The current state and future of accessory mineral research

Abstract

Over the past decade, there have been many significant advances in the area of accessory minerals research, notably permitted by the development of imaging and in situ measurement techniques. In this paper, we review some recent developments and suggest areas on which to focus future research. The magmatic stability of key accessories like zircon and monazite is now reasonably well known and the past decade has seen a large improvement of the knowledge on the metamorphic stability of monazite, epidote, sphene, and zircon. However, other stability domains such as supergene and hydrothermal conditions remain poorly known. Such data are nevertheless essential as the occurrence or transformations of accessory minerals are being increasingly used as probes of the conditions and timing of their host rock transformations. The stability of accessories plays also a key role on the mobility of geochemically important trace elements, often predominantly hosted by these phases in rocks. The recent years have also seen extended efforts to improve our knowledge on the crystal chemistry, crystallographic substitutions, and the mechanisms of element mobility within accessory mineral lattices, based both on natural cases and experimental studies. Zircon, monazite, and apatite were the main targets of these investigations. These researches resulted in the derivation of new metamorphic geothermometers, allowed to improve our knowledge of the behaviour of radiometric systems hosted by accessory minerals, and investigated the nature and quantity of nuclear waste that could be stored in ceramic waste forms with structures and compositions similar to those of monazite or zircon, for example. Much remains to be done in this area, however. Geochronology is another major incentive for accessory mineral research. Recent years have shown the multiplication of geochronological investigations carried out in situ with combined microtextural and microchemical investigations. These researches illustrated the wealth of chronological information locked in accessory minerals. This is, however, a rapidly evolving field, which will strongly benefit from improved understanding of internal mineral textures, mechanisms of element mobility within crystals, and future development of in situ analytical techniques like Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) and ion microprobe.