CHEMICAL CHARACTERIZATION OF OSCILLATORY ZONING AND OVERGROWTHS IN ZIRCON USING 3 MEV MU-PIXE

We have used 3 MeV mu-PIXE to chemically characterize mm-scale oscilla tory zoning and overgrowths in large crystals of zircon from the Silin jarvi carbonatite, Finland, and a granitic pegmatite from Brazil. The zircon crystals are optically continuous in plane-, cross-polarized an d reflected light; oscillatory zoning and overgrowths are distinguishe d using cathodoluminescence microscopy. Purple-, green- and yellow-lum inescent oscillatory zones are usually sharply defined and vary from a bout 1 to 400 mu m in width; rare gradational zoning also is observed. In one crystal, external overgrowths cross-cut earlier oscillatory zo nes and show embayed contacts with earlier growths. New overgrowths al so show oscillatory zoning. Analysis of the oscillatory zones and over growths by mu-PIXE (3 MeV proton beam 5 mu m in diameter) shows the gr een- and yellow-luminescent zones in the Finnish samples to have simil ar Zr/Hf ratios of 49 - 54, whereas the Zr/Hf ratio of new overgrowths range from 43 to 46. The yellow-luminescent zones have lower Sc conte nts (similar to 53-65 ppm) than the green zones (similar to 98-154 ppm ). Overgrowths have low Sc contents (21 - 49 ppm). In the Brazilian sa mple, the purple-luminescent core has the highest rare-element content (where Y, Th, U, Gd, Dy, Er and Yb are about 2500, 1500, 1000, 130, 3 00, 340 and 480 ppm, respectively); this grades to a zoned region with yellow-luminescent bands where the Y, U and Th levels are generally l ower (1900-2300, 360-900 and 400-700 ppm, respectively). In the outerm ost zoned overgrowth, Y and Yb contents are about 300 and 80 ppm, resp ectively, with all other rare elements being below their limits of det ection.