Accessory minerals as tracers of crustal processes

This Chemical Geology special issue stems from oral presentations given at two symposia held in 2007; Accessory minerals as tracers of crustal processes, at The Goldschmidt Conference in Köln, 19–24 August, 2007, and Internal textures, and trace element and isotope geochemistry in accessory minerals: Advances in imaging and in situ microanalysis, at Frontiers in Mineral Science held in Cambridge, 26–28 June, 2007. Accessory minerals are, by definition, a volumetrically minor component of most crustal rocks, but they comprise a rich and sometimes unique repository of geological information. For example, the oldest Earth materials known are ancient zircon, which provide glimpses into the evolution and environment of the early Earth. A more comprehensive understanding of the links between igneous, hydrothermal, and metamorphic processes, and geochronology and petrology, however, remains one of the major interdisciplinary challenges in modern geosciences. In addition to opening a window to the distant past, emerging techniques, in tandem with new generations of analytical instruments that combine geochronology, trace element geochemistry, and radiogenic tracer isotopes continue to revolutionise the fields of provenance fingerprinting, geochronology, petrology, and petrogenetic processes. In response to these developments, accessory mineral studies have become increasingly sophisticated and wider ranging in recent years, and this is well encapsulated and amplified by the diverse ensemble of papers of this special issue. Progress has been made on several fronts: First, the microanalytical tool box has grown ever more sophisticated, and as such more of the chemical and isotopic information harboured by these mineral archives is being extracted with greater spatial resolution, accuracy, and precision; Second, analytical strategies are increasingly aligned towards integrating different strands of chemical and isotopic data acquired from the same micro-volumes of individual mineral grains. Much greater emphasis has also been placed on extracting in situ chemical information from accessory minerals in their textural context from the nano- to micro-meter scale, which is an important step forward for the interpretation of rocks with complex histories. This allows geochronological and petrological data to be more directly linked with metamorphic and igneous processes; and Third, a better understanding is being gained of the physiochemical conditions that govern the stability and growth/dissolution of accessory phases during crustal processes. This in turn promotes robust interpretations of geochronological data and also provides new insights into trace element and isotopic behaviour during metamorphism and fluid activity and oxygen fugacity during late magmatic evolution. This special issue, which includes eleven papers from several of the talks presented at these two symposia, provides fresh views on different strategies for how to meet these challenges. In brief, the manuscripts highlight a broad range of topics covering a variety of analytical techniques, and a diverse array of accessory minerals. Recent advances in analytical techniques, provenance tracing, and the ability to unravel igneous and metamorphic processes are emphasised