Experimental shock deformation in zircon: a transmission electron microscopic study

In recent years, apparently shock-induced and, thus, impact-characteristic microdeformations, in the form of planar microdeformation features and so-c alled strawberry (granular) texture, have been observed in zircons in rocks from confirmed impact structures and from the K/T boundary. The nature of the planar microdeformations in this mineral is, however, still unknown, an d critical information is needed regarding the shock pressure range in whic h these deformation effects are produced. We experimentally shock deformed two series of thin zircon (ZrSiO4) target plates, cut perpendicular to the c-axis, at shock pressures of 20, 40, and 60 GPa. The recovered samples wer e characterized by optical and scanning electron microscopy. In addition, o ne sample series was studied by transmission electron microscopy (TEM). Mic rodeformation effects observed at 20 GPa include pervasive micro-cleavage a nd dislocation patterns. Plastic deformation is indicated by a high density of straight dislocations in glide configuration. The dominant glide system s are < 100 >{010}. Micro-cleavages, induced by shear stresses during the c ompression stage, occur mostly in the {100} planes. The large density of di slocations at crack tips shows that plastic deformation was initiated by th e micro-cracking processs. At 40 GPa, the sample was partly transformed from the zircon (z) to a schee lite (CaWO4)-type (s) structure. Planar deformation features (PDFs) contain ing an amorphous phase of zircon composition are present in the not yet tra nsformed zircon relies. The phase with scheelite structure, initiated in th e {100} planes of zircon, consists of thin (0.1 to several mu m) bands that crosscut the zircon matrix. The phase transformation is displacive (marten sitic) and can be related by {100}(z)//{112}(s) and [001](z)//< 110 >(s). T he scheelite structure phase is densely twinned, with twins in the (112) pl ane. The 60-GPa sample consists completely of the scheelite structure phase . Crosscutting and displacing relationships between twins and PDFs demonstr ate that PDFs are formed in the zircon structure, i..e., before the phase t ransformation to the scheelite structure occurred, most likely at the shock front. Crystallographic orientations of optically visible planar features in zircon, in comparison with orientations of planar defects at the TEM sca le, suggest that the optically visible features are more likely planar micr ofractures than PDFs. (C) 1999 Elsevier Science B.V. All rights reserved.