Quantitative characterization of plastic deformation of zircon and geological implications

Abstract

The deformation-related microstructure of an Indian Ocean zircon hosted in a gabbro deformed at amphibolitegrade has been quantified by electron backscatter diffraction. Orientation mapping reveals progressive variations in intragrain crystallographic orientations that accommodate 20 of misorientation in the zircon crystal. These variations are manifest by discrete low-angle (<4) boundaries that separate domains recording no resolvable orientation variation. The progressive nature of orientation change is documented by crystallographic pole figures which show systematic small circle distributions, and disorientation axes associated with 0.5-4 disorientation angles, which lie parallel to rational low index crystallographic axes. In the most distorted part of the grain (area A), this is the [100] crystal direction. A quaternion analysis of orientation correlations confirms the [100] rotation axis inferred by stereographic inspection, and reveals subtle orientation variations related to the local boundary structure. Microstructural characteristics and orientation data are consistent with the lowangle boundaries having a tilt boundary geometry with dislocation line [100]. This tilt boundary is most likely to have formed by accumulation of edge dislocations associated with a <001> {100} slip system. Analysis of the energy associated with these dislocations suggest they are energetically more favourable than TEM verified <010>{100} slip. Analysis of minor boundaries in area A indicates deformation by either [010] (001) edge, or [100](100) and [001](100) screw dislocations. In other parts of the grain, [110] cross slip on (111), (111) and (112) planes seems likely. These data provide the first detailed microstructural analysis of naturally deformed zircon and indicate ductile crystal-plastic deformation of zircon by the formation and migration of dislocations into low-angle boundaries. Minimum estimates of dislocation density in the low-angle boundaries are of the order of ~3.1010 cm-2. This value is sufficiently high to have a marked effect on the geochemical behaviour of zircon, via enhanced bulk diffusion and increased dissolution rates. Therefore, crystal plasticity in zircon may have significant implications for the interpretation of radiometric ages, isotopic discordance and trace element mobility during high-grade metamorphism and melting of the crust.