Comments1-1-new

Discussion 1 (Accepted by Editor Ben A. van der Pluijm, and published) Eclogite rheology: Implications for subducted lithosphere: CommentsShaocheng JiDépartément des Génies Civil, Géologique et des Mines, École Polytechnique deMontréal, C.P. 6079, Succursale Centre-Ville, Montréal, Québec, H3C 3A7, CanadaUsing a modified Griggs apparatus with a claimed accuracy of ±50 MPa in differential stress, Jin et al. (2001) deformed hot-pressed quartz eclogite (50 % garnet, 40% omphacite and 10% quartz), garnet aggregate and omphacite aggregate. They found that "the flow properties of eclogite are very similar to those of peridotite" and that "garnetite is only 2-3 times as strong as harzburgite". Based on these data, they suggested that delamination of oceanic crust from the underlying lithosphere due to contrasting theologies is unlikely to occur during subduction either at shallow mantle depths or in the lower part of the mantle transition zone". The purpose of this comment is to raise some questions about the experimental evidence used by Jin et al. (2001) in reaching their conclusions.The mechanical strength of a material depends on operating deformation mechanisms. For example, quartz is stronger than feldspar when quartz deforms by dislocation creep while feldspar by microfracturing and cataclastic flow, whereas quartz is weaker than feldspar when both quartz and feldspar deform by dislocation creep (Dell'Angelo and Tullis, 1996). Careful scrutiny of Jin et al. (2001) Fig. 1c and d reveals that garnet grains in their deformed samples are extensively fractured rather than flattened, indicating little plastic deformation of garnet. Yet Jin et al. (2001) did not provide any evidence from either the TEM or electron-backscattered diffraction (EBSD) fabrics for intracrystalline plasticity of garnet in their deformed eclogite or garnetite. Although the authors attributed the observed strain-softening (they inferred as "steady-state creep") to an increase of "specimen misalignment" with increasing strain, the softening may alternatively be due to progressive microfracturing of garnet grains at confining pressures much lower than the value they claimed.Eclogites from the Dabie-Sulu UHP metamorphic terrane show remarkable evidence of plastic deformation: well-developed foliation, pronounced stretching lineation and deformation-metamorphism-induced, alternating garnet- and omphacite-rich compositional layers (Ji et al., 1998). These layers are millimeter- to decimeter-scale inthickness. Both garnet (Fig. 1) and omphacite grains are flattened, elongated and sheared. TEM observations of deformed garnets from UHP eclogites (Ando et al., 1993; Voegelé et al., 1998) display well-organized dislocations and regularly spaced subgrain boundaries. EBSD measurements (Saruwatari et al., in preparation) demonstrated that garnets of the UHP eclogites developed significant lattice preferred orientations with the activity of slip systems 1/2<111>{110} and <100>{010}. The above observations all indicate recovery-accommodated dislocation creep for garnet under UHP conditions.Jin et al. experiments failed to duplicate the typical microstructures observed optically and with the TEM in naturally deformed UHP eclogites. This strongly suggests that deformation mechanisms in their deformed samples are not the same type as those operated in natural UHP eclogites from former subducting slabs. Thus, there is no basis to make any meaningful extrapolation of their experimental data to the much longer times and lower temperatures of natural deformation. For the same reason, their comparisons of flow strengths between eclogite and peridotite, and between garnetite and ringwoodite or omphacite become inappropriate, and any implications resulting from such comparisons are meaningless.The eclogite specimens of Jin et al. contain of 10% intergranular quartz. As shown in their Fig. 1c and d, quartz is strongly flattened and takes the much greater part of the strain with reference to its volume fraction. The flow strength of quartz eclogite can hardly be applied to bimineralic (garnet and omphacite) eclogites in the subducting slab because the UHP eclogites associated with garnet-bearing ultramafic rocks contain generally <50% SiO2 and <1-2% coesite or quartz pseudomorphs mainly as inclusions in strong minerals such as garnet (Zhang et al., 1994; Zhai and Cong, 1996). The addition of 10% quartz into eclogites as intergranular "lubricant" should result in significant weakening. Furthermore, the rheology of polyphase rocks is affected not only by the flow properties and volume fraction of each constituent phase but also by the geometrical distribution of phases. For example, the presence of a thin-layered structure significantly enhances the layer-normal compressive strength of polyphase rocks (Ji et al., 2000). Therefore, the thin-layered eclogites should be remarkably stronger than the quartz eclogite investigated by Jin et al. (2001).Based on the above discussion, I conclude that Jin et al. (2001) have exceeded the competence of their data in the hypothesis proposed for the rheology and tectonic deformation of subducted lithosphere.ReferencesAndo, J., Fujino, K., and Takeshita, T. 1993, Dislocation microstructures in naturally deformed silicate garnets: Physics of the Earth and Planetary Interiors. 80, 105-116.Dell'Angelo, L.N., Tullis, J. 1996, Textural and mechanical evolution with progressive strain in experimentally deformed aplite: Tectonophysics. 256, 57-82.Ji, S., Wang, Z.C., and Saruwatari, K. 1998, Plasticity of eclogite: Implications for rheology and seismic reflectivity of the subducting slab, in Canada LITHOPROBE Report, No. 64, p. 86-91.Ji, S., Wirth, R., Rybacki, E., Jiang, Z.-T., 2000. High temperature plastic deformation of quartz-plagioclase multilayers by layer-normal compression: Journal of Geophysical Research. 105, B7, 16651-16664.Jin, Z.M., Zhang, J., Green II, H.W., and Jin, S. 2001, Eclogite rheology: Implications for subducted lithosphere: Geology. 29, p. 667-670.Voegelé, V., Cordier, P., Sautter, V., Sharp, T.G., Lardeaux, J.M., and Marques, F.O., 1998, Plastic deformation of silicate garnets II. Deformation microstructures in natural samples: Physics of the Earth and Planetary Interiors. 108, 319-338.Zhai, M.G. and Cong, B.L. 1996, Major and trace element geochemistry of eclogites and related rocks. In Cong, B.L., ed., Ultrahigh-Pressure Metamorphic Rocks in the Dabieshan-Sulu Region of China. Kluwer Academic Publishers, Boston, p. 69-89.Zhang, R.Y., Liou, J.G., and Cong, B.L. 1994, Petrogenesis of garnet-bearing ultramafic rocks and associated eclogites in the Sulu ultrahigh-P metamorphic terrane, eastern China: Journal of Metamorphic Geology. 12, 169-186.Figure captionsFigure 1. Typical structure of deformed garnetite layers in a foliated eclogite from the Sulu UHP metamorphic terrane (Donghai, China). Lineation-normal extensional fractures formed during late uplifting. XZ section. Length of the photography = 1.5 cm.。