THE EVOLUTION AND FUTURE OF METHODS AND APPLICATIONS OF MONAZITE GEOCHRONOLOGY AND GEOCHEMISTRY TO EARTH SCIENCE

Monazite geochronology has been around for many decades, but the last 25 years have seen major advances in understanding its occurrence in metamorphic and igneous rocks, its ability to provide metamorphic and igneous histories through U-Th-Pb dating, its integration into mainstream metamorphic petrology and thermometry, its applications to diagenesis, and in using dating, trace elements and Sm-Nd isotope geochemistry to study earth history. If not for its more limited occurrence and lower durability during weathering, it may have rivalled zircon as a key mineral in the study of tectonics and earth history. Isotope geochronology of monazite has evolved from bulk isotope dilution TIMS analysis to be complemented by the high sensitivity, high spatial-resolution area of SIMS, LA-ICP-MS, and EMPA analysis that now dominates U-Th-Pb geochronology. This paper will give a seasoned review of monazite geochronology and examples of how improved analysis methods using both TIMS and in situ methods have opened up new areas of science, particularly in metamorphism and tectonics, provenance analysis of sediments, and chronology of magmatism. However, challenges remain in fully understanding how to best measure and interpret ages to take account of its high Th content, its proclivity to contain excess radiogenic ²⁰⁶Pb, and its often problematic behaviour in terms of both analysis, internal zoning via back scatter (BSE) imaging, and U-Th-Pb systematics, all of which have a bearing on its application to use in tectonics. The most significant current analytical challenge is to improve the accuracy and precision of in situ interelement U-Th-Pb measurements to approach counting statistics and to increase sensitivity so that precise and accurate ages can be measured on smaller and smaller domains. With increasing access to measurement technology, there is a need to ensure that measurement and behavior complexities are not ignored in arriving at age interpretations that may be overly simplistic. The frontiers of monazite U-Th-Pb dating, isotope geochemistry and chemical imaging will be discussed in the context of where monazite applications are going in the future. Increased use of monazite and other accessory minerals is essential to better understand earth history, to redress the predominance of zircon-centric studies.