Zircon formation and stability under hydrothermal conditions

• Applicant: Dr. Christian Bergemann
• Subject Area: Mineralogy, Petrology and Geochemistry
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 465952655

Zircon is arguably the most important mineral for the geochronological and isotopic investigation of most geologic environments. It is used in the study of magmatic activity, metamorphism, as well as sedimentary units. There is, however, a notable exception to this wide use of zircon geochronology, and that is its application to hydrothermal environments, where zircon is actually far more abundant than commonly assumed. The study of zircon in hydrothermal environments is key to understand late-magmatic and (retrograde) metamorphic, as well as ore-forming processes.To fill this apparent gap, the principal goal of this project is to quantify the geochemical and pressure-temperature conditions controlling zircon crystallization in hydrothermal environments. The chemical and isotopic composition of hydrothermal zircon should reflect the conditions of precipitation from aqueous fluids. To investigate this, three major environments where hydrothermal zircon may readily form are targeted. Firstly, hydrothermal zircon as an overgrowth on pre-existing zircon in retrograde Alpine-type fissures. Secondly, rodingites (a metasomatic mafic rock) where multiple generations of hydrothermal zircon formed during initial rodingitization and subsequent deformation. Thirdly, using the insights gained in the first project parts, a variety of hydrothermal mineral deposits will be targeted.Already acquired clearly hydrothermal zircon crystals from these environments will be complemented by sample material to be collected in a field campaign. Combined with state-of-the-art micro-analytical methods, this will allow constraining the thermodynamic and geochemical conditions favoring hydrothermal zircon crystallization. To this end, the following goals are identified: (1) defining the properties of zircon that are diagnostic of hydrothermal growth, (2) testing how hydrothermal zircon formation and chemistry depend on the composition of the precipitating fluid, (3) determining how zircon formation and chemistry depend on the surrounding country rock and (4) identifying the sources of Zr for zircon crystallization in hydrothermal fluids. In order to characterize hydrothermal zircon and its host rocks through their chemistry, age, and isotopic composition, a multi-method approach will be applied.The outcome of this three-year project will significantly impact the field of geochronology by improving the understanding of a variety of settings in which hydrothermal zircon may form. In providing a new tool for dating fluid-induced alteration processes of hydrothermal environments the proposed research will greatly benefit the study of mineral deposits, as it would permit to directly date hydrothermal ore-forming processes. This will pave the way for long-term research in collaboration with the mineral exploration industry in order to test and enhance the criteria for identifying hydrothermal zircon, both under controlled experimental conditions and in mineral deposit research.

• DFG Programme: Research Grants
• International Connection: Spain, Switzerland
• Cooperation Partners: Dr. Kalin Kouzmanov; Dr. Casto Laborda Lopez; Professor Dr. Josef Mullis; Professorin Dr. Daniela Rubatto