 ppm H2O, respectively. These converted water contents roughly correspond to the values measured via IR spectroscopy (1500 ppm at the maximum at the sample boundary; Fig. 9). However, fine zoisite grains also develop along the anorthite grain boundaries (Fig. 6c). The quantity of these fine grains would also be high along the sample boundaries, and water would also be trapped in these grains.
The deformation of the feldspar group by reactions involving solution–precipitation creep has been reported for naturally deformed samples under lower-middle crustal conditions (Fitz Gerald and Stünitz 1993; Ree et al. 2005; Menegon et al. 2008; Brander et al. 2012; Fukuda et al. 2012). In such cases, the fluid phase was likely supplied externally. Our study provides information on fabric development and strain localization during the water-introduction process. The anorthite grains in the water-added experiment would have been partially dissolved in the reaction in Eq. (1), which is also indicated by the vermicular features (Fig. 6d). The crystallographic orientation of anorthite was random in the bulging portion, indicating the presence of grain-size-sensitive creep in the anorthite aggregate (Fig. 8). The IR spectra around the bulging part show stretching bands of mainly hydroxyl in the zoisite (Fig. 9). Thus, the localization of plastic deformation is directly related to the diffusion of water into the sample, even though the mechanical data between the dry and water-added experiments may be identical (Fig. 3). Thus, when water is introduced into dry polycrystalline anorthite, its deformation mechanism switches from cataclastic flow to plastic flow due to grain-size-sensitive creep assisted by the reaction and melt, thereby causing strain localization. Melt-assisted grain-size-sensitive creep was also confirmed by Dell’Angelo et al. (1987) for fine-grained granitic aggregates with melt contents of 3–5% via transmission electron microscopy; in those samples, the deformation switched from the dislocation creep of quartz and feldspar with a melt content of < 1%.
Implications for crustal dynamics
We performed deformation experiments on polycrystalline anorthite samples both with and without water. Fractures dominated in the dry experiments, while plastic deformation occurred in the 0.15 wt% water-added experiment and was concentrated within the water-diffused area. Zoisite was formed as the reaction product, and water was incorporated within it. The maximum water content was 1500 wt ppm H2O, which is consistent with the amount of added water, and the water contents gradually decreased toward the inner part of the sample. The profile of the water concentration gradient fits well with the solution of one-dimensional diffusion. The determined diffusion coefficient was ~ 10−12 m2/s, which agrees with previous data. We calculated the evolution of the plastic deformation strain rate controlled by water diffusion at different rock mass and temporal scales under lower-middle crustal conditions with a representative water diffusion coefficient of 10−13 m2/s. The calculated water-diffusion-based strain rates were within a geologically reasonable range from 10−10 to 10−15 s−1 at rock mass scales from < 1 m to 1 km with a temporal increase from < 1 year to 10,000 years. This implies that water diffusion likely controls the deformation of the initially dry and strong lower-middle crust.
JM performed the deformation experiments. JF performed all of the analyses and drafted the main part of the manuscript. JF, JM, and HN interpreted all of the data and drafted the manuscript.
The authors thank J. Tullis, G. Hirth, and R. Cooper for their experimental support at Brown University and M. Nakamura and S. Okumura for conducting the IR measurements at Tohoku University. Detailed reviews by T. Okudaira and two anonymous reviewers greatly improved the manuscript. We also thank T. Takeshita for his editorial handling and comments.
The authors declare that they have no competing interests.
Ethics approval and consent to participate
JF was supported financially by a Grant-in-Aid for Scientific Research for Young Scientists (233694) provided by the Japan Society of the Promotion of Science (JSPS), by a Grant-in-Aid provided by the Fukada Geological Institute, and by MEXT KAKENHI grant (15K21755). In addition, this work was supported financially by MEXT KAKENHI grant (26109005) to JM.
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