Omphacite grains from UHP eclogite of the Dabie Mountains in eastern China are elongated and show strong lattice preferred orientations (LPOs). Observations by the transmission electron microscopy (TEM) identified not...Omphacite grains from UHP eclogite of the Dabie Mountains in eastern China are elongated and show strong lattice preferred orientations (LPOs). Observations by the transmission electron microscopy (TEM) identified not only structures of plastic deformation occurring as free dislocation, dislocation loops and dislocation walls, but also bubbles of water present in the deformed omphacite. The bubbles attach to the dislocation loops which are often connected to one another via a common bubble. Using infrared spectroscopy (IR), two types of hydrous components are identified as hydroxyl and free-water in the omphacite. An analysis of deformation mechanism of microstructure in omphacite suggests that the mineral deformed plastically under UHP metamorphic conditions by dislocation creep through hydrolitic weakening.展开更多
The rheological characters of omphacites in Dabie ultra-high-pressure eclogite have been studied in terms of fabric, dislocation and micro-structures. 1. The eclogite has undergone high-temperature deformation, thus f...The rheological characters of omphacites in Dabie ultra-high-pressure eclogite have been studied in terms of fabric, dislocation and micro-structures. 1. The eclogite has undergone high-temperature deformation, thus forming omphacite lattice preferred orientation. In addition to creep dislocation, the omphacite ductile deformation may have other mechanisms, such as diffusion creep and grain boundary migration. 2. The main-phase deformation of eclogite is coaxial, but asymmetry strain also exists due to strain partitioning in the Dabie orogenic belt. 3. The twin measured by the universal stage is (100), indicating that omphacite high-T deformation was superimposed by low-T deformation. 4. Subgrain structure is common in omphacite, but the deformation features of the omphacites in the Shuanghe area and Bixiling area are different, the latter being dominated by dynamic recrystallization. 5. The Flinn plots show that the strain of omphacite belongs to the constriction ellipsoid and stretching strain, which is similar to the result of the omphacite fabric analysis.展开更多
Omphacite microstructures are important to decipher the metamorphic pressuretemperature(P-T)conditions of eclogite during subduction and exhumation processes.Here we present a systematic microstructural investigation ...Omphacite microstructures are important to decipher the metamorphic pressuretemperature(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 C)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.展开更多
Transmission electron microscope (TEM) and high resolution transmission electron microscope (HRTEM) analyses have been performed on omphacite from ultra-high pressure (UHP) eclogites at the locality of Shima, Dabie Mo...Transmission electron microscope (TEM) and high resolution transmission electron microscope (HRTEM) analyses have been performed on omphacite from ultra-high pressure (UHP) eclogites at the locality of Shima, Dabie Mountains, China. TEM reveals that the microstructures consist dominantly of dislocation substructures, including free dislocations, loops, tiltwalls, dislocation tangles and subboundaries. They were produced by high-temperature ductile deformation, of which the main mechanism was dislocation creep. Antiphase domain (APD) boundaries are common planar defects; an age of 470 ± 6 Ma for UHP eclogite formation has been obtained from the equiaxial size of APDs in ordered omphacites from Shima, coincident with ages given by single-zircon U-Pb dating (471 ± 2 Ma). HRTEM reveals C2/c and P2/n space groups in different parts of one single omphacite crystal, and no exsolution is observed in the studied samples, which is attributed to rapid cooling. It is suggested that the UHP eclogites underwent a long period of annealing at high temperatures, followed by relatively rapid cooling. These data provide valuable information for the formation and exhumation mechanism of UHP eclogites in the Dabie high-pressure (HP) and UHP metamorphic belt.展开更多
Distributions of the rare-earth elements (REE) in omphacite and garnet and REE behaviors during metamorphic processes were discussed. The REE concentrations of garnet and omphacite in six eclogite samples from the Dab...Distributions of the rare-earth elements (REE) in omphacite and garnet and REE behaviors during metamorphic processes were discussed. The REE concentrations of garnet and omphacite in six eclogite samples from the Dabie Mountain, central China, were measured by inductively coupled plasma-mass spectrometry (ICP-MS). The correlation of δEu ratios between garnet and omphacite indicated that chemical equilibrium of REE distribution between garnet and omphacite could be achieved during ultra-high pressure (UHP) metamorphism. Most of the partition coefficients (Kd=CiOmp/CiGrt) of light rare-earth elements (LREE) are higher than 1. However the partition coefficients of heavy rare-earth elements (HREE) are lower than 1. This indicated that the LREE inclined to occupy site M2 in omphacite, but the HREEs tended to occupy eightfold coordinated site in garnet during the eclogite formation. The REE geochemistry of the eclogites indicated that LREE could be partially lost during the prograde metamorphic process of protolith, but be introduced into the rocks during the symplectite formation. LREE are more active than HREE during the UHP metamorphism. The results are favorable to highlighting the REE behavior and evolution of UHP metamorphic rocks.展开更多
基金funded by the Ministry of Science and Technology of China(No.G19990755-01)the National Natural Science Foundation of China,Postdoctoral Science Foundation of China,the Chinese Academy of Sciences Wong K.C.Post-doctoral Research Award Fund and the State Key Laboratory of Mineral Deposits,Nanjing University
文摘Omphacite grains from UHP eclogite of the Dabie Mountains in eastern China are elongated and show strong lattice preferred orientations (LPOs). Observations by the transmission electron microscopy (TEM) identified not only structures of plastic deformation occurring as free dislocation, dislocation loops and dislocation walls, but also bubbles of water present in the deformed omphacite. The bubbles attach to the dislocation loops which are often connected to one another via a common bubble. Using infrared spectroscopy (IR), two types of hydrous components are identified as hydroxyl and free-water in the omphacite. An analysis of deformation mechanism of microstructure in omphacite suggests that the mineral deformed plastically under UHP metamorphic conditions by dislocation creep through hydrolitic weakening.
文摘The rheological characters of omphacites in Dabie ultra-high-pressure eclogite have been studied in terms of fabric, dislocation and micro-structures. 1. The eclogite has undergone high-temperature deformation, thus forming omphacite lattice preferred orientation. In addition to creep dislocation, the omphacite ductile deformation may have other mechanisms, such as diffusion creep and grain boundary migration. 2. The main-phase deformation of eclogite is coaxial, but asymmetry strain also exists due to strain partitioning in the Dabie orogenic belt. 3. The twin measured by the universal stage is (100), indicating that omphacite high-T deformation was superimposed by low-T deformation. 4. Subgrain structure is common in omphacite, but the deformation features of the omphacites in the Shuanghe area and Bixiling area are different, the latter being dominated by dynamic recrystallization. 5. The Flinn plots show that the strain of omphacite belongs to the constriction ellipsoid and stretching strain, which is similar to the result of the omphacite fabric analysis.
基金This work was supported by the National Natural Science Foundation of China(Nos.41272224,41972231).
文摘Omphacite microstructures are important to decipher the metamorphic pressuretemperature(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 C)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.
基金This work was supported by the Research Fund for the Doctoral Program of Higher Education of China(Project No.9349101)National Natural Science Foundation of China grants 49572146 and 49872069.
文摘Transmission electron microscope (TEM) and high resolution transmission electron microscope (HRTEM) analyses have been performed on omphacite from ultra-high pressure (UHP) eclogites at the locality of Shima, Dabie Mountains, China. TEM reveals that the microstructures consist dominantly of dislocation substructures, including free dislocations, loops, tiltwalls, dislocation tangles and subboundaries. They were produced by high-temperature ductile deformation, of which the main mechanism was dislocation creep. Antiphase domain (APD) boundaries are common planar defects; an age of 470 ± 6 Ma for UHP eclogite formation has been obtained from the equiaxial size of APDs in ordered omphacites from Shima, coincident with ages given by single-zircon U-Pb dating (471 ± 2 Ma). HRTEM reveals C2/c and P2/n space groups in different parts of one single omphacite crystal, and no exsolution is observed in the studied samples, which is attributed to rapid cooling. It is suggested that the UHP eclogites underwent a long period of annealing at high temperatures, followed by relatively rapid cooling. These data provide valuable information for the formation and exhumation mechanism of UHP eclogites in the Dabie high-pressure (HP) and UHP metamorphic belt.
基金financially supported by the National Natural Science Foundation of China(Grant No.40873049) and Kyoto University,Japan
文摘Distributions of the rare-earth elements (REE) in omphacite and garnet and REE behaviors during metamorphic processes were discussed. The REE concentrations of garnet and omphacite in six eclogite samples from the Dabie Mountain, central China, were measured by inductively coupled plasma-mass spectrometry (ICP-MS). The correlation of δEu ratios between garnet and omphacite indicated that chemical equilibrium of REE distribution between garnet and omphacite could be achieved during ultra-high pressure (UHP) metamorphism. Most of the partition coefficients (Kd=CiOmp/CiGrt) of light rare-earth elements (LREE) are higher than 1. However the partition coefficients of heavy rare-earth elements (HREE) are lower than 1. This indicated that the LREE inclined to occupy site M2 in omphacite, but the HREEs tended to occupy eightfold coordinated site in garnet during the eclogite formation. The REE geochemistry of the eclogites indicated that LREE could be partially lost during the prograde metamorphic process of protolith, but be introduced into the rocks during the symplectite formation. LREE are more active than HREE during the UHP metamorphism. The results are favorable to highlighting the REE behavior and evolution of UHP metamorphic rocks.
