Plastic mechanism of deformation of garnet-- Water weakening

Vol. 45 No.10SCIENCE IN CHINA (Series D)October 2002Plastic mechanism of deformation of garnetW ater weakeningSU Wen (苏文)', CONG Bolin (从柏林)", YOU Zhendong (游振东),ZHONG Zengqiu (钟增球)2 & CHEN Daizhang (陈代章)1. Laboratory of Lithosphere Tectonic Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences,Beijing 100029, China;2. China University of Geosciences, Wuhan 430074, China;3. China University of Geosciences, Beiing 100083, ChinaCorrespondence should be addressed to Su Wen (email: suwenwen0649@sina.com)Received March 12, 2002Abstract The strongly deformed eclogites are well developed in ultra-high pressure jadeite-quartzite zone of the Dabie Mountains, Eastern China, and garnets had been deformed strongly.Observations by transmission electron microscopy identified not only structure of plastic deforma-tion occurring as free dislocation, dislocation loops and dislocation walls, but also clusters of watermolecules present in the deformed garnet. Using infrared spectroscopy, two types of hydrous com-ponents are identified as the hydroxyl and free-water in the garnet. Based on analysis of micro-structure mechanism of deformation in garnets, and experimental data of petrology, the clusters ofwater molecules were considered to lead strong plastic deformation of garnet by dislocations be-cause of mechanical weakening. .Key words: clusters of water molecules, water weakening of garnet, Dabie Mountains.Plastic mechanism of deformation of garnet has attracted special attention and discussionamong scholarsI- 1] for the following reasons: (i) Garnet is a major mineral of ultra-high pressure(UHP) metamorphic rocks. It influences and decides rheological properties of the UHP metamor-phic rock. Therefore, it is a key mineral to decide rheological properties of the subduction belt andinfluence whole dynamic processes; (i) Garnet is an isoaxial mineral, which cannot be determinedby mechanism of deformation such as subgrain, LPO, wavy extinction, and twisting band by opti-cal-microscope like the other minerals. Mostly, natural garnet is a rigid-body, and its deformationchanges stress by rotating. Full-plastic deformed garnet can be rarely found in natural rock. Ji andMartignole (1994, 1996)4.2] suggested that garnet is softer than quartz and feldspar and easy forplastic deformation to take place at T> 900C after studying on elongated deformed garnet in theMorin shear zone. And they proposed that garnets developed dislocation creep by TEM. Prior etal.(2000)4), Brok and Kruhl (1996)3, Blackburm et al.中国煤化工(1968)61 suggested that diffusional creep (pressure solution) is plastid:TYHCNMHGnofgarnet.Recently, we found strongly deformed eclogite in the jadeite -quartzite zone of the UHPmetamorphic belt from the Dabie Mountains, and garnets had been elongated strongly. According886SCIENCE IN CHINA (Series D)Vol. 45to calculation of the metamorphic PT condition by omphacite garnet in the presence of coesite (P≥2.8 GPa), deformation of garnets occurred at about T= 700°C (< 900"C). The garnets still showstrong plastic deformation. Now, what mechanism makes the garnets lead to deformation mecha-nism within garnet remains unclear. Observation in both deformed and undeformed garnets byTEM shows that cluster of water molecules presents only in the deformed garnet. McI aren et al.and McL aren (1983, 1991)12.13] found clusters of water molecules in‘ wet' synthetic quartz whilethey studied deformation mechanism of‘wet synthetic quartz. TEM observations by McL aren etal. and McI aren (1983, 1991)12.13] indicated that clusters of water molecules lead to water weak-ening, resulting in plastic deformation of quartz by dislocations. Here, we discuss the rheologicalproperties of the UHP garnet in the Dabie Mountains based on analysis by TEM, measurement ofstress and experimental data of petrology.116*30'E1 Geological background and sample30*50'NGarnets for this study are from foli-ated eclogite in a jadeite-quartzite beltnear Changpu County, in the eastern endof the Dabie Mountains UHP metamor-源、、ShuihouA Xinjansphic belt. The jadeite-quartzite belt ex-WumiaoYezhaig/Qianshan30*40'Ntends 40 km long in the east-west direc-tion and 1km wide (fig. 1) and consists ofHengchoneHanschingchong105jadeite-quartzite,eclogite,kyanite-国2四6Pailou王] 30quartzite, marble and gneiss. Coesite in-02 km口4clusion was found in the eclogite andFig. 1. Sketch map showing the distribution of jadeite- quartzite zonejadeite-quartzitell4)，indicating that theyin the Dabie Mountains.1, Eclogite facies rocks; 2, amphibolite facies were subducted down to great depthrocks; 3, granite; 4, eclogite and garnet-peridotite; 5, marble; 7, sam-(about 80一100 km), experienced UHPple location.metamorphism and subsequently exhumed to the surface.The eclogite (98121E) is composed of garnet (45%), omphacite (38%), quartz (10%), diop-side (5%) and a few coesite, rutile, zircon, apatite and retrograde metamorphic minerals. All theeclogitic minerals such as garnet, omphacite and even rutile are strongly elongated. Some elongat-ed minerals, especially omphacite, retrograde symplectite. The undisturbed long axis of thesesymplectite minerals is perpendicular to the crystal boundary of the host mineral, indicating thatthe deformation did not take place in the host minerals after their elongation or during the sym-plectite-forming process, and that the deformation of the garnet. omphacite and rutile took place中国煤化工under the condition of UHP eclogite facies metamorphisite in the garnetindicated that the peak metamorphic pressure conditioniYHc N M H Gak metamorphieT= 703- -726°C by clinopyroxene gartI5.16.No.10WATER WEAKENING887Garnet crystals are ranged from 0.6 to 3.5mm in size, and usually strongly elongated (fig.2(a)). The garnet grains on the hand specimengenerally are slightly shape-preferred orientation.And its long axes are parallel to the stretchinglineation (fig. 2(b)). Inclusions of rutile and rarecoesite are found in garnet. The composition of1 mmthe garnet from core to rim is homogeneous, asdetermined by microprobe, showing Alm 57.45,Gro 19.61, Pyr 17.87, and Spe 5.07 mol%.2 Analysis methods and results福学2.1 Microstructure and measurement strainAccording to character of the foliation, sixthin sections were cut in a foliated eclogite forstrain analysis of the garnet. Three thin sectionsare parallel to the mineral elongation lineation(XZ section), and the other three are perpen-Fig.2. (a) Photomicrographs showing garnet crystalsstrongly elongated and associated with omphacite, quartz anddicular to the mineral elongation lineation (YZ nutile in the flialed elogle (98121B); (b) ganet ciystls areslightly shape-preferred orientation and its long axes aresection). Measurement of the garnet grain-size parallel to the stretching lineation (Changpu County iand aspect ratios on XZ and YZ sections had beenYuexi).done. The strain axes of stretched garnets are expressed asY : Z= 2.43,X : Z= 3.83 ratios (fig. 3).The Flinn coefficient of the plastic deformed garnets is 0.403. All of these imply that the plasticdeformation in garnet is more or less an oblate strain and may be associated with simple shear.16 r16pr=306n=2632F12-意g6 8-回)12162016 20YignidX/gridFig. 3. Aspect ratio of deformed garnets in the foliated eclogite.中国煤化工2.2TEM analysisDoubly polished thin sections of 0.02- -0.03 mmHHCNMHGboth the foli-ated eclogite and the weakly strained eclogite. They were then ion-milled by Gatan 600. TEM888SCIENCE IN CHINA (Series D)Vol. 45investigations were performed with a 200 kV HATACH H8100.Observation by TEM shows that cluster of water molecule presents only in the deformedgarnet of the foliated eclogite (fig. 4). The cluster of water molecule is distributed unhomogene-ously in the garnet (fig. 4(a)). It is up to 0.063 um and its image consists of two sets of lobes andshows characteristic of a lens-shaped inclusion whose plane is normal to the foil surface. Thus, theimage could be due to strain around a tiny spherical inclusion and its strain field is clearly seen(fig. 4(b)). This is similar to the cluster of water molecule shown in synthetic quartz by McLarenet al. and McI aren (1983, 1991)121. We suggest that the tiny spherical inclusion is cluster ofwater molecule within garnet, combining with results of IR spectroscopy analysis (see sec. 2.3).0 Sumb)Fig. 4. BF micrographs showing cluster of water molecule presenting in the deformed garnet. (a) BF micrographsshowing cluster of water molecule distributing unhomogeneously in the deformed garnet; (b) BF micrographs of thesame area shown in fig. 4(a). Image of the cluster of water molecule consists of two sets of lobes symmetrically placedacross a line with no contrast.The strain field of the cluster of water molecule is characteristic of a lens-shaped inclusionwhose plane is normal to the foil surface.On the other hand, TEM analysis also reveals intracrystalline plastically deformed micro-structures (fig. 5), including free dislocation, dislocation loops and dislocation walls. A number ofTEM observations have shown that the density of dislocations in deformed garnet is at least3.63X 108 cm-2.2.3 IR spectroscopy analysisThe double-polished thin section with thickness of 0.2- 0.3 mm was prepared for IR spec-troscopy analysis. Several inclusion- and crack-free garnet crystals with at least 150- 200 μm intheir shortest dimension were extracted from the thin section, and then cleaned with ethanol. Using a Nicolet Impact 410 and IR-RLAN ADVANTAGE Fourier Transform Infrared MicroscopicSpectrometer, we conducted measurement for garnet with the wave-number range from 4000 to650 cm-', at room temperature. The resolution of measu中国煤化工Considering a representative of measured result,:MYHCNMHGandthenthreepoints of every grain were analysed for sample of 98121E. Single-crystal IR spectrum of the de-formed garnet is shown in fig. 6. The bands of hydroxyl and free water are located in 3570-No.10WATER WEAKENING89a)Fig. 5. Bright-field electron micrographs showing dislocations in the naturally deformed garnet. (a) Dislocationloops and bubble; (b) dislocation networks and bubbles; (C) dislocation loop is connected to one another via acommon bubble; (d) disloc ation walls and free dislocations.3580 cm-1 and 3440 cm-1 regions in IR spec-0.3 ;trum[17- -231. Two absorption bands in fig. 6 are con-3576入sistent with the hydroxyl and free water, respec-号 0.2tively (approximately 3576 cm-1 for OH and 3443cm-1 for free water), indicating that OH and free虽0.1-water are present in the garnet.3 Discussion0.040003750350032503000UHP metamorphic garnets in the DabieWave number/cm-lMountains seem to be unique as compared to thoseFig. 6. IR spectra of the garnet in the eclogite (98121E)recent publications. Because the data presented in OH- stretching region.here show not only the existence of clusters of water molecules in the garnet, but also developedstrong elongation, high density of dislocation and high aspect ratios, adding microprobe data de-monstrate that the studied garnet crystals have no compositional zoning. Therefore, this indicatesthat the diffusion process of the grain boundary mobility did not take place in these garnets. Onthe basis of the above features, we consider that it can only be explained by plastic deformation ofthe garnets. According to the interpretation of Ji and Martignole (1994, 1996)121, plastic deforma-tion garnets exclusively reflect high temperature deformation. And they suggested that garnet issofter than quartz and feldspar and easy for plastic de中国煤化工> 900^C, butgarnet does not cause plastic deformation at T<700(TYHCNMHG this study weredeformed at temperatures between 703C and 726 C, showing strong plastic deformation withingarnet. Therefore, there must be some mechanism leading to plastic deformation of garnet under890SCIENCE IN CHINA (Series D)Vol. 45lower temperature.Now, the role of water in promoting plastic deformation is the critical problem. Studies onexperimental and natural garnets suggest that mechanism of deformation of garnet displays in twoways: diffusion creep (pressure solution) and/or dislocation creep. However, it has not been notedin these studies about the deformation behavior and mechanism of garnet under hydrous condi-tions because garnet is, so far, considered as anhydrous mineral. In fact, previous studies of hy-drous component in nominally anhydrous minerals were mainly focused on water in quartz be-cause weakening of quartz affects the mechanical strength of this mineral. The hydrolytic weak-ening of synthetic quartz crystals was first observed by Griggs and Blacic[24] and they proposedthat mechanisms of the hydrolytic weakening involved the hydrolysis of the Si-O- Si bond:H2O+=Si←O←Si=→=Si-OHOH-Si =.Although this hypothesis has been challenged concerning the atomicitic mechanism of thehydrolytic weakening of quartz, the experimental studies of water-bearing synthetic quartz havedemonstrated that the diffusivity or solubility of water- related point defects in solid solution ismuch lower than that required by the Griggs model of Peierls stress reduction and easy dislocationis possible only when the dislocations are hydrolyzed. More recently, TEM observations byMcI aren et al. and McL aren! 12,13] and Gerretsen et al. [251 indicated that the clusters of water mo-lecules lead to point-related defects and induce water weakening in single quartz crystals. Themicrostructures evolved in these crystals during both creep and constant strain-rate experimentswere observed by TEM and show that the clusters act as highly efficient sources of the disloca-tions which must have been nucleated before plastic flow can be induced.Our observation by TEM indicates that the clusters of water molecules can be found only inthose strongly plastic deformed garnets, implying that deformation of the garnet was related to theclusters of water molecules. The crystals characteristically exhibit clusters of water molecules,indicating the presence of water-related defects, which is effectively 2H2O molecules replacingone SiO2 unitlI,121. The clusters of water molecules, which act as water reservoirs, were initiallypresent and then evolved to form small, and stress-free water bubbles on prismatic dislocationloops, which are similar to those in“wet”synthetic quartz of McI aren et al. and McI aren (1983,1991)[1213]. Yet, due to the very large Burgers vector, dislocation in garnet may differ in the sameprocess in the synthetic quartz. But, their mechanism of water weakening is the same. Therefore,we suggest that the water-related defects, whose presence is essential for the weakness of thesecrystals, are clusters of water molecules. It plays an important role in the cluster acting as highlyefficient sources of the dislocations involved in the deformation process of the garnet.4 Conclusion中国煤化工(1) The mechanism of deformation of the garnet.MHC N M H Gie Mountains isplastic deformation and may be associated with simple shear under the condition of UHP eclogitefacies metamorphism.No.10WATER WEAKENING891(2) The clusters of water molecules present in garnet were directly associated with mechani-cal weakening and inducing in plastic deformation of garnet by dislocations.Acknowledgements We thank CAS Academician Ye Danian, Drs. Ye Kai and Liu Jingbo, Profs. Zhai Mingguo, WangQingcheng, Yang Jianjun and Zhao Zhongyan for comments on this manuscript. This study was funded by the CMST (Grant No.G19990755-01), the NNSFC, PSFC, the CAS K C. Wong Post-doctoral Research Award Fund, and State Key Laboratory ofMineral Deposit, Nanjing Unive rsity.References1. Ji, S., Martignole, J.. Ductility of garnet as an indicator of extremely high temperature deformation, Joumnal of StructuralGcology, 1994, 16: 985- -996.2. Ji, S., Martignole, J.. Ductility of garnet as an indicator of extremely high temperature deformation: Reply, Joumal ofStructural Geology, 1996, 18: 1375- -1379.3. Brok, B. D., Kruhl, J. H Ductility of garnet as an indicator of extremely high temperature deformation: Discussion, Jour-nal of Structural Geology, 1996, 18: 1369- -1373.4. Prior, D. J,, Wheeler, J.. Brenker, F. E. et al, Crystal plasticity of natural garnet: New microstructural evidence, Geology,2000, 28: 1003- -1006.5. Blackbumn, W. H, Dennen, W. H., Flattened garnets in strongly foliated gneisses from the Grenville Series of the Ganano-que area, Ontario, American Mineralogist, 1968, 53: 1386- -1393.6. Dalziel, I. W. D. Bailey, s. w.. Deformed garnets in a mylonitic rock from the Grenville front and their tectonic signif-cance, American Jourmal Science, 1968, 266: 542- 562.7. Voegele, V.. Cordier, P.. Sautter, V. et al. Plastic deformation of silicate garnets II:I Deformation microstructures in naturalsamples, Phys. Earth Planet Inter., 1998, 108: 319- 338.8. Karato, S., Wang, Z, Liu, B. et al, Plastic deformation of garnets: Systematics and implications for the rheology of themantle transition zone, Earth Planet Sci. Lett, 1995, 130: 13- -30.9. Kleinschrodt, R., McGrew, A., Garnet plasticity in the lower continental crust: Implications for deformation mechanismsbased on microstructures and SEM-electron canelling pattern analysis, J. Str. Geol., 2000, 22:795- -809.10. Den Brok, B., Kruhl, J, Ductility of garnet as an indicator of extremely high temperature deformation: Discussion, J.Structural Geol,. 1996, 18: 1369- -1373.1. You Zhendong. Zhong Zengqiu, Suo Shutan et al, A petrogenetic study of the garnet pyroxenite enclaves in spinel peni-dotite, North Dabieshan, China, Sciences in China, Ser. D, 2000, 43(supp): 189- -199.2. McLaren, A. C., Cook, R. F, Hyde, S. T. et al, The mechanisms of the formation and growth of water bubbles and asso-cicated dislocation loops in synthetic quartz, Physics and Chemistry Minerals, 1983, 9: 79- 94.13. McLaren, A., Transmission electron microsocopy of minerals and rocks, in Cambridge Topics in Mineral Physics andChemistry (eds. Putnis, A, Liebermann, R. C.), Cambridge: Cambridge University Press, 1991, 549.14. Su Wen, Xu Shutong. Jang Laili et al, Coesite from the quartz jadeitite, Easterm China, Mineralogical Magazine, 1996,60: 659- 662.15. Ellis, D. J, Green, D. H., An experimental study of the effect of Ca upon garnet- clinopyroxene Fe-Mg excharge equilibria,Contribution to Mineralogy and Petrology, 1979,71:13- 22.6. Krogh, E. J. The garet-clinopyroxene Fe-Mg geothermometer: A reinter pretation of existing experimental data, Con-tribution to Mineralogy and Petrology, 1988, 99:44- -48.17. Rossman, G. R., Aines, R. D., The hydrous components in garnets: Grossular-hydrogrossular, Am. Miner., 1991, 76:1153- 1 164.中国煤化工18. Bell, D. R.. Rossman, G. R., Water in the earth' s mantle: The rold, Science, 1992, 255:1391- 1397.MYHCNM HG9. Bell, D. R., Rossman, G. R., The distribution of hydroxyl in garnets from the subcontinental mantle of southern Africa,Contrib. Miner. Petrol, 1992, 11:161- -178.892SCIENCE IN CHINA (Series D)Vol. 45.20. Langer, K., Robarick, E. Sobolev, N. V. et al, Single-rystal spectra of garmets from diamondiferous high-pressuremetamorphic rocks from Kazakhstan: Indications for OH, HO, and FeTi charge transfer, European Jourmal of Mineralogy,1993, 5: 1091- -1 100.21. Withers, A. C, Wood, B. J., Carroll, M. R., The OH content of pyrope at high pressure, Chemical Geology, 1998, 147:161-171.2. Xia Qunke, Chen Daogong, Rossman, G. et al, An important high. utrahigh pressure fluid: Nominally anhydrous mineral,Geological Review, 2000, 46(5): 56- 61.23. Zhang Junfeng, Jin Zhenming, Green I, H. W. et al, Water in continental subduction belt: Evidence from ultra-high pres-sure eclogite, Chinese Science Bulletin (in Chinese), 2000, 45(17): 1889一1 894.24. Griggs, D. T, Blacic, J. D.. Quartz: Anomalous weakness of synthetic crystals, Science, 1965, 147: 292- -295.25. Gerretsen, J.. McI aren, A. C, Paterson, M. S., Evolution of water inclusions in wet synthetic quartz as a function of tem-peraturee and pressure, EOS, 1987, 68: 1453.中国煤化工MHCNMHG