TRACE-ELEMENT ZONING AND INCIPIENT METAMICTIZATION IN A LUNAR ZIRCON - APPLICATION OF 3 MICROPROBE TECHNIQUES

We have determined major (Si, Zr, Hf), minor (Al, Y, Fe, P), and trace element (Ca, Sc, Ti, Ba, REE, Th, U) concentrations and Raman spectra of a zoned, 200 mu m zircon grain in lunar sample 14161, 7069, a quar tz monzodiorite breccia collected at the Apollo 14 site. Analyses were obtained on a thin section in situ with an ion microprobe, an electro n microprobe, and a laser Raman microprobe. The zircon grain is optica lly zoned in birefringence, a reflection of variable (incomplete) meta mictization resulting from zonation in U and Th concentrations. Variat ions in the concentrations of U and Th correlate strongly with those o f other high-field-strength trace elements and with changes in Raman s pectral parameters. Concentrations of U and Th range from 21 to 55 ppm and 6 to 31 ppm, respectively, and correlate with lower Raman peak in tensities, wider Raman peaks, and shifted Si-O peak positions. Concent rations of heavy rare earth elements range over a factor of three to f our and correlate with intensities of fluorescence peaks. Correlated v ariations in trace element concentrations reflect the original magmati c differentiation of the parental melt similar to 4 b.y. ago. Degradat ion of the zircon structure, as reflected by the observed Raman spectr al parameters, has occurred in this sample over a range of alpha-decay event dose from similar to 5.2 x 10(14) to 1.4 x 10(15) decay events per milligram of zircon, as calculated from the U and Th concentration s. This dose is well below the similar to 10(16) events per milligram cumulative dose that causes complete metamictization and indicates tha t laser Raman microprobe spectroscopy is an analytical technique that is very sensitive to the radiation-induced damage in zircon.