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Volume 31 Issue 6
Dec.  2020
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Zhonghang Wang, Feng Shi, Junfeng Zhang. Effects of Water on the Rheology of Dominant Minerals and Rocks in the Continental Lower Crust:A Review. Journal of Earth Science, 2020, 31(6): 1170-1182. doi: 10.1007/s12583-020-1307-9
Citation: Zhonghang Wang, Feng Shi, Junfeng Zhang. Effects of Water on the Rheology of Dominant Minerals and Rocks in the Continental Lower Crust:A Review. Journal of Earth Science, 2020, 31(6): 1170-1182. doi: 10.1007/s12583-020-1307-9

Effects of Water on the Rheology of Dominant Minerals and Rocks in the Continental Lower Crust:A Review

doi: 10.1007/s12583-020-1307-9
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  • As an important part of the continental lithosphere, the continental lower crust can influence and control many important geodynamic processes, which are of great significance to the evolution of the lithosphere. Extensive plastic deformation is common in continental lower crust. There have been many studies focusing on the rheology of the continental lower crust in the past few decades. This paper provides a review on the effects of water on the rheology of dominant minerals (clinopyroxene, plagioclase and garnet) and rocks in the continental lower crust. The water contents in continental lower crustal minerals and rocks are in general rich and very heterogenous from sample to sample and region to region. Water can significantly reduce the strength of clinopyroxene, plagioclase, garnet and lower crustal rocks. Water can also have a profound influence on fabric development and slip systems in lower crustal minerals. Quantitative experimental investigations and extensive natural studies of water effect on rheology are necessary to refine the classic lithosphere strength profile models and to address the existing controversy on strength of the continental lower crust.
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Effects of Water on the Rheology of Dominant Minerals and Rocks in the Continental Lower Crust:A Review

doi: 10.1007/s12583-020-1307-9

Abstract: As an important part of the continental lithosphere, the continental lower crust can influence and control many important geodynamic processes, which are of great significance to the evolution of the lithosphere. Extensive plastic deformation is common in continental lower crust. There have been many studies focusing on the rheology of the continental lower crust in the past few decades. This paper provides a review on the effects of water on the rheology of dominant minerals (clinopyroxene, plagioclase and garnet) and rocks in the continental lower crust. The water contents in continental lower crustal minerals and rocks are in general rich and very heterogenous from sample to sample and region to region. Water can significantly reduce the strength of clinopyroxene, plagioclase, garnet and lower crustal rocks. Water can also have a profound influence on fabric development and slip systems in lower crustal minerals. Quantitative experimental investigations and extensive natural studies of water effect on rheology are necessary to refine the classic lithosphere strength profile models and to address the existing controversy on strength of the continental lower crust.

Zhonghang Wang, Feng Shi, Junfeng Zhang. Effects of Water on the Rheology of Dominant Minerals and Rocks in the Continental Lower Crust:A Review. Journal of Earth Science, 2020, 31(6): 1170-1182. doi: 10.1007/s12583-020-1307-9
Citation: Zhonghang Wang, Feng Shi, Junfeng Zhang. Effects of Water on the Rheology of Dominant Minerals and Rocks in the Continental Lower Crust:A Review. Journal of Earth Science, 2020, 31(6): 1170-1182. doi: 10.1007/s12583-020-1307-9
  • It has been generally accepted that the composition of the continental lower crust is dominated by granulite under high temperature and high pressure conditions (700–900 ℃, 0.6–1.2 GPa). The continental lower crust can be divided into the upper felsic granulite part and the lower mafic granulite part (Rudnick and Gao, 2003; Christensen and Mooney, 1995; Rudnick and Fountain, 1995). The continental lower crustal minerals mainly include plagioclase, pyroxene, quartz and garnet with plagioclase and clinopyroxene being the dominant minerals in lower crusts of normal thickness (~35 km) and garnet being an additional dominant mineral in overthickened lower crust (> 35 km). The density is between 2.8 and 3.1 g/cm3, and the P-wave seismic velocity is between 6.9 and 7.2 km/s (Rudnick and Fountain, 1995).

    The continental lower crust was previously thought to be "dry" due to lack of fluid (Yardley and Valley, 1997; Frost and Bucher, 1994). However, the discovery of a small amount of hydrogen existing in the form of structural hydroxyl (OH) as defects in lower crustal minerals has blurred the definitions of hydrous and anhydrous conditions (e.g., Skogby et al., 1990). Although the studies of water contents in minerals and rocks of the continental lower crust are still sparse, it has been shown that they can contain quite significant amount of water (mostly hundreds of ppm and up to 1 900 ppm) (Table 1). In general, the lower crustal minerals and rocks are rich in water comparing to those in the continental upper lithospheric mantle (e.g., Wang, 2010; Bell and Rossman, 1992). Previous results also suggest that water content and water content partition among granulitic minerals are very heterogenous from sample to sample and region to region (e.g., Zhang et al., 2018, 2016; Yang G C et al., 2012; Yang X Z et al., 2008; Xia et al., 2006).

    Rocks Water content (ppm) References
    Clinopyroxene Orthopyroxene Plagioclase Garnet Quartz
    Nüshan granulite 200–2 360 130–1 170 140–880 - - Xia et al. (2006)
    Hannuoba granulite (terrain) 585–1 480 230–1 875 175–895 291–620 - Yang et al. (2008)
    Hannuoba granulite (xenolith) 275–720 60–185 65–205 - -
    Daoxian granulite 285–500 65–140 205–350 - -
    Yushugou granulite - 1–54 172–533 63–215 34–66 Zhang et al. (2016)
    Himalayan granulite 193–547 - 335–1 053 188–432 125–185 Zhang et al. (2018)
    Junan granulite 300–1 180 80–169 717–1 239 - - Yang et al. (2012)
    Tongbai granulite - 210–993 - - -
    Songshugou granulite - 465–733 - - -

    Table 1.  Water contents in continental lower crustal minerals

  • It has been widely accepted that the deformation of the continental lithosphere is stratified vertically (e.g., Deng and Tesauro, 2016; Bürgmann and Dresen, 2008; Kohlstedt et al., 1995). The strengths of fault/quartz, plagioclase and olivine were used to represent the mechanical strengths of the upper crust, the lower crust and the upper mantle, respectively (e.g., Bürgmann and Dresen, 2008; Ranalli, 2000; Kohlstedt et al., 1995), resulting in three contrasting continental lithosphere strength profile models (Fig. 1). In the "Jelly Sandwich" model (Fig. 1a), the continental lower crust is considered a weak layer sandwiched between relatively strong upper crust and upper mantle in the continental lithosphere (Kohlstedt et al., 1995; Ranalli and Murphy, 1987). This model is widely accepted because it is consistent with many geological and geophysical observations. The "Crème Brȗlée" lithosphere strength profile model suggests a strong crust on top of a weak upper mantle (Fig. 1b). In this model, the strength of the lithosphere is mainly concentrated in the continental lower crust. Hence, it is consistent with the observation of earthquake distribution mostly in the continental crust but not in the upper mantle (Burov and Watts, 2006; Jackson, 2002a, b; Maggi et al., 2000a, b). The "Banana Split" lithosphere strength profile model (Fig. 1c) indicates a weak continental lithosphere. The supporting evidence includes weakness of major fault zones existing at all depth levels and down to the bottom of the lithosphere (Zoback et al., 1987). These contrasting models imply rheology of the continental lithosphere may have significant spatial and temporal heterogeneities due to the variation in composition, geothermal gradient and water content. The "Jelly Sandwich" and the "Banana Split" models interpret that a "wet" continental lower curst dominates while the "Crème Brȗlée" model indicates a "dry" continental lower crust. The evaluation and application of these models to natural conditions require profound knowledge on the effects of water on the rheology of continental lower crustal minerals and rocks.

    Figure 1.  Continental lithosphere strength profile models (after Bürgmann and Dresen, 2008). (a) "Jelly Sandwich" model: friction strength dominated upper crust+wet lower crust+dry upper mantle; (b) "Crème Brȗlée" model: friction strength dominated upper crust+dry lower crust+dry upper mantle; (c) "Banana Split" model: friction strength dominated upper crust+wet lower crust+wet upper mantle. Strength of upper crust: Byerlee's Law (Byerlee, 1978); flow laws: wet plagioclase, Rybacki et al. (2006); dry plagioclase, Rybacki and Dresen (2000); dry and wet olivine: Hirth and Kohlstedt (2003). Temperatures at Moho are 680 ℃ in (a) and 790 ℃ in (b), respectively. Strain rate: 10-14 s-1.

  • Numerous studies have confirmed that trace amounts of water can significantly affect the physical and chemical properties of rocks and minerals (Zhao and Yoshino, 2016; Hirth and Kohlstedt, 1996; Karato, 1990; Kushiro, 1972) and geological processes in deep Earth (Asimow and Langmuir, 2003; Bercovici and Karato, 2003). Previous studies have demonstrated qualitatively that minerals are strong under anhydrous or dry conditions and weak under hydrous or wet conditions, respectively. With increasing water content, mechanical strength exponentially decreases while strain rate increases (Fig. 2). This water weakening effect can be quantified as the water fugacity/ content exponent in a flow law of an empirical form

    Figure 2.  Schematic mechanical strength and strain rate dependence on water content. With increasing water content, mechanical strength decreases exponentially (a) while strain rate increases exponentially (b). COH, r, n are water content/fugacity, water content/fugacity exponent, and stress exponent in a flow law, respectively.

    where $\dot{\varepsilon }$ is the strain rate, A is the pre-exponential factor related with material, $C_{\text{OH}}^{r}$is the water fugacity or the water content, r is the water fugacity or water fugacity exponent, d is the grain size, m is the grain size exponent, σ is the steady-state flow stress, n is the stress exponent related to deformation mechanism, R is the gas constant, T is the absolute temperature, Q=∆E+PV is the activation Gibbs free energy, P is the pressure, ∆E and ∆V are the activation energy and the activation volume, respectively.

    The larger the water fugacity or content exponent, the stronger the influence of water on strength reduction or strain rate increase. Previous flow-law parameters of continental lower crustal mineral aggregates and rocks determined experimentally in the laboratory under hydrous (wet) and anhydrous (dry) conditions are summarized in Table 2. In addition to the reduction effect on mechanical strength, water can also affect the slip system in minerals, giving rise to different fabric formation. We summarized below the effects of water on the rheology of continental lower crustal rocks and their major constituent minerals (clinopyroxene, plagioclase and garnet).

    Mineral/rock Lg A (MPa-n-r) n r m Q (kJ/mol) Deformation regime References
    Quartz -3.0 2.0 0 0 167 Dislocation, dry Shelton and Tullis (1981)
    -4.0 4.0 1 0 223 Dislocation, wet Gleason and Tullis (1995)
    -11.2 4.0 - 0 135 Dislocation, wet Hirth et al. (2001)
    Clinopyroxene -4.1 4.3 - 0 284 Dislocation, wet Avé Lallemant (1978)
    8.1 1 - 3 337 Diffusion, wet Dimanov and Dresen (2005)
    0.8 5.5 - 0 534 Dislocation, wet
    1.2 2.6 0 0 335 Dislocation, dry Shelton and Tullis (1981)
    6.1 1 1.4 3 340 Diffusion, wet Hier-Majumder et al. (2005)
    6.7 2.7 3 0 670 Dislocation, wet Chen et al. (2006)
    -2 3.5 - 0 310 Dislocation, wet Zhang et al. (2006)
    9.8 4.7 - 0 760 Dislocation, dry Bystricky and Mackwell (2001)
    1.5 4.2 0 0 413 Dislocation, dry Moghadam et al. (2010)
    -3.3 3.7 - 0 326 Dislocation, wet Orzol et al. (2006)
    Orthopyroxene -0.5 2.4 0 0 293 Dislocation, dry Raleigh et al. (1971)
    -2.2 2.8 - 0 271 Dislocation, wet Ross and Nielsen (1978)
    8.6 3 - 0 600 Dislocation, dry Bystricky et al. (2016)
    1.5 1 0.7 3 200 Diffusion, wet Zhang et al. (2017)
    Plagioclase 12.1 1 0 3 467 Diffusion, dry Rybacki and Dresen (2000)
    12.1 1 0 3 460 Diffusion, dry Rybacki et al. (2006)
    12.7 3 0 0 648 Dislocation, dry Rybacki and Dresen (2000)
    12.7 3 0 0 641 Dislocation, dry Rybacki et al. (2006)
    1.7 1 - 3 170 Diffusion, wet Rybacki and Dresen (2000)
    -0.7 1 1 3 159 Diffusion, wet Rybacki et al. (2006)
    2.6 3 - 0 356 Dislocation, wet Rybacki and Dresen (2000)
    0.2 3 1 0 345 Dislocation, wet Rybacki et al. (2006)
    Garnet -5.3 3.5 1 0 215 Dislocation, wet Xu et al. (2013)
    - 3.4 2.4 0 - Dislocation, wet Katayama and Karato (2008)
    Gabbro 10.3 4.0 0 0 644 Dislocation, dry Zhou et al. (2012)
    Diabase -3.7 3.4 - 0 260 Dislocation, wet Shelton and Tullis (1981)
    0.9 4.7 0 0 485 Dislocation, dry Mackwell et al. (1998)
    Granulite -2.1 3.1 - 0 243 Dislocation, wet Wilks and Carter (1990)
    4.2 4.2 - 0 445 Dislocation, wet
    -2.0 3.2 - 0 244 Dislocation, wet Wang et al. (2012)
    Eclogite 3.3 3.4 - 0 480 Dislocation, wet Jin et al. (2001)
    3.3 3.5 - 0 403 Dislocation, wet Zhang and Green (2007)
    Note: the flow parameters of A, n, r, m, Q are the same as described in Eq. (1).

    Table 2.  Rheological parameters of continental lower crustal minerals and rocks

  • As one of the most abundant constituent minerals in the continental lower crust, modal proportions of clinopyroxene vary greatly in different tectonic settings. It has been suggested that clinopyroxene is a major hydroxyl-rich constituent nominal anhydrous mineral in the lower crust (Ingrin and Skogby, 2000; Bell and Rossman, 1992). Hence, clinopyroxene can be a major contributor to low mechanical strength of the continental lower crust due to its high volume fraction in the lower crust.

    There have been many experimental studies on the rheological strength of clinopyroxene aggregates. Avé Lallemant (1978) documented for the first time that water promoted deformation of diopside and websterite (68% clinopyroxene+ 32% enstatite). Under a pressure of 300 MPa and temperatures of 1 200–1 270 ℃, Boland and Tullis (1986) found that hydrous clinopyroxene aggregates with 0.5 wt.%–1.0 wt.% water were weaker than the "dry" ones in the dislocation creep regime. Bystricky and Mackwell (2001) reported the strength of "dry" clinopyroxene aggregates exceeded that of the "dry" olivine in both the dislocation and the diffusion creep regimes. Meanwhile, the water weakening effect on strength was also observed in undried samples. Dimanov et al. (2003) investigated the strength of fine-grained clinopyroxene in the diffusion creep regime and found that water significantly reduced the strength of clinopyroxene. All these studies demonstrate clearly that the strength of clinopyroxene can be significantly reduced under hydrous or water saturated conditions compared to anhydrous conditions.

    In addition to water, mineral composition can also play an important role in controlling mechanical strength of clinopyroxene. Zhang et al. (2006) studied the rheology of omphacite (Di58Jd42) under the conditions of temperatures of 1 300–1 500 K, a confining pressure of 3 GPa, and strain rates of 10-5–10-4 s-1. The results suggest the strength of omphacite is between those of diopside and jadeite, demonstrating that Na-bearing clinopyroxene is significantly weaker than Na-free clinopyroxene. Orzol et al. (2006) investigated the strength of synthetic jadeite aggregates and concluded the strength of jadeite was much lower than that of diopside in the dislocation creep regime. It is therefore important to take into consideration the effect of mineral composition when evaluating the effects of water on clinopyroxene rheology.

    Hier-Majumder et al. (2005) and Chen et al. (2006) conducted systematic deformation experiments on clinopyroxene aggregates under different water fugacity conditions. Their experimental results showed the strength of clinopyroxene was significantly reduced under hydrous conditions with water fugacity exponents of r=1.4±0.2 and r=3.0±0.6 in the diffusion creep regime and in the dislocation creep regime, respectively. These results also demonstrated clearly that the water weakening effect was stronger in clinopyroxene than in olivine (Mei and Kohlstedt, 2000a, b) and plagioclase (Rybacki and Dresen, 2000). Figure 3a shows the experimental results of the rheological strength of clinopyroxene under both hydrous and anhydrous conditions in the dislocation regime. Most hydrous clinopyroxene samples have much weaker strength compared with the anhydrous ones. In addition, hydrous clinopyroxene samples can have a strength variation up to 10-fold.

    Figure 3.  Rheological strength of clinopyroxene (a) and plagioclase (b) extrapolated to a natural strain rate of 10-14 s-1 from experimental flow laws. The dashed and solid lines represent strength under hydrous and anhydrous conditions, respectively. Flow laws are from: (a) 1-wet jadeite (Stockhert and Renner, 1998); 2-wet omphacite with 100 ppm–200 ppm water (Zhang et al., 2006); 3-wet diopside (Avé Lallemant, 1978); 4-dry jadeite (Moghadam et al., 2010); 5-wet diopside with 0.5 wt.%–1 wt.% water (Boland and Tullis, 1986); 6-wet diopside with 0.05±0.02 wt.% water (Dimanov and Dresen, 2005); 7-dry diopside (Bystricky and Mackwell, 2001); 8-wet diopside with 0.004±0.001 wt.% water (Dimanov and Dresen, 2005); (b) 1-wet anorthite with ~700 ppm water (Rybacki and Dresen, 2000); 2-dry albite (Shelton and Tullis, 1981); 3-dry anorthite (Rybacki et al., 2006).

    Bürgmann and Dresen (2008) extrapolated the experimental results of clinopyroxene to natural conditions, and established the deformation mechanism map for wet clinopyroxene (Fig. 4a). In addition to grain size, stress and temperature, water also plays an important role in the deformation regime of clinopyroxene. For example, the diffusion creep boundary moves to the region of larger grain size in a wet sample compared with that in a dry sample at 900 ℃. In addition, the stress exponent (n) decreases from 4.7 under anhydrous conditions (Bystricky and Mackwell, 2001) to 2.7 under hydrous conditions (Chen et al., 2006) for the deformation of clinopyroxenes of a similar composition. Likewise, the activation energy also decreases from 760 kJ/mol under anhydrous conditions (Bystricky and Mackwell, 2001) to 670 kJ/mol under hydrous conditions (Chen et al., 2006), possibly caused by water enhanced dislocation climb.

    Figure 4.  Deformation mechanism maps of wet clinopyroxene (a) and wet plagioclase (b) (after Bürgmann and Dresen, 2008). Flow laws are from: (a) wet clinopyroxene (Dimanov and Dresen, 2005), dry clinopyroxene (Bystricky and Mackwell, 2001); (b) wet and dry plagioclase (Rybacki et al., 2006). For comparison, the dotted line denotes dry clinopyroxene and dry plagioclase at 900 ℃. A natural strain rate of 10-14 s-1 was used for the calculation of deformation mechanism maps.

    Numerous studies have been carried out on fabric and slip system developed in natural and experimentally deformed clinopyroxene. It is generally accepted that plastic deformation can lead to fabric formation (Godard and van Roermund, 1995). Raterron et al. (1994) conducted experiments on single diopside crystal under high temperature and high pressure conditions. Transmission election microscope (TEM) results showed that there were mainly three types of slip system: [001](100), [001]{110}, and 1/2 < 110 > {110}. The [001](100) slip system is likely to operate when the temperatures are at 800–1 000 ℃ while the [001]{110} and the 1/2 < 110 > {110} slip systems are likely to operate when the temperatures are higher than 1 000 ℃. Mauler et al. (2000) investigated clinopyroxene fabric with torsion experiments. The results showed that the [001] axes were parallel to the shear direction, and the [100] axes were perpendicular to the shear direction. Kanagawa et al. (2008) reported in the Pankenushi gabbro that coarse-grained clinopyroxene deformed in dislocation creep regime with the [001] axes forming a broad girdle in the foliation plane and the (100) planes parallel to the foliation, implying a [001](100) slip system. The fine-grained clinopyroxene deformed dominantly by grain boundary sliding, forming a weak fabric. Puelles et al. (2009) found that clinopyroxene developed strong fabric with the [001] axes forming a girdle in the foliation plane and the poles of the (010) forming high concentrations normal to the foliation in the Bacariza Formation. These natural and experimental results reveal that the [001](100) slip system operates dominantly in clinopyroxenes of the lower crust.

    In natural and experimentally deformed omphacite, the fabric pattern are characterized with the [001] axes being subparallel to the lineation, and the [010] axes being perpendicular to the foliation, respectively (e.g., Zertani et al., 2019; Kim et al., 2018; Moghadam et al., 2010; Shi et al., 2010; Zhang et al., 2006) (Fig. 5). In samples with strong foliation and no lineation, omphacite fabrics are featured with the [010] axes normal to the foliation, and the [001] axes forming a girdle in the foliation plane, forming the S-type fabric (Fig. 5a). In samples with strong lineation and no foliation, omphacite fabrics are characterized by the [001] axes parallel to the lineation, and the [010] axes forming a girdle perpendicular to the lineation, forming the L-type fabric (Fig. 5b). In samples with both pronounced lineation and foliation, omphacite fabrics are characterized by the [001] parallel to the lineation, and the [010] perpendicular to the foliation, forming the SL-type fabric (Fig. 5c). Assuming that fabrics formation is controlled by a dominant slip system, these omphacite fabrics suggest that the (010) and the [001] denote the slip plane and the slip direction, respectively. A [001](010) slip system is indicated. However, this slip system had not been previously reported as a dominant slip system either in natural or experimental samples, leading to the conclusion that these omphacite fabrics could not be explained by the [001](010) slip system (e.g., Zhang and Green, 2007). The L-type omphacite fabric is common in natural eclogites (e.g., Park and Jung, 2019; Kim et al., 2018; Shi et al., 2010; Bascou et al., 2001), which has now been generally interpreted as the result of operation of the [001](100), [001]{110}, 1/2 < 110 > {110} slip systems (Bascou et al., 2002). Zhang et al. (2006) reported omphacite formed the S- and L-type fabrics in axial compression and simple shear experiments, respectively, demonstrating that omphacite fabric variation between the S-type and the L-type was caused by strain field difference rather than the change in omphacite space group. In addition, Zhou and He (2015) reported for the first time the operation of the [001](010) slip system in clinopyroxene at 1 000–1 150 ℃ with TEM evidence.

    Figure 5.  Sketch of typical omphacite fabrics (after Zhang et al., 2006). (a) S-type fabric; (b) L-type fabric; (c) SL-type fabric; S. foliation; L. lineation.

  • Although plagioclase is one of the most abundant constituent minerals in the continental lower crust, studies on its rheological properties are still sparse comparing to quartz, pyroxene and olivine. Tullis et al.(1996, 1979) and Tullis and Yund(1991, 1980) demonstrated with experimental studies that water could significantly reduce the strength of albite. Shelton and Tullis (1981) found the strength of anorthite (An96) may be significantly affected in the dislocation creep regime by water. Dimanov et al. (1999) and Stünitz and Tullis (2001) reported that anorthite was weaker under hydrous conditions comparing with those under anhydrous conditions. Under the conditions of temperatures of 1 140–1 480 K, a confining pressure of 300 MPa and strain rates of 2×10-6–1×10-3 s-1, Rybacki and Dresen (2000) found that the strength of plagioclase decreased with increasing water content in both the dislocation and the diffusion creep regimes under a hydrous condition (700 ppm water). Rybacki et al. (2006) firstly reported the water fugacity exponent r=1.0±0.3 in the diffusion creep regime under different water fugacity conditions. Extrapolated the experimental results to natural conditions of the continental lower crust, the strength of plagioclase would be reduced to 1/3 under hydrous conditions. All these studies demonstrated unambiguously that water could play a significant role in weakening the strength of plagioclase. However, the rheological flow laws of plagioclase in the dislocation creep regime are still very sparse (Fig. 3b).

    The deformation mechanism map for wet plagioclase is shown in Fig. 4b. Water plays an important role on the deformation mechanism of plagioclase. For example, the diffusion creep boundary of wet plagioclase at 900 ℃ shifts to the region of smaller grain size and higher flow stress when comparing to that of dry plagioclase at 900 ℃ (Dimanov and Dresen, 2005). This process is likely to play a crucial role in forming mylonite in the continental lower crust. Previous experimental results showed that water had a limited effect on stress exponent, but could significantly reduce activation energy. For example, for plagioclase samples of 40 ppm and 700 ppm water, their activation energies decrease from 648 to 467 kJ/mol in the dislocation creep regime, and from 356 to 170 kJ/mol in the diffusion creep regime, respectively (Rybacki and Dresen, 2000).

    There have been many studies on plagioclase fabrics from natural gabbroic mylonite. Results of these natural plagioclase fabrics are still controversial. Kruse et al. (2001) reported plagioclase fabrics in the Jotun nappe characterized with the [001] axes nearly subparallel to the lineation and the (010) planes subparallel to the foliation. Xie et al. (2003) found plagioclase fabrics in a highly deformed anorthositic mylonite showing the [100] axes forming high concentrations parallel to the lineation and the (010) planes roughly parallel to the foliation. Mehl and Hirth (2008) investigated mylonites in the Southwest Indian Ridge, and found strong fabrics of plagioclase in the plagioclase mylonite layers characterized by high concentrations of the [100] axes subparallel to the lineation and the (010) planes subparallel to the foliation. In contrast, the plagioclase fabrics are weak in the mylonite layers of mixed plagioclase and pyroxene. It had been proposed that the strong plagioclase fabrics were caused by dislocation creep while the weak fabrics were attributed to phase mixing that led to a transition to diffusion creep assisted by grain boundary sliding. Getsinger et al. (2013) had a similar observation as reported by Mehl and Hirth (2008) in a lower crustal shear zone exposed in gabbro. However, the plagioclase fabrics were different. Plagioclase in the plagioclase monophase region displayed a pronounced fabric with high concentrations of the [010] axes parallel to the lineation and the (100) parallel to the foliation. The weak plagioclase fabrics in the polyphase region were attributed to diffusion and dislocation accommodated grain boundary sliding. Homburg et al. (2010) investigated the plagioclase fabric in the Oman shear zone and found that the [100] axes of plagioclase formed high concentrations in the foliation plane at ~45° to the lineation, while the (001) planes were subparallel to the foliation. Satsukawa et al. (2013) summarized the fabrics of plagioclase in deformed gabbroic rocks and classified them into three types (Fig. 6): the axial-B fabric characterized by the (010) planes approximately parallel to the foliation, and the [100] axes and the poles of the (001) forming girdles in the foliation; the type-P fabric characterized by the [100] axes parallel to the lineation and the (010) planes parallel to the foliation; the axial- A fabric with the [100] axes parallel to the lineation and the poles of the (010) and (001) planes forming girdles perpendicular to the foliation.

    Figure 6.  Sketch of typical plagioclase fabrics (after Satsukawa et al., 2013). (a) Axial-B fabric; (b) type-P fabric; (c) axial-A fabric.

    Ji et al. (1999) found in torsion experiments that the [100] axes of anorthite aggregates were subparallel to the shear direction while the (010) planes were parallel to the shear plane, implying a [100](010) slip system. Heidelbach (2000) also reported the important role of the [100](010) slip system in forming plagioclase fabrics. However, under similar conditions with lower strain rates, Stünitz et al. (2003) reported the [001](010) being the dominant slip system with TEM observations. In fact, most fabric analyses (Ji et al., 1988; Ji and Mainprice, 1988) and TEM results (Kruse and Stünitz, 1999; Montardi and Mainprice, 1987; Olsen and Kohlstedt, 1984) showed that the [001](010) slip system was the dominant slip system in plagioclase. Ji et al. (2000) speculated that the [100] and the [001] axes were the dominant sliding directions at high and low temperatures, respectively. In addition, the increase of water fugacity could also reduce the critical temperature at which the sliding direction changed from the [001] axes to the [100] axes.

  • Garnet can be an abundant constituent mineral in the overthickened continental lower crust (Johnson et al., 2017). Under such conditions, garnet would have an important effect on the rheology of the continental lower crust. Quantitative studies of garnet creep have been sparse. There exist only a limited number of experimental studies on plasticity of garnets (e.g., Xu et al., 2013; Zhang and Green, 2007; Wang and Ji, 2000, 1999; Voegelé et al., 1998; Cordier et al., 1996; Karato et al., 1995). Karato et al. (1995) conducted a series of studies on the plastic deformation of a suite of garnet single crystals at high temperature and room pressure. A unified rheological law for garnets was established. Their results showed that the resistance to plastic deformation in garnets was significantly higher than for other minerals in the Earth's mantle. They found the creep strength of garnet was largely controlled by the resistance to dislocation glide rather than by recovery processes. Similar results were achieved by Wang and Ji (1999). They extrapolated their results to natural conditions and estimated that garnet became ductile at around 1 073–1 173 K and that the rheological contrast between garnet and clinopyroxene was minimal at temperatures higher than that. However, it is worth mentioning that most silicate garnets are not stable under these room pressure and high temperature experimental conditions. Hence, these results should only be treated as the lower bound of the garnet strength. Cordier et al. (1996) and voegelé et al. (1998) studied the plastic deformation of garnet single crystals with TEM under a high confining pressure of 6.5 GPa. They suggested brittle deformation of garnet below 1 273 K and ductile deformation above 1 273 K, characterized by dislocation creep with significant recovery. However, no correlation was found between dislocation microstructure and hydrous components in garnet. The mechanical strength of dry garnet aggregates has been demonstrated to very high (Zhang and Green, 2007; Jin et al., 2001), making it technically very difficult to determine the flow law of dry garnet. Xu et al. (2013) determined for the first time using a DDIA deformation apparatus the flow law of garnet under water saturated conditions and attained a water fugacity exponent of 1. It has been proposed that water can significantly reduce the strength of garnet comparing to the reduction effects on olivine and diopside (Muramoto et al., 2011).

    Dislocation creep and diffusion creep of garnets have been proposed respectively based on various evidence from previous studies. TEM studies revealed the long lengths of the dominant Burgers vector of garnet dislocations, (1/2)[111] and [100] (Allen et al., 1987). This, together with the high creep strength of garnet determined in the laboratory, has led to a generalized assumption that garnet remains essentially rigid during natural deformation (Jin et al., 2001; Karato et al., 1995). Garnet with strong shape preferred orientation (SPO) has been proposed to form by grain boundary sliding under hydrous conditions (e.g., Zhang and Green, 2007; Brok and Kruhl, 1996). The discovery of dislocations in elliptically-shaped garnets led to the opposite argument that garnet could deform by dislocation creep under high temperatures (Xie et al., 2019; Ji et al., 2003; Ji and Martignole, 1994). However, electron backscattered diffraction (EBSD) analyses revealed random distributions of crystallographic preferred orientation (CPO) of elliptical garnets in deformed granulites and eclogites (e.g., Zhang and Green, 2007; Kleinschrodt and Duyster, 2002; Kleinschrodt and McGrew, 2000). It is inconsistent with the numerical simulation results that garnet shall develop strong fabrics (Fig. 7) through dislocation creep. We currently still understand little about how natural garnets are deformed and what mechanism dominates the deformation processes-dislocation creep, diffusion creep or grain boundary sliding.

    Figure 7.  Fabrics of garnet by numerical simulation (VPSC modeling) (after Mainprice et al., 2004). The pole figures are predicted fabric diagrams for the < 100 > , < 111 > and < 110 > axes (lower hemisphere; scale is in multiples of a uniform distribution) after plastic deformation as indicated. Dominant slip system is assumed to be {110} < 111 > . Equivalent strains are 1.0 for all simulations. Axial compression: (a) (α=1), (b) (α=100); simple shear: (c) (α=1), (d) (α=100).

  • Natural granulite xenoliths uplifted by magmatism, and the lower crustal cross sections exposed by tectonic movements provide opportunities to directly study rheology of the continental lower crust (e.g., Soret et al., 2019; Homburg et al., 2010; Percival et al., 1992), including foliation and lineation (e.g., Dumond et al., 2010; Chardon and Jayananda, 2008), microstructures such as lattice preferred orientation (LPO), shape preferred orientation (SPO) and deformation under optical microscope (e.g., Puelles et al., 2009; Lund et al., 2006; Martelat et al., 1999). Past studies have indicated extensive plastic deformation in the continental lower crust in both macroscopic and microscopic scales.

    Compared with monophase aggregates, experimental investigations of polyphase rocks that can best represent the continental lower crust are still sparse. Wilks and Carter (1990) established the flow laws of natural felsic and mafic granulites. Mackwell et al. (1998) studied the rheological strength of diabase with different proportions of clinopyroxene and plagioclase. The results showed that diabase rich in plagioclase was much weaker than that rich in clinopyroxene, indicating that different proportions of mineral components had a significant effect on the rheological properties of polyphase lower crustal aggregates. Wang et al. (2012) conducted systematic deformation experiments with synthetic mafic granulite and established the flow law of mafic granulite in the dislocation creep regime under hydrous conditions. Extrapolation of the flow law of mafic granulite to natural conditions suggests a weaker continental lower crust comparing with the upper crust and the upper mantle, supporting the "Jelly Sandwich" continental lithosphere strength profile model. Zhou et al. (2012) established for the first time the flow law of fine-grained gabbro containing partial melt. In addition, the experimental results of Jin et al. (2001) and Zhang and Green (2007) showed that the mechanical strength of eclogite was comparable to that of dry harzburgite. All these experimental results provide important constraints on the rheological properties of continental lower crust.

  • Although there are three classic continental lithosphere strength profile models to describe the first-order rheological properties of the continental lithosphere, these existing models are unlikely to accurately estimate the mechanical strength of lithosphere because of the heterogeneity in water contents in minerals and rocks of the continental lower crust from different tectonic settings. As we have summarized above, they cannot be simplified as anhydrous or hydrous. The current continental lithosphere strength profile is either too weak under hydrous conditions (Fig. 1a) to sustain longevity of the continental root or too strong under anhydrous conditions (Fig. 1b) to support the observations of extensive continental deformation. Therefore, quantitative experimental investigations on water effects of rheology of the continental lower crustal minerals and rocks are necessary to refine these classic models. In addition, the rheology of plagioclase has been widely used to represent the mechanical strength of the continental lower crust in these models. However, the experimental starting materials of these experiments are mostly synthetic end-member components (calcium- or sodium- plagioclase) while the representative constituent plagioclase in the continental lower crust is mafic labradorite (Rudnick and Gao, 2003; Rudnick and Fountain, 1995). This difference in composition would lead to inaccuracy in the estimation of rheology as well.

    Due to the limitations of experiments in Paterson-type deformation apparatus, water fugacity exponents of clinopyroxene and plagioclase calculated from such experiments (Chen et al., 2006; Rybacki et al., 2006) may have large uncertainty when extrapolating to natural conditions. For example, the water content in clinopyroxene was 9 ppm–31 ppm for the extrapolation of water fugacity exponents in Chen et al. (2006). Such limited water contents in clinopyroxene could not well represent the large span of water content (a few hundreds to thousands of ppm water) in natural samples of the continental lower crust (Table 1). Meanwhile, these previous experiments were performed under water-saturated conditions, which is inconsistent with the natural water-unsaturated environment in deep Earth (e.g., Karato, 2010). Water-saturation can lead to molecular water droplets or film along grain boundaries, giving rise to a large uncertainty in estimation of the water content thus the water weakening effect. Up to now, studies on the rheological strength of minerals under water-unsaturated conditions are still very scarce (e.g., Faul et al., 2016; Fei et al., 2013). Although dislocation creep is generally believed to dominate in deep Earth (e.g., Karato, 2010), there is currently no experimental investigation on the water fugacity exponent of plagioclase in the dislocation creep regime.

    Finally, considering the heterogeneity in composition and water content, extensive natural and experimental investigations on water content, fabric and deformation mechanism of plagioclase and clinopyroxene are necessary to address the existing controversy and to better decipher the complex rheology of the continental lower crust.

  • We appreciate constructive comments from three anonymous reviewers that have greatly improved the manuscript. This research was funded by grants from the National Natural Science Foundation of China (Nos. 41425012, 41590623) and the MOST special fund from the State Key Laboratory of GPMR at China University of Geosciences, Wuhan. The final publication is available at Springer via https://doi.org/10.1007/s12583-020-1307-9.

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