Alpha-decay event damage in zircon

Abstract

Based on density measurements, X-ray diffraction analysis, and high-resolution trans-mission electron microscopy of a suite of natural zircon samples from Sri Lanka (0.06 xl0r5 to 6.3 x l0rs a-decay events/mg), three stages of damage accumulation may bedelineated. Stage I ( 8 x 10'5 a-decayevents/mg) consists of material that is entirely aperiodic as far can be determined byX-ray or electron diffraction. There was no evidence for the formation of ZtO, or SiO, asfinal products during the last stage of metamictization. Based on rnodeled density changes,aperiodic regions continue to experience a change in structure as they are redamaged.During stage II of the process, the modeled density of aperiodic regions changes from 4.5g/crn3 lo 4.1 g/cm3. Fission fragment damage does not contribute to the process of meta-mictization. The amorphization process is consistent with a model for the multiple overlapof displacement cascades, suggesting amorphization occurs as a result of defect accumu-lation rather than directly within a single displacement cascade.Comparison of results for natural zircon with those for Pu-doped zircon showed thatdose-rate variations (even as great as a factor of 108) had no substantial effect on thedamage accumulation process. Unit-cell parameters increased and density decreased morefor the Pu-doped zircon than for natural zircon in early stages of damage accumulation(<3 x 10,5 a-decay events,/mg), suggesting that annealing of point defects in the earlystages of the damage accumulation process occurs in natural zircon under ambient con-ditions. This accounts for the distinct sigmoidal shape of the damage cnryes for naturalzircon and the apparent incubation period before onset ofamorphization.IxrnonucrroN ppm U and2to 2000 ppm Th (Ahrens etal.,1967;Gorz,Zircon (ZrSiOo, I4r/amd, Z : 4) is a common acces- 1974), it is one of the most important phases used insory mineral in crustal igneous, metamorphic, and sedi- U-Th-Pb dating techniques (Faure, 1977;JiLger and Hun-mentary rocks, as well as in lunar materials, meteorites, z1ke4 1979). Of particular interest are processes by whichand tektites. Because zircon typically contains 5 to 4000 the U-Th-Pb systematics are disturbed (e.g., Silver and0003-004x/9 1/09 l 0- l 5 I 0$02.00 l 5 r0