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Volume 31 Issue 2
Apr.  2020
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Zhanjun Xie, Xiangwen Liu, Zhenmin Jin, Zhuoyue Li. Microstructures and Phase Transition in Omphacite: Constraints on the P-T Path of Shuanghe Eclogite (Dabie Orogen). Journal of Earth Science, 2020, 31(2): 254-261. doi: 10.1007/s12583-019-1279-9
Citation: Zhanjun Xie, Xiangwen Liu, Zhenmin Jin, Zhuoyue Li. Microstructures and Phase Transition in Omphacite: Constraints on the P-T Path of Shuanghe Eclogite (Dabie Orogen). Journal of Earth Science, 2020, 31(2): 254-261. doi: 10.1007/s12583-019-1279-9

Microstructures and Phase Transition in Omphacite: Constraints on the P-T Path of Shuanghe Eclogite (Dabie Orogen)

doi: 10.1007/s12583-019-1279-9
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Microstructures and Phase Transition in Omphacite: Constraints on the P-T Path of Shuanghe Eclogite (Dabie Orogen)

doi: 10.1007/s12583-019-1279-9
    Corresponding author: Xiangwen Liu

Abstract: Omphacite microstructures are important to decipher the metamorphic pressure-temperature (P-T) conditions of eclogite during subduction and exhumation processes. Here we present a systematic microstructural investigation of omphacite using transmission electron microscopy (TEM) in an ultrahigh-pressure (UHP) eclogite sample from Shuanghe, Dabie orogen, China. The omphacite can be divided into two phases:(1) Omp-1, exhibits as the main part of omphacite with P2/n space group containing small amount of dislocations and numerous antiphase domains (APDs). (2) Omp-2, shows as banded subgrains with C2/c space group containing large amount of dislocations. These two phases of omphacite have almost the same crystallography orientation and bounded by dislocation walls. Along these dislocation walls, we found some barroisite and albite microcrystals. The barroisite microcrystals contain large amount of dislocations and show a topological relation with host omphacite. While the albite microcrystals contain small amount of dislocations and does not show topological relation with host. These microstructures could give us the following metamorphic information to constrain the P-T path of eclogite:Firstly, this two omphacite space groups should be evolved from the same precursor C2/c omphacite, which had been underwent strongly plastic deformation during the syn-to late-peak metamorphic stage. Secondly, the precursor C2/c omphacite began to be retrograded and altered by small amount of barroisite microcrystals along its dislocation walls under the P-T condition of 2.2-2.6 GPa and 650-700℃ at the early amphibole eclogite stage. Thirdly, large amount of Na and other elements were exsolved from some precursor C2/c omphacite subgrains and crystallized numerous albite microcrystals at their boundaries, which is the necessary for the space group of C2/c surviving under the lower P-T conditions (1.7-1.9 GPa and 630-690℃) during the middle amphibole eclogite stage. Lastly, the rest of precursor C2/c omphacite subgrains were transformed into P2/n polymorph and formed the APDs structures under the P-T condition of ~1.5 GPa and 650-680℃ during the late amphibole eclogite stage.

Zhanjun Xie, Xiangwen Liu, Zhenmin Jin, Zhuoyue Li. Microstructures and Phase Transition in Omphacite: Constraints on the P-T Path of Shuanghe Eclogite (Dabie Orogen). Journal of Earth Science, 2020, 31(2): 254-261. doi: 10.1007/s12583-019-1279-9
Citation: Zhanjun Xie, Xiangwen Liu, Zhenmin Jin, Zhuoyue Li. Microstructures and Phase Transition in Omphacite: Constraints on the P-T Path of Shuanghe Eclogite (Dabie Orogen). Journal of Earth Science, 2020, 31(2): 254-261. doi: 10.1007/s12583-019-1279-9
  • Eclogite is one of the eyewitnesses of geodynamic processes like subduction, continent-continent collision, or exhumation of oceanic or continental crust (Bukała et al., 2018; Wang et al., 2009; Godard, 2001). Omphacite, as one kind of the mainly constituent mineral in eclogite, has the following properties to make it provide huge amount of information about these geodynamic processes: (1) The strength of omphacite is much weaker than that of garnet leading the rheological properties of eclogite are mainly dominated by omphacite (Zhang and Green, 2007; Jin et al., 2001; Piepenbreier and St ckhert, 2001). (2)he space group of omphacite would transform between disordered C2/c and ordered P2/n at temperatures about 800 ℃ (depending on its composition) (Carpenter, 1982; Fleet et al., 1978; Matsumoto et al., 1975). (3) The ordering transition of P2/n omphacite could form the microstructure of antiphase domains (APDs) whose mean size depends on the temperature, time, cooling rate and composition (Brenker et al., 2003; Carpenter, 1982). (4) The exsolutions, inclusions, and precipitated minerals, such as quartz and its polymorph phases, K-feldspar, clinoamphibole, etc., in omphacite have been regarded as the indicators of metamorphic pressure-temperature (P-T) conditions (Liu et al., 2018; Xu et al., 2015; Müller et al., 2004; Skrotzki, 2001; Becker and Altherr, 1992; Smith, 1984). Therefore, in addition to the widely used geothermobarometric methods based on element partitioning between minerals, the microstructures in omphacite and other minerals from eclogite have become an important key to decipher its actual P-T path (Müller et al., 2011; Liou et al., 2009; Müller and Compagnoni, 2009; Neufeld et al., 2008; Ji and Martignole, 1994).

    It has been confirmed that the eclogites and their surrounding country rocks in the Dabie orogen were subjected to in-situultrahigh-pressure (UHP) metamorphism based on the widespread coesite and diamond inclusions and exsolution textures in eclogitic and gneissic minerals (Liu et al., 2007; Xu et al., 2005; 1992; Okay et al., 1989). However, the reports about mineral microstructures evidence to corroborate the peak metamorphic conditions and the P-T path are still scarce. In this paper, we investigated the coexisting microstructures including different space groups, dislocation patterns, APDs, and precipitated minerals in omphacite from the Shuanghe UHP eclogite by transmission electron microscopy (TEM) and discussed their geological implications.

  • In this study, the eclogite sample DS12 was collected from the Shuanghe UHP metamorphic belt at the eastern Dabie orogen, China (Fig. 1). The Dabie orogen belt is bounded to the east by the Sulu orogen belt, and the boundary between them is the strike-slip Tancheng-Lujiang fault (T-L fault) (Wang et al., 2010; Zhang et al., 2009). Detailed geological, petrological and geochemical studies showed that the age of peak metamorphism is 234–226 Ma at pressures above 2.7 GPa and temperatures of 700–850 ℃ (Liu et al., 2006; Zhang et al., 2003; Li et al., 2000; Liou et al., 1997; Cong et al., 1995; Okay, 1993).

    At Shuanghe region, the UHP metamorphic rocks form an elongated tectonically-bounded slab with NNW-SSE trend, which is surrounded mostly by orthogneisss (Fig. 1). This UHP metamorphic slab exhibits an evident compositional layering which is approximately parallel to the metamorphic foliation. The compositional layering is composed of the following main rock types: eclogite, schist, gneiss, marble, jadeite-quartzite, and amphibolite.

    The studied eclogite sample was taken from the eastern side of the reservoir (Fig. 1). The eclogites at this site are exposed within biotite-feldspar gneiss rocks and show obvious metamorphic foliation with a medium-grained texture (Fig. 2). The garnet and omphacite grains were slightly elongated indicating that they experienced a plastic deformation process. The rim of some omphacite grains is replaced by a narrow fine-grained symplectite belt indicating that they were retrograded slightly.

    Figure 1.  Geological map of the Shuanghe region, eastern Dabie orogen, China (modified after Xie et al. (2019)).

    Figure 2.  Photomicrograph of the studied eclogite sample DS12 under cross polarized light. The garnet and omphacite grains were slightly elongated. Grt. Garnet; Omp. omphacite; Qtz. quartz; Rt. rutile; Sym. symplectite.

  • Firstly, the petrological and mineralogical characteristics of our eclogite thin section were studied by an optical microscopy. Then, the selected omphacite grains for TEM observation were mounted on the 3-mm-diameter copper rings and removed from the thin section. These specimens were thinned to electron transparency via Ar+ ion milling in a Gatan-600 ion mill working at an accelerating voltage of 4 kV and an ion beam current of 1 mA. After that, a conductive carbon film was coated on the specimen surface using a JEOL-JEE4X vacuum evaporator. The microstructures of omphacite were analyzed using a Philips CM12 TEM, which has equipped with an EDAX PV9100 X-ray energy dispersive spectroscopy (EDS). Selected area electron diffraction (SAED) patterns and bright field (BF) observations were carried out at the accelerating voltage of 120 kV, spot sizes of 1.4 μm for imaging and 100 nm for EDS. The diffraction patterns were analyzed by the software package of TEMlab1.0 (Xu and Huang, 2014). All experiments were carried out at the State Key Laboratory of Geological Processes and Mineral Resources (GPMR) at China University of Geosciences (Wuhan).

  • In this study, we have observed the coexisting microstructures of different space groups, dislocation patterns, APDs, and precipitated minerals in 3 omphacite crystals from the thin section of eclogite sample DS12. As shown in Figs. 3, 4, the microstructural observations and SAED patterns suggest that our omphacite can be divided into two phases: (1) Omp-1, exhibits as the main part of omphacite with P2/n space group containing small amount of dislocations. At some special orientations, it can be found numerous irregular or roundish APDs with the size of 0.1–0.4 μm distributed in this area, which is the consequence of cation ordering (Brenker et al., 2003, 2002). (2) Omp-2, shows as banded subgrains with a size of 1–4 μm in the matrix of Omp-1, which has the C2/c space group and contains large amount of dislocations. The SAED patterns indicate that these two phases bear almost the same crystallography orientations, such as [010]Omp-1∥[010]Omp-2 and [121]Omp-1∥[121]Omp-2, and bounded by the subparalleled [100]* dislocation walls. The EDS spectra suggest that the Na and Al content of Omp-1 are much higher than that of Omp-2. This coexistence of C2/c and P2/n space groups in omphacite was also founded in the eclogite from Shima, Dabie orogen, which is attributed to the metastable transition of omphacite during a rapid cooling process (Wu et al., 2010).

    Figure 3.  Bright field photos of omphacite, barroisite and albite microcrystals. (a) and (b) are the same measured area viewed from different orientations showing the omphacite subgrain (Omp-2) distributes in the host of omphacite (Omp-1). Barroisite (Brs) and albite (Ab) microcrystals precipitated at the boundaries of Omp-2. Numerous antiphase domains (APDs) only founded in Omp-1. (c) Small albite microcrystal accompanied with Omp-2 in the host of Omp-1. (d) The Omp-2 subgrain containing large amount of dislocations is separated from the dislocation-free matrix of Omp-1 by [100]* dislocation wall.

    Figure 4.  EDS spectra and SAED patterns of Omp-1 and Omp-2 from the area of Fig. 3a. (a) EDS spectrum of Omp-1 shows high Na and Al containing; (b) and (c) are the same measured area of Omp-1 viewed from the orientations of [010] and [121] indicating it belongs to P2/n space group; (d) EDS spectrum of Omp-2 shows low Na and Al contenting; (e) and (f) are the same measured area of Omp-2 viewed from the same orientation to (b) and (c) respectively indicating it belongs to C2/c space group.

  • We found some mineral microcrystals precipitated around the C2/c omphacite subgrains (Fig. 3). The EDS spectra of these mineral microcrystals indicated they are belong to the amphibole or plagioclase (Figs. 5a, 5d). Based on the SAED analysis, we confirm that the cell parameters of amphibole microcrystals are consistent with that of barroisite (Binns, 1967), which bear a topological relation of [010]Omp-1∥[010]Omp-2∥[010]Brs and [121]Omp-1∥[121]Omp-2∥[110]Brs with the host omphacite. The plagioclase microcrystal's cell parameters correspond to that of low albite (Armbruster et al., 1990), which does not show certain topological relation with the host and other minerals. The barroisite microcrystals show as ~0.5 μm in width and ~1.5 μm in length with numerous dislocations. In contrast, the albite microcrystals exhibit as a size of 1–2 μm in width and 1.5–2.5 μm in length with rarely dislocation structures.

    Figure 5.  EDS spectra and SAED patterns of barroisite and albite from the area of Figs. 3a, 3c respectively. (a) EDS spectrum of barroisite shows it contains low Na content; (b) and (c) are the same barroisite crystal viewed from the orientations of [0-10] and [1-10]; (d) EDS spectrum of albite shows it contains high Na content; (e) and (f) are the same albite crystal viewed from the orientations of [-1-3-5] and [-1-1-3].

  • The semi-quantitative calculation based on the EDS spectra of omphacite show that the value of XNa/(Na+Ca) in Omp-1 approximate to 0.41. This composition is similar to that of omphacite inclusions in zircons from eclogite lens in marble (Liu et al., 2006) and omphacite from banded eclogite in gneiss (Wang, 2005), whose variation range of XNa/(Na+Ca) is much smaller than that of omphacite from the jadeite-quartzite from our studied area (Wang et al., 2010). In contrast, the value of XNa/(Na+Ca) in Omp-2 is less than 0.22, which is much lower than that of Omp-1. Combined with their SAED analysis, the data of Omp-1 and Omp-2 are plotted in the P2/n and C2/c space group field of omphacite phase diagram, respectively (Fig. 6) (Rossi et al., 1983). The omphacite with C2/c space group is known as its fully disordered phase under high temperatures condition, which would transform into P2/n phase if temperatures lower than 730 ℃ when considering the composition of Omp-1 (Carpenter, 1982; Fleet et al., 1978; Matsumoto et al., 1975). The coexistence of P2/n and C2/c omphacite space groups and their topological relations, dislocation patterns and precipitated microcrystals indicate that these two space groups should be evolved from the same precursor C2/c omphacite phase, which had experienced a complex evolution history including plastic deformation, chemical diffusion, space group transition and phase alteration during the exhumation process of subducted slab.

    Since the Omp-2 subgrains contain lower Na and Al (esp. Na) content than that of Omp-1 and the microcrystals of barroisite and albite precipitated at their boundaries (Figs. 3, 4, 6), the exsolved Na and Al and other elements from Omp-2 would be the main materials source for these Na- and/or Al-bearing microcrystals crystallized. Considering the garnet and omphacite from Shuanghe UHP eclogites could contain water content up to > 2 000 wt. ppm and ~500 wt. ppm H2O respectively (Xie et al., 2019; Liu et al., 2016; Sheng et al., 2007), the host omphacites could provide the water for small amount of barroisite crystallized at the early stage of retrogradation. Furthermore, the topological relation of [010]Omp//[010]Brs, where [010]Brs=[010]Brs, showed in Fig. 5 had been reported in most previous studies (Müller et al., 2004; Skrotzki, 2001; Veblen and Buseck, 1981). In addition, Müller et al. (2004) found another topological relation of [101]Omp//[101]Brs. These topological relations between barroisite microcrystals and host omphacite indicating they may have a genetic relationship. However, these two precipitated minerals should not be ascribed to the typical exsolution phenomenon because this would be an inequality chemical system that the external water and other elements could diffuse into the inner of omphacite crystal through its subgrain boundaries. In other words, these precipitated minerals should be ascribed to the metasomatism or alteration phenomenon, which belongs to the retrogradation process (Müller et al., 2004; Veblen and Buseck, 1981). The distinction between them and exsolution is chemical, rather than structural (Liu et al., 2009). The double chains silicate of amphibole microcrystals would preferentially to nucleate at the deformation-induced [010] dislocations of single chain silicate omphacite (Skrotzki, 2001).

    Figure 6.  Evolution path of omphacite in the diopside-jadeite (Di-Jd) system (modified after Rossi et al. (1983)). The red boxes with solid line are the data measured in this study. The blue solid lines and red boxes with dotted line boundaries are the inferred evolution path and precursor composition of omphacite, respectively. The green, canary yellow and light blue boxes are results from Wang (2005), Wang et al. (2010) and Liu et al. (2006), respectively.

    Considering the peak metamorphic temperature of Shuanghe UHP slab is 700–800 ℃ (Liu et al., 2006; Liou et al., 1997; Cong et al., 1995), the stable temperature condition of precursor C2/c omphacite that containing high Na content would be up to ~800 ℃ (Fig. 6). The large amount of dislocations in barroisite microcrystals indicate that they were precipitated earlier than P2/n omphacite (Omp-1) and albite grains and experienced a plastic deformation process together with the precursor C2/c omphacite. Due to the low Na content of barroisite, its crystallization could not obviously impede the major trend of its disordered C2/c phase to an ordered P2/n state. However, the Na-rich albite microcrystals precipitated around the Na-poor C2/c omphacite (Omp-2) subgrains suggests that their precursor phase experienced a Na and other elements exsolution event and crystallized these albite microcrystals. This reaction could strongly decrease the Na content of these precursor C2/c omphacite subgrains and would be the key point of their C2/c space group and dislocation patterns survived under the low P-T conditions. In contrast, those precursor C2/c omphacite subgrains that did not exsolved Na element were transformed into the P2/n omphacite. This phase transition is a process of diffusion-controlled ordering of Mg and Al on the M1 positions, which could promote the dislocation recovery and the APDs formation (Brenker et al., 2003; Carpenter, 1982). These would be the main reasons that the two space groups of omphacite and their microstructures can coexist in our sample.

  • Based on the mineralogical and textural relationships of eclogite and its surrounding rocks, the metamorphic history Shuanghe UHP slab can be divided into five stages (Table 1, Fig. 7) (Li et al., 2000; Cong et al., 1995). Numerous researchers had been investigated the metamorphic P-T conditions of these stages based on the geochemical data, and their results show that the peak P-T values obtained from the omphacite and garnet inclusions in zircons (Liu et al., 2006) are much higher than that from the common mineral assemblages in eclogite (Wang, 2005; Liou et al., 1997; Cong et al., 1995). In addition, the widely distributed coesite inclusions in minerals from Shuanghe UHP rocks also suggest that the peak P-T values could be up to 2.7 GPa at 700 ℃ (Wang, 2005). In addition, Wu et al. (2004) and Wu et al. (2008) discovered the monalbite in jadeite from jadeite quartzite of our studied area, indicating that the host rock experienced a high temperature metamorphism over 930 ℃.

    Figure 7.  The omphacite microstructures formed events and P-T path of Shuanghe eclogite. The P-T boundaries of various metamorphic facies are modified after Liou et al. (2009). Abbreviations: GR. granulite; AM. amphibolite; EA. epidote amphibolite; BS. blue schist; GS. green schist; EC. eclogite, which are subdivided into amphibole (Amp) eclogite, epidote (Ep) eclogite, lawsonite (Lws) eclogite and dry eclogite.

    In this study, the barroisite and albite microcrystals precipitated at the dislocation walls of omphacite suggests that our eclogite had went into the amphibole eclogite retrograde stage. High pressure and temperature experiment results show that the barroisite would appear under the P-T conditions of 2.2–2.4 GPa and 650–700 ℃ under H2O-saturated condition (Forneris and Holloway, 2003), which could correspond to the early amphibole eclogite stage. This is consistent with the inference that the barroisite microcrystals should be precipitated earlier than that of P2/n omphacite (Omp-1) and albite grains based on their dislocation features. Previous studies show that the low albite microcrystals would be stable at the temperatures of less than 700 ℃ (Deer et al., 2001). Considering the high-pressure conditions when these low albite microcrystals precipitating, we infer their precursor phase would be high albite phase corresponding to a P-T conditions of 1.7–1.9 GPa and 630–690 ℃ (Liou et al., 1997), which should be assigned to the middle amphibole eclogite stage.

    Metamorphic stage Mineral assemblage Age (Ma) Pressure (GPa) Temperature (℃) References
    Pre-eclogitic stage Grt+Omp > 241 1.7–1.8 588–668 Liu et al. (2006)
    Ep+Amp 0.6–0.8 Cong et al. (1995)
    Peak coesite-eclogite stage Coe+Grt+Omp±Qtz 239–231 > 5.5 784–849 Liu et al. (2006)
    > 2.7 650–750 Cong et al. (1995)
    Liou et al. (1997)
    > 2.9 780–840 Wang (2005)
    Quartz-eclogite stage Grt+Omp+Qtz > 1.8 700–950 Cong et al. (1995)
    Amphibole eclogite stage
    (Symplectite stage)
    Sym(Na-Ca Amp±Na-Pl)±Grt±Omp 219–211 0.8–1.4 550–720 Liu et al. (2006)
    2.2–2.4 650–680 Liou et al. (1997)
    Brs+C2/c Omp 2.2–2.4 650–700 Present study
    Low Ab+C2/c Omp 1.7–1.9 630–690 Present study
    APDs+P2/n Omp ~1.5 650–680 Present study
    Amphibolite stage Amp+Pl±Grt 0.6–0.8 470–570 Cong et al. (1995)

    Table 1.  Metamorphic P-T conditions of the Shuanghe eclogite

    As mentioned above, the size of APDs has been regarded as a time-temperature indicator of omphacite bearing metamorphic rocks (Brenker et al., 2003; Carpenter, 1982). The coarsening rate law of APDs in omphacite can be expressed as

    where δ0 is the initial APD size, δ is the resulting size of APDs after a respective time interval ∆t corresponding to the peak metamorphic temperature T0 in Kelvin with a temperature variation of ∆T, n=8 and $K=6\times {{10}^{35-n}}{{{\AA}}^{n}}/yr$ are constant, ∆H=95 kcal/mol is the activation energy of Fe-bearing omphacite, and R=8.314 is the gas constant. The isotopic chronology and cooling history studies of Shuanghe UHP metamorphic rocks show that they have experienced a rapid exhumation or decompression process from the peak metamorphism age of 234–226 Ma to the amphibole eclogite stage age of 219 Ma, corresponding to the temperatures reduced from ~800–850 ℃ to ~500 ℃ with a cooling rate of about 40 ℃/Ma (Liu et al., 2006; Li et al., 2000). Thus, the APDs should be formed during the cooling period and their initial size δ0 could be neglected and the time interval ∆t is about 7–15 Ma. Carpenter (1982) suggested that the roundish equally-axed antiphase domains are of informational value for an estimation of the thermal history. The irregular and larger APDs should be attributed to dislocation assisted coarsening mechanism. In this study, the mean size of roundish APDs is about 0.15–0.20 μm in Omp-1. Using these parameters, we obtained the temperatures recorded by APDs size is about 650–680 ℃, which are plotted in the amphibole eclogite stage temperature field. Since the C2/c omphacite phase areas contain large amount of dislocations and do not contain any P2/n omphacite subgrains, we infer that their Na exsolution (albite crystallized) event would be occurred slightly earlier than omphacite space group transition. Therefore, the pressures of APDs formed should be slightly lower than that of albite crystallized and are inferred to be ~1.5 GPa, which would be occurred at the late amphibole eclogite stage.

    Despite the precipitated barroisite and albite microcrystals are not the typical exsolutions, their crystallized thermodynamic condition can also be used to constrain the P-T path of their host eclogite rocks. As shown in Fig. 7, after plotting the above omphacite microstructures formed events on the P-T diagram, we obtain a P-T path of our eclogite sample. The metamorphic temperatures at amphibole eclogite stage revealed by our omphacite microstructures is up to 700 ℃. This temperature is slightly higher than that estimated by Liou et al. (1997), even it is obviously lower than that of other previous results (Liu et al., 2006; Wang, 2005; Cong et al., 1995).

  • The P2/n space group phase is the main part of omphacite, which contains small amount of dislocations and numerous antiphase domains (APDs). The C2/c space group phase with large amount of dislocations shows as the subgrains that are surrounded by some barroisite and albite microcrystals. These coexisting microstructures indicate that this two omphacite space groups should be evolved from the same precursor C2/c omphacite, which had experienced a strongly plastic deformation process during the syn- to late-peak metamorphic stage. At the early amphibole eclogite stage, this precursor C2/c omphacite began to retrograde and altered small amount of barroisite microcrystals along its dislocation walls. And then, some precursor C2/c omphacite subgrains were exsolved large amount of Na and other elements and crystallized some albite microcrystals at their boundaries during the middle amphibole eclogite stage, which is the key point for their C2/c space group survived under low the P-T conditions. Finally, the rest of precursor C2/c omphacite subgrains were transformed into P2/n phase and formed the APDs structures at the late amphibole eclogite stage.

  • We would like to thank the two anonymous reviewers for their insightful comments and suggestions. This work was supported by the National Natural Science Foundation of China (Nos. 41272224, 41972231). The final publication is available at Springer via https://doi.org/10.1007/s12583-019-1279-9.

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