MINOR AND TRACE-ELEMENT ANALYSIS OF NATURAL ZIRCON (ZRSIO4) BY SIMS AND LASER-ABLATION ICPMS - A CONSIDERATION AND COMPARISON OF 2 BROADLY COMPETITIVE TECHNIQUES

Naturally occurring zircon (ZrSiO4) is the most important mineral for U-Pb geochronology. Due to its exceptional stability in most geologica l conditions a single crystal may possess multiple internal structures which may reflect and record a succession of individual geological ev ents. SIMS and laser ablation ICPMS microprobe techniques are consider ed and compared as tools for sensitive, accurate and precise analysis of internal structures in zircon for minor and trace elements. The com parison is generally applicable to all geological and solid inorganic materials. LA-ICPMS is a rapid technique with excellent precision (5-1 0%), but as revealed by analysis of a natural zircon, results may be c ompromised by sample heterogeneity. SIMS analyses require a much small er analyte volume and are less susceptible to averaging of sample hete rogeneity. Replicate analyses on standard zircons reveal that SIMS is sensitive to even micro-scale heterogeneity so that mean results of re plicate analyses may have large standard errors. For routine analytica l purposes the two technologies are competitive, but may be complement ary under non-routine analytical conditions or depending upon sample c haracter as illustrated by textural features and SIMS analyses of comp lex igneous/re-crystallized composite zircon from Queensland, Australi a. SIMS and LA-ICPMS analyses of extraordinary zircons from the ultram afic MARID suite of nodules, South Africa, reveal characteristics of a n igneous-melt origin. The zircons have HREE-enriched chondrite normal ised patterns and Th/U ratios typical of igneous-melt precipitated zir con. These features as well as calculated melt compositions provide su pport for an igneous-melt residue origin for the MARID suite, although an alternative origins model involving an alkali-rich aqueous fluid c annot be ruled out.