NATURAL AND LABORATORY EXAMPLES OF ANNEALING RADIATION DAMAGE IN ZIRCON

Radiation damage in Precambrian zircons from the Chugach-Prince William (CPW) terrane in Alaska was studied by Raman spectroscopy to quantify radiation damage and how that damage anneals. The primary goal of this research is to understand the temperature at which disorder in damaged zircons becomes annealed and then relate these findings to geologic settings. A natural annealing experiment involved detrital Precambrian zircon from the Baranof Schist (Whale Bay, Alaska) that show increasing metamorphic grade increasing from prehnite-pumpellyite to andalusite adjacent to a 50 Ma pluton. The Raman active modes in the Precambrian zircon show increasing order with proximity to the pluton (and increasing metamorphic grade). Precambrian zircons from units in prehnite-pumpellyite grade rocks (≥300-350°C) have significant disorder, they have less disorder at biotite grade (~400°C), and they are the most crystalline (least disorder), in rocks with andalusite (≥500°C). Our hypothesis is that the zircon crystals with high radiation damage begin the annealing process in greenschist facies (≥300-350°C) and radiation damage becomes fully annealed (or crystalline) in amphibolite facies (≥500°C). Sri Lanka Precambrian zircons (widely used standard) were heated between the temperatures of 400°C–1000°C to determine the temperature in which zircon becomes annealed in the laboratory. To accomplish this, 16 fragments of a single Precambrian Sri Lanka zircon (564 Ma) were analyzed. Crystallinity of each fragment was first measured using Raman Spectroscopy and two zircons were allocated for each temperature step. After Raman measurements, samples were heated to 400°C, 600°C, 800°C, 900°C and 1000°C in an oven for 22,500 seconds. The Raman active modes where then again measured for each sample after heating. At 400°C, no significant change in the radiation damage was present. Our highest temperature step of 1000°C resulted in v₃SiO₄ of ~1006.2 cm^-1, a number that is the most crystalline in this study, but not fully crystalline. The most dramatic annealing occurs between 400 and 800°C.