Quantitative structural analyses and numerical modelling of ...

B10210ZÁVADA ET AL.: EXTREME DUCTILITY OF FELDSPAR AGGREGATESB10210Figure 17. PT plot with contours of molar volume in Jbar 1 (equivalent to 10 1 cm 1 mol1 ) calculated for thesame system as shown in Figure 13. Mineral abbreviationsare after Kretz [1983].which soaks up the melt produced within plagioclase bands(the melt is passively redistributed, Figure 15). Consequently,plagioclase could not deform any further due to the loss ofintergranular melt that worked as a creep enhancing medium.[35] It has been shown that creation of melt-bearingporosity can be induced by fast melt production due tobreakdown of some hydrous phase in the rock (e.g., viadiscontinuous reaction) or due to high heating rate causingsubstantial overstepping of continuous melting reactionsand thus fast melt production as a result of the system’seffort to reach equilibrium state [e.g., Connolly et al., 1997;Rushmer, 2001]. In contrast, change in molar volume of therock during slow progress of continuous reactions will beprobably small.[36] In order to test this hypothesis, the P-T phasediagram section in Figure 13a was contoured for molarvolume of the system to see, whether the observed meltingreaction at estimated PT conditions may lead to rapidvolume increase. The result (Figure 17) shows that the onlyrapid increase in molar volume of the rock in given PTrange results from dehydration melting of muscovite at PTconditions higher than those reached by studied orthogneisssamples. In addition, the role of melt volume change andvelocity of the reaction is speculative for aggregates with‘‘incoherent’’ grain boundaries like both feldspars undergoingGBS in studied orthogneiss than rather ‘‘intact’’ solidphases around reaction sites in the experiments of Connollyet al. [1997].[37] The calculated volume of melt (2–4 vol %) suggeststhat the melt formed isolated melt films, pools or pockets. Inthe case of ‘‘overpressure’’ model, sufficient amount of meltneeds to be produced to induce considerable melt overpressureand melt redistribution. This critical melt volumecorresponds to creation of interconnected network of meltbetween the framework of grains, so that the melt can startincreasing its hydrostatic pressure toward the level ofmaximum compressive stress [Renner et al., 2000]. Thismelt volume is given by the melt connectivity transition(MCT) of F =0.07[Rosenberg and Handy, 2005] or similar‘‘liquid percolation threshold’’ (LPT) of F =0.08[Vigneresseet al., 1996], although this critical level is likely to depend onseveral variables (e.g., dihedral angles). It is possible that themelt migrates from locally overpressured isolated pockets athigh angle to the maximum compressive stress (s 1 )tointergranular boundaries subparallel with this direction(s 1 ). However, it is unlikely that the cohesion of grainboundaries and tensile strength of (001) intragranular cleavageof K-feldspar are significantly smaller than intergranularcohesion of plagioclase, which would be necessary conditionfor preferential hydraulic failure of K-feldspar, although themelt was produced in plagioclase bands.[38] In contrast, during cavitation, underpressure arisingfrom opening of incipient intergranular voids in K-feldsparis theoretically susceptible to ‘‘extract’’ melt from isolatedpools and films within plagioclase aggregates, in spite of itslittle amount. If distortion of plagioclase grains is easierthan for K-feldspar grains to accommodate GBS, then plagioclaseaggregates should also ‘‘extrude’’ excessive intergranularmelt into possible sink sites. This is in agreementwith analogue modeling results of Walte et al. [2005], whereaggregates composed of ‘‘weaker’’ grains (more susceptibleto plastic deformation) show higher ‘‘GBS locking transition’’than relatively stronger grains. Melt migration alongmutual boundaries of quartz was probably negligible incomparison to both feldspars. This is again controlled byrelative ‘‘weakness’’ of quartz grains with respect to ‘‘stronger’’grains of both feldspars and ‘‘welded’’ boundaries ofquartz. The viscosity ratio between melt and host aggregategrains thus increases from quartz to plagioclase and ishighest for K-feldspar [Walte et al., 2005].[39] Considering little amount of melt produced by themelting reaction and absence of intergranular melt pocketsperpendicular to the stretching lineation in plagioclase, weconsider the ‘‘cavitation’’ model to be the most likelymicrophysical mechanism explaining the dilation, microfracturationand preferential melt distribution in K-feldsparaggregates of type 3 microstructure.8.2. Impact of Impurities and MechanicalAnisotropy of Crystals on GBS[40] Experiments with aluminum in GBS regime withrelatively high-purity metals (e.g., aluminum of purity 4N(where N expresses the degree of purity of a material; 2N =99% purity, 4N = 99.99% purity, etc.), 650°K, strain rate10 9 s 1 ) or at high strain rates (e.g., 4N copper at 10 2 s 1 ,873°K [Čadek, 1988]) are marked by high relative elongationse, distinct contraction at the area of failure (necking)associated with intracrystalline fracturing. On theother hand, experiments with relatively low-purity metals(2N aluminum at 10 9 s 1 , 650°K) or at low strain rates(4N copper at 10 9 s 1 , 873 K) show small relativeelongations to failure, regular contraction along the deformedspecimen (homogeneous strain) and intergranularfracturing (Figure 18) [Sklenička et al., 1977; Čadek, 1988;Mohamed, 2002]. Therefore the higher purity material canwithstand higher relative elongations to creep failure (and atfaster strain rates) than its lower purity equivalent. Incontrast, stress accumulations around impurities and grainboundary ledges enhance diffusion-driven cavitation, cavitieseasily form at dislocation pileups around impuritieswithin the grains and creep failure will occur after smallerrelative elongations (and slower strain rates).12 of 15290